Antennas & Feedlines

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Diamond Vertical Loop Antenna - 2020

Coming Soon.

Magnetic Loop Antenna - July 12, 2020

The magnetic loop is one of the more unique and fascinating antenna designs in existence. It seems impossible that such a small antenna could perform as well as it does. I've been toying with the idea of building a mag loop for a few years and finally completed my antenna. There are a ton of good resources on the web about mag loops and I'll leave it to them to go into the details of its theory of operation, design techniques, etc.

• http://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx

• https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm

• https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf

• http://www.kk5jy.net/magloop/

• http://www.aa5tb.com/loop.html

The main idea of the antenna is that it generates a magnetic field to propagate radio waves instead of an electric field like most antennas. These antennas have good directivity, lower noise reception, and interesting radiation patterns featuring both low and high angle radiation making them good for both DX and regional communication. The design I chose is fairly standard: 10ft of 0.5" copper tubing (0.6" OD) bent into a circle, an inductive feedpoint made from 2ft of solid 12 AWG wire, and a split stator air variable tuning capacitor. All of this is mounted on a cross types structure made of 3/4" PVC conduit. To make tuning the antenna easier I mounted a vernier reducer to the shaft of the capacitor. I also wanted to be able to tune the antenna remotely; this helps prevent issues with the operator coupling to the antenna and affecting the match as well as keeping people away from the high voltage parts of the antenna (even with only 10 watts input this antenna has roughly 1000V across the tuning capacitor).

Remote tuning was accomplished using a stepper motor controlled by an arduino microcontroller with a stepper control board. The remote control box is connected to the arduino using 30 feet of cat5 cable. I used 5 wires in the cat5 (common, 4 inputs) and wired them to the arduino board. This gives me the ability to run the motor at two different speeds as well as forward & reverse commands. The stepper motor has 200 steps per revolution. Since the capacitor goes through its entire range in a little under 180 degrees, that leaves less than 100 steps to tune the antenna. The vernier reducer provides a 6:1 reduction resulting in about 600 tuning steps. High speed is useful when changing bands and low speed moves 1 step at a time for fine tuning. In practice the system works pretty well and once I get close to the match point I can adjust the SWR for the best match easily. With that said, it would be even better to have more steps available to really fine tune the match, but it is still totally functional as it is. One thing I discovered while programming the controls is that the stepper motor controller will actively hold the motor in position. This uses power and heats up the motor. To avoid this potential issue I programmed the controller to "release" the motor after a few seconds without a command. This should ensure that the system draws minimal power (the arduino is powered by a 12V battery) and doesn't overheat the motor.

Part of the fun of building this antenna was experimenting and learning about its characteristics. The matchpoint is adjusted using the tuning capacitor, however, the lowest achievable SWR is limited by the position of the feed point. By moving the feed loop vertically on its support I was able to lower the minimum SWR I could tune. I originally had it closer to the main loop, but I found that moving it away a couple of inches improved the SWR. The capacitor itself has two 180pF sections connected in series through the shaft of the capacitor itself. This reduces resistive losses and improves the efficiency of the antenna. It also halves the effective capacitance of the capacitor leaving me with only 90pF of range. Due to this limitation I can only tune from the 20 meter to the 10 meter band. If my capacitor was a little larger I could tune the 30 meter band as well, though with reduced efficiency. For use on the lower bands I would really need to increase the size of the loop, both the circumference and the size of the conductor.

This has been a fun test bed for experimentation and it also works as an antenna. Setup in my driveway I made a couple of contacts on 20 meters using FT8 while running at 5W. Nothing spectacular, but still proof that the antenna radiates.

• Download Arduino Code

End Fed Half-Wave Antennas v2 - July 5, 2020

A few years ago I built some end fed antennas that worked reasonably well. Since then I've learned of an alternate design that is far superior. The primary difference is the turns ratio on the impedance transformer used to match the antenna to 50 ohm coax. The old design used a 27:3 turns ratio resulting in an 81:1 impedance transformation. The preferred design uses a 14:2 or 21:3 turns ratio resulting in a 49:1 impedance transformation. The new design also changes the 150pF capacitor to a 100pF capacitor (two 220pF in series is another alternative to double the voltage rating).

The theory of operation of this antenna is that by feeding a resonant half-wave antenna at the high impedance end point, it allows the antenna to be easily matched on all multiples of the fundamental frequency. Consequently an antenna cut for the bottom of 80 meters (about 3.55 MHz) will also resonate at 7.1 MHz, 10.65 MHz, 14.2 MHz, 17.75 MHz, 21.3 MHz, 24.85 MHz, and 28.4 MHz. This places the resonant point within or very close to every ham band in the HF spectrum! This unique ability makes this antenna design incredibly useful as a multi-band antenna for home or field use. By being end fed it also make setup much easier than a typical dipole since the feedpoint and heavy matchbox are near the ground instead of suspended high in the air.

I have been running a version of this antenna at my home station for about 2 years now in an inverted L configuration between two trees. The matchbox is mounted about 3 feet off the ground at the base of one tree. The wire runs up the tree and through a 90 degree PVC conduit elbow attached to the tree with a bungee to act as a strain relief. The wire then continues to another tree in my backyard where it is secured by a rope. The total antenna wire length is about 133 feet and is usable from 80-10 meters. The antenna is a good match on multiple bands and I use a tuner to clean up the ones that aren't an acceptable match (definitely needed for matching the voice section of 80 meters). There are a few other tricks people use to shift the resonant points of various bands: a capacitor in the center to shift the 80 meter frequency without affecting the higher bands, and a small coil near the feedpoint to pull down the 10 meter resonant point, however, I find this simpler configuration works fine for me.

The construction of the impedance transformer involves winding a primary (smaller) and secondary (larger) winding on a type 43 ferrite core. One side of the primary & secondary are shorted together and connected to the shield side of the coax connector and a ground stud. This can then be connected to a ground rod and/or counterpoise system. The other side of the primary is connected to the center pin of the coax connector. The other sided of the secondary is connected to the antenna wire. Finally the 100pF capacitor (or equivalent) is connected between the coax connector's center pin and shield.

So far I've made 3 different versions of this matchbox. The first is a medium power design using a single FT240-43 ferrite toroid wound with 18 AWG enamel wire with 2 primary and 14 secondary turns and a 100pF 1kV capacitor. This design can handle about 100 watts SSB and 30 watts digital.

For some extra overhead I also built a heavier duty version using two FT240-43 toroids stacked on top of one another. This design used the same 14:2 turns ratio, but with 14 AWG enamel wire and two 220pF 3 kV capacitors in series. This should be able to handle over 300 watts SSB and about 85 watts digital. For this model I used larger drain holes covered with screen to allow ventilation while keeping out insects. I use this as my primary matchbox at my home station.

For lower power operation I built a version using a single FT140-43 toroid wound with 18 AWG enamel wire. This version uses a 21:3 turns ratio and a 100pF 1kV capacitor. This design should handle about 60 watts SSB and 15 watts digital. Finally I built an even smaller matchbox for true QRP operation (5 watts digital & 15 watts SSB). This one uses the same 21:3 turns ratio using 22 AWG enamel wire on a small ferrite bead. As you can see this could have fit in an even smaller enclosure, however, I couldn't find one I liked and I already had this one on hand.

The larger models are mounted in 4"x4"x2" NEMA 4X plastic boxes available at Lowes or Home Depot and use #10-32 stainless hardware. The smaller boxes are ABS Plastic, 3.25"x2.1"x1.5" for the FT140 based matchbox, and a 3.25"x2.1"x1" for the smalles matchbox; both use #8-32 stainless hardware.

K1RF made a great presentation discussing this antenna design in detail. It very much worth checking out if you are interested.

QRP Link Dipole Antenna - September 23, 2017

Dipole antennas are some of the simplest antennas to build in addition to being very efficient and solid performers. I wanted to make a simple dipole antenna for QRP portable operation that could be used on multiple bands. I also wanted it to be light enough to be supported by my light-duty 31 ft Jackite mast in an inverted V configuration.

Link dipoles are a great way to make a lightweight multi-band antenna because you only have one run of wire (vs a fan dipole), there are no traps or coils, and you don’t need a complicated and heavy balun (vs an off-center-fed dipole). Band selection is achieved by connecting or disconnecting the appropriate links to make the antenna as long or short as needed to work on the band you want. To keep the antenna as light as possible I used 26 AWG insulated stranded copper-weld wire from The Wireman (#534). This wire is small, but because it has a steel core it is stronger than pure copper wire would be. It also has very tough insulation that protects the wire very well and is exceedingly lightweight (1000 ft weighs under 1 lb, and I am only using about 66 ft).

For the physical links in the dipole I used Nite Ize MicroLock S-Biners. These are small polycarbonate double-carabiners that are more than strong enough and very light. I made a loop at each end of every antenna section and secured it using adhesive lined heatshrink tubing. While not the strongest connection this should be adequate for this application since the antenna is so light that there isn’t much strain on any one point. The electrical links in the dipole were made using Anderson Powerpoles. The antenna itself was cut for the 20, 30, and 40 meter bands. I am considering adding an 80 meter section in the future, but that may add too much weight.

The center feed-point was made using a small piece of 1/8″ acrylic. This was drilled for antenna wire strain relief as well as #8 bolts for the connection between the antenna and the coaxial feedline. I decided to forego using a 1:1 balun at the feed-point to save weight. The feed assembly is secured to the mast using wire ties. Since every mast section is tapered the assembly can only slide so far down from the top before it fits tightly to the mast. I adjusted the wire ties to place the peak a couple of feet down from the top where the mast is somewhat more substantial and can more easily support the weight of the antenna. For coax I have been using RG-58, again to keep the weight down. I may end up moving to RG-8X in the future if the mast can support it since it is much lower loss.

A handy feature of this antenna is how easy it is to put up. The mast is a good match for my car’s flag pole hitch mount (using a 2″ PVC spacer) and is strong enough to be freestanding. To erect the antenna I just have to secure the upper three sections of mast (the sections friction lock together), place the mast in the mount on my car, unroll the antenna, slide the feed-point onto the mast, connect the coax, push the rest of the mast up, and spread out & secure the antenna ends. I can be on the air in about 5 minutes.

All of my efforts to keep the antenna as light as possible definitely paid off. Together the antenna and winder weigh only 15 oz (not including feedline). This makes it a perfect match for the lightweight mast and a small QRP radio. I look forward to getting a lot of use out of this setup.

WSPR Antenna Comparison (Loop vs Dipole vs End Fed) - February 20, 2017

Over the last few months I have been playing with WSPR and I wanted to be able to use it as a rough way to evaluate the relative performance of different antennas. My goal was to test how well two different antennas that I made for field use compare to my base station antenna.

Antennas

My base station’s loop skywire was tested in its current configuration, 270 feet of wire strung between several trees. The two test antennas were put up in the same configurations that I intended to use them in the field.

• Loop Skywire (average height of about 35 feet)

• 80/40 Loaded Dipole (inverted V, center at 21 feet, ends at 5 feet)

• 80/40 Resonant End Fed (sloper, loaded end at 25 feet, feedpoint at 3 feet)

Test Procedure

I did 24 hour WSPR runs using 5 watts of power with each antenna on successive days. The idea was to test each antenna in as equivalent band conditions as possible. By using the WSPRnet Database, I was able to collect signal reports from every station who heard me over the 24 hours. I then put all of this data into a spreadsheet. I calculated the average, median, maximum, and minimum dB signal reports for each antenna on every band I did the test. I also generated the same results for stations within a 500 mile radius as well as within a 300 mile radius. The purpose of these additional calculations was to evaluate each antenna’s performance for EMCOMM situations. Finally I calculated the same data for the distances from the receiving stations.

Results

See PDF Attached Below.

Conclusions

The first thing that I noticed when looking at the WSPR data was how closely the end fed and loaded dipole performed on both bands. The end fed seems to have a slight edge, but I would put this down to it being higher in the air rather than any inherent design advantage. On both bands the end fed is about 1dB better than the dipole, but given band fluctuations from day to day and the inaccuracy inherent to WSPR signal reports I’m going to call this a draw. Even the receiving station distance numbers were strikingly close to one another. This makes sense since both of these antennas are essentially identical, except one is fed in the center and the other is not.

On 80 meters the loop is clearly the best performer, besting both of the test antennas by at least 2dB overall and by several dB for regional contacts. It also reached much further out with almost double the average and more than double the maximum distance to a receiving station. Add in that it had the most spots from almost 40% more stations and it is clearly the most effective antenna.

On 40 meters the loop’s results are more complicated. Looking solely at the signal report data the loop is the worst performer of the three. I found this to be a ridiculous assertion because of how well this antenna performs in my personal experience for both regional and DX communications. One explanation for the relatively poor overall report is that the loop easily outperformed the other antennas in average and median distance to receiving station in addition to receiving 27% more signal reports from 33% more receiving stations. This could have skewed the results because more distant stations with additional spots would give weaker signal reports.

To test this idea I dug a little deeper into the data and looked at stations that heard all three antennas. What I found was that the loop generally had more spots from the same station than the other two antennas. These extra spots always came at the poorest times of day for propagation and consequently resulted in very low signal reports. When the receiving station had spots for all three antennas at the same time of day, the loop almost always had the highest signal report, usually by multiple dB. This combination of factors pulled down the average and median signal reports and masked how well the loop performs on 40 meters. I think this information also points to just how good of a performer the loop is on 80 meters because in spite of having a similar problem to overcome it still received the best signal reports by far.

Using WSPR to evaluate antennas is not an exact science and I am far from an expert statistician, so these results are by no means definitive. That said, I think this was a worthwhile exercise and resulted in some interesting data that generally correlates with my first hand experience using these antennas.

80/40 Meter Loaded Dipole Antenna - February 19, 2017

After having success with my resonant end fed antennas I decided that I wanted to build a more traditional resonant half-wave antenna that was also considerably shorter than normal. The plan for this antenna was to build a lightweight 80/40 meter antenna for field use (as part of my Go Kit) that wouldn’t overload my 21 foot telescoping fiberglass mast. The antenna also needed to be capable of handling 50 watts at 100% duty cycle for digital operation as well as 100 watts of SSB.

Design

Similar to my 80/40 resonant end fed antenna, the goal for this antenna was to achieve resonance on both 80 and 40 meters by using loading coils large enough to isolate the 40 meter element of the antenna while simultaneously greatly shortening the space required for 80 meter operation. Several vendors sell antennas of this design (MFJ, Alpha Delta, etc.), however, I always prefer to build my own since I can build the antenna exactly how I want, save money, and learn something in the process.

There are a lot of good resources regarding how to build this type of antenna. K7MEM has a loaded dipole calculator that lets you play with different parameters to determine how big the loading coils should be and how far they should be placed from the feedpoint. This works best for single band designs, but it also serves as a good way to double check antenna dimensions. I also found this design, as well as an article in the April 1961 issue of QST that both provide a great starting place for antenna dimensions and what size loading coils to use. The coils used tend to be in the 80uH to 130uH range. Larger coils allow for a shorter antenna, however, they also reduce the available bandwidth. I went for somewhat of a middle-ground with 111uH coils. Due to the antenna’s limited bandwidth I planned to use extension stubs to shift the antenna’s resonance from the top of 80 meters for voice work to the bottom for digital operations.

Construction

In order to keep the antenna as light as possible I used 18AWG stranded copper wire. The coils were wound using 22AWG enamel wire. Each 111uH coil was made using 65 turns of the enamel wire on a 1.25″ PVC form (I used K7MEM’s coil designer to figure out the details). I used stainless steel screws and 8-32 hardware to secure the enamel wire and provide a connection point for the antenna wires. I then coated each coil with two coats of polyurethane for weather sealing and to secure the coil to the PVC.

For the 40 meter elements, I first connected the two antenna halves to the center balun (I used a Unadilla W2DU 1:1 balun that I had laying around). Then I connected the other end of the wires to the loading coils. The short 80 meter elements were then wired to the other side of the loading coils. The 40 meter elements were trimmed for resonance at 7.1MHz which resulted in a span of about 67.5 feet for the 40 meter section.

I then began trimming the 80 meter elements. While there is minimal interaction between the 40 and 80 meter sections of the antenna due to the choking effect of the loading coils, when the 80 meter section is trimmed is does slightly effect the 40 meter section’s resonance. For this reason the 40 meter section was left a little long so that when the 80 meter section is the correct length, the 40 meter section resonates on the desired frequency.

After trimming, the 80 meter elements were about 4 feet long for a total antenna span of about 76.5 feet (including the coils). This resulted in resonant frequencies at 7.15MHz and 3.977MHz. I found that by adding 18.5 inch stubs (using Anderson powerpoles) to the end of the antenna resulted in a resonant frequency of 3.583MHz. The 40 meter 2:1 SWR bandwidth effectively covers the entire band. On 80 meters the antenna has about 40kHz of 2:1 bandwidth and 60kHz of 3:1 bandwidth. One major advantage of this antenna over my resonant end fed is that it does not use any complex matching system, only a 1:1 balun. This allows for more aggressive use of an antenna tuner without the risk of damaging the matching system, which increases the usable bandwidth of the antenna. Using the internal tuner in my Yaesu FT-450D I can increase the antenna’s 80 meter 2:1 SWR bandwidth to about 130KHz.

This antenna turned out about as well as I had hoped it would. With the winder it weighs only 3lbs, 1lb less than my 80/40 end fed. It is also a very good match for my Go Kit’s fiberglass mast as this combination held up well even when loaded down with some ice and snow and with wind gusts over 30mph. The loaded dipole makes a nice balance between size and performance and will be my Go Kit’s primary HF antenna going forward.

HF Random Wire Antennas - November 10, 2016

Resonant antennas have a lot of advantages: they are efficient, impedance matched to your transmitter and require minimal tuning. The main disadvantage of resonant antennas is that they are nearly always only usable over a single frequency band. Non-resonant antennas do not present a match on any band by default, however, they can be easily matched to a wide range of frequencies. One of the most common ways to match a transmitter to a non-resonant antenna is to use a 9:1 Unun combined with an antenna tuner.

100W Random Wire

I built this version for field use and wanted to make the design as flexible as possible. To this end I built the antenna such that I can easily lengthen it when extra room is available. The default length is 53 feet and the antenna can be extended to 124.5 feet. These lengths were chosen because they are not resonant on any ham band. The 9:1 Unun for this antenna uses a FT240-K ferrite toroid wound with 18AWG enamel wire. The Unun is mounted to a DX Engineering Balun Bracket to provide a mounting point and antenna wire strain relief. The antenna extension was made by using two DX Engineering Wire End Insulators that are be bolted together for strain relief and Anderson Powerpoles for the electrical connection of the 14AWG antenna wire. For a counterpoise I made two 50 foot lengths using 24AWG speaker wire. I can also use the shield of the feedline coax and then isolate the antenna from the transmitter using a 1:1 Balun/Choke. I have used this antenna using only the 53 foot section of wire and was able to tune all of HF and made a few contacts using my HF Go Kit, although some bands required adjustment of the counterpoise length in order to be in range of the Yaesu FT-450’s antenna tuner.

QRP Random Wire

After experiencing some success with my high power version I decided to build a QRP version. The QRP 9:1 Unun uses a FT140-43 ferrite toroid and is wound using 24AWG enamel wire. This combination should easily handle 10 watts. The physical construction of the Unun itself uses the same strain relief technique as my End Fed Half Wave Matchbox, where 1/8″ acrylic is epoxied to the enclosure used to house the toroid. For this antenna I used 26AWG stranded copperweld and cut it to 29.5 feet with an additional extension to 53 feet. This should allow for quick and easy field deployment using my 31 foot lightweight fiberglass mast. I did some experiments with my QRP transceiver and my QRP Autotuner and was able to tune all of HF using this configuration and two 50 foot counterpoises.

Overall I think these random wire antennas are a good addition to my antenna arsenal. They are not necessarily the best option, however, they are very versatile and can prove useful when a simple multi-band antenna is required.

End Fed Half-Wave Antennas - November 8, 2016

Half wave dipole antennas are generally considered the reference point for all antennas in ham radio, especially on HF. When fed from the center, a dipole makes for an easy impedance match to 50 ohm coax. When fed off-center at an appropriate location (typically the 1/3 point) and fed with a 4:1 balun, the dipole becomes a solid multi-band antenna. Feeding a half wave antenna from the end, however, presents additional challenges because the impedance is in the thousands of ohms. In spite of this, end feeding antennas can be an incredibly convenient configuration because you only need one support (like a tree) and you can easily place your operating position at or very near to the feedpoint of the antenna. This has led to this antenna design to being very popular with portable operators and others who want an antenna that is easy to erect quickly.

QRP Matchbox

While researching this type of antenna I found a couple of blogs (here and here) that have a lot of good information regarding end fed half wave antenna designs. These designs rely on the principles used by the PAR Endfedz which consist of an impedance transformer between the antenna and transmitter as well as a capacitor across the feedpoint. Based on this design I made an impedance transformer using a FT140-43 ferrite toroid (this size toroid is overkill for a QRP application) with 27 turns on the secondary and 3 turns on the primary (24AWG enamel wire). This is then wired such that the start of both the secondary and primary are connected to the coax connection shield. The other side of the primary is connected to the coax center pin. The remaining secondary connection is the attachment point for the antenna. A 150pF capacitor is then wired across the coaxial connection. I used a 1000V mica capacitor since very high voltages are present at the feedpoint.

The matchbox was constructed using a 3.25″ x 2.125″ x 1.5″ ABS plastic box and 8-32 stainless steel hardware. To provide strain relief I epoxied a piece of 1/8″ acrylic to the back of the matchbox enclosure. I also made a strain loop at the end of the antenna wire for attachment to the acrylic sheet using an S hook. This allows the acrylic to carry the load of the antenna, not the antenna connection point. I also used pieces of acrylic for the end insulators since it is the perfect material to weave small wire through and lock it in place.

I wanted to experiment with the effectiveness of this matchbox with different antenna designs. I also wanted to test the antennas in a typical field installation configuration; in this case they were erected as a sloper with one end in a tree about 25 feet in the air and the feedpoint about 5 feet off the ground.

40/20 Meter Half-Wave

This antenna is a full size 40 meter half-wave with a tuning stub in the center to adjust the resonance of the antenna as a 20 meter full-wave. The tuning of this antenna was very straightforward; I simply tuned the main element for the center of the 40 meter band and then adjusted the 20 meter stub for the center of the 20 meter band. With the 26AWG stranded copperweld wire that I used the antenna ended up being about 62 feet long with a 2 foot long stub in the center. This antenna exhibits great bandwidth and easily covered both bands with under 2:1 SWR.

40/30 Meter Loaded Half-Wave

This antenna is a full size 30 meter half-wave with a loading coil/choke and tuning stub at the end of the antenna to provide resonance on 40 meters as well. The loading coil/choke consists of 55 turns of 24AWG enamel wire on a piece of 3/4″ PVC pipe. This coil is approximately 47uH of inductance, which should have an impedance of almost 3000 Ohms at 10MHz. The purpose of the coil is to choke off the current flow and electrically shorten the antenna on the 30 meter band while providing the necessary inductance to resonate the full antenna on the 40 meter band since it is shorter than a full half-wave on that band.

Tuning this antenna required a fair amount of trial and error because the 30 meter element and tuning stub length interact and affect the resonance on both bands. I initially trimmed the main element without the loading coil and had a good match with 42.5 feet of wire. After attaching the loading coil and several feet of tuning stub I found that the antenna appeared to be too short for 30 meter resonance and too long for 40 meter resonance. Eventually after several trimmings I found that a stub length of about 3 feet resulted in the 30 and 40 meter resonances tracking each other when I adjusted the length of the main element. I then added wire to the main element until I achieved a good match on both bands, in this case a main element of 48 feet works well. 30 meters is a narrow band and this antenna easily covers the entire band with under 2:1 SWR. Because of the loading coil, this antenna does not exhibit particularly high bandwidth on 40 meters, however, the purpose of this antenna is for QRP digital operation which does not involve a lot of tuning around, so it was trimmed to provide the best match at the low end of 40 meters and should have plenty of bandwidth for PSK and JT65 operation.

100W Matchbox

After my successful experiments with the QRP matchbox I wanted to build a more robust version for higher power applications. This requires the use of thicker gauge wire and a larger toroid to handle the higher currents and and more powerful magnetic fields. In this case I used 18AWG enamel wire wound on a FT240-43 ferrite toroid, which should easily handle 100 watts of power. A 27:3 turns ratio was used again as well as the same 150pF 1000V mica capacitor. I mounted the completed toroid in a 4″ x 4″ x 2″ NEMA 4X box and mounted it to a DX Engineering Balun Bracket. For the antenna connection I used 10-32 stainless steel hardware.

80/40 Meter Loaded Half-Wave

This antenna is constructed similarly to the 40/30 meter version described above. This time, however, the loading coil consists of 67 turns of 20AWG enamel wire on a piece of 1″ PVC pipe. This coil has an inductance of about 66uH which is required to achieve an appropriate amount of current choking at 7MHz. For strain relief I used 1/4″ Lexan sheet to make the connection points for the antenna and coil wires. I then epoxied the Lexan to the PVC coil form and bolted the connections using 10-32 stainless steel hardware. The coil was sealed using two coats of polyurethane.

I found this antenna to be easier to tune than the 40/30 meter version. This is most likely due to the larger difference in frequency ratio between the 80 and 40 meter bands vs the 40 and 30 meter bands. After trimming I found that a main element length of about 67 feet gave a good match across the 40 meter band. As anticipated this antenna has a limited 2:1 SWR bandwidth on the 80 meter band (about 90KHz). I decided to construct a way around this by adding a tuning stub to the end of the 80 meter section to allow for adjustment of which portion of 80 meters I wanted to operate in. Since the primary usage of this antenna would be for field deployment and emergency communications I would most likely need to be able to use it in the digital portion of the band (3.583MHz or so) as well as the higher end of the band (3.983-3.99MHz) where the ACS nets in my area take place. After some experimentation I found that the antenna was resonant in the voice section I wanted with an 80 meter stub 9 feet in length. I then made an additional 3 foot length of wire that I can add to the end using Anderson Powerpoles that shifts the resonance of the antenna to the digital portion of the band. This allows me to easily change the section of the 80 meter band I want to use by simply adding or removing this small section of wire. This additional wire has a very minimal effect on the 40 meter resonance of the antenna (around 10KHz) and does not prevent the antenna from achieving an SWR of under 2:1 across the entire band whether it is installed or not.

To assess the end fed’s performance I did a side by side comparison with my 80 meter loop skywire. I setup the end fed as a sloper with the loaded end supported by a tree about 25 feet in the air and the feedpoint about 3 feet off the ground. I then observed the signal strength when listening to the local ACS net on 80 meters. I found that I could copy everyone easily with the end fed half-wave, however, they were generally 1 or 2 S-units weaker than with my loop. I also used the End Fed Half-Wave during Winter Field Day and was able to easily make contacts on both 40 and 80 meters using both SSB and PSK31. With the winder this antenna weighs 4lbs, not bad considering the weight and bulk added by the matchbox. Overall I think this antenna is a very solid semi-compromise antenna for field use and will definitely be part of my Go Kit going forward.

Folded Skeleton Sleeve Antennas - June 30, 2016

I am always interested in trying different antenna designs, especially if they are simple to construct and provide increased functionality. While perusing some old issues of QST magazine online I found a series of articles that discuss a design called the Folded Skeleton Sleeve. The design is a unique way to build a dual-band resonant dipole or groundplane vertical. The articles appear in the May 2011, October 2011, October 2012, December 2013, and March 2015 issues of QST magazine.

I was particularly interested in this antenna design because a simple resonant dual-band antenna could be very useful for deployment at Field Day or for EMCOMM purposes. Other multi-band antenna designs exist and can perform quite well (windoms, off-center-fed dipoles, G5RVs, non resonant end feds, dipoles fed with window line, etc.), however, most of these require a wide range antenna tuner to achieve a decent SWR on multiple bands. Other designs, such as trap dipoles, can be heavy and cumbersome with multiple points of failure. The folded skeleton sleeve design exhibits non of these limitations.

Design

The folded skeleton sleeve at first looks like a standard folded dipole, however, the top radiator is not continuous. Two notches are cut along the top of the window line to create the parasitic element that allows for operation on the higher frequency band.

A 75M / 40M antenna should be perfect for both EMCOMM (these are the most common HF bands used for emergency communications) and Field Day. A 40M / 20M antenna is equally perfect for Field Day and the combination of the two provides a lot of operating versatility from two simple antennas that cover the three busiest Field Day bands. I also decided to construct a 40M / 30M antenna for use as a portable antenna for digital communications.

Construction

I built the antennas using 18AWG stranded copper-weld 450 Ohm window line (Wireman #553) and folded dipole insulator kits (Wireman #804) which make fantastic strain reliefs for securing the window line. I also made my own 1:1 baluns in a similar design to what I have done before, except this time I used FT-150A-K toroids and 18AWG wire which allowed me to make the baluns smaller in size while still being adequate to handle 100W. To house the baluns I used Bud Industries PN-1322-DGMB NEMA 4X enclosures. These are well made boxes and they feature convenient mounting tabs that are easily bolted to the center insulator.

75/40 Bandwidth

75 Meter Band

• 2:1 SWR: 3.68-3.785

• 3:1 SWR: 3.63-3.86

40 Meter Band

• 2:1 SWR: 7.18-7.238

• 3:1 SWR: 7.1-7.3

While the bandwidth of this antenna is not particularly wide, it is easily matched to the radio’s 50 ohm output with practically any antenna tuner.

My ham radio club used the 75/40 at our Field Day site for the duration of the event. While obviously intended for use on 75 & 40 meters, the antenna was used on the higher bands as well with the help of a wide range antenna tuner. Over the course of field day this setup resulted in over 350 CW contacts.

40/30 Bandwidth

40 Meter Band

• 2:1 SWR: 7.158-7.33

• 3:1 SWR: 7.073-7.448

30 Meter Band

• 2:1 SWR: 9.93-10.24

This antenna exhibits better bandwidth than the 75/40 and even reaches an SWR of 1.1:1 on 30 meters.

40/20 Bandwidth

This antenna is by far the best design of the bunch. This configuration results in an SWR of under 2:1 across the entirety of both the 40 and 20 meter bands.

Antenna Winders

Since ladder line can be annoying to work with since it doesn’t coil easily, I decided to build some winders from 1/2 inch PVC pipe to keep the finished antennas organized. I built a larger one for the 75/40 antenna and smaller ones for the 40/20 and 40/30 antennas. I am really pleased with how these turned out and plan to build more for use with other antennas; they are a fantastic way to avoid a tangled mess.

6 Meter Quad Turnstile Antenna - June 13, 2016

A quad turnstile consists of two cubical quad loops oriented in a diamond configuration and angled 90 degrees apart from one another with both diamonds sharing the same top and bottom points. The advantage of this design over a single quad loop is that when phased 1/4 wavelength (90 degrees) apart the combination of the two antennas creates an omnidirectional radiation pattern. This type of antenna can also be made from two crossed dipoles, however, using full wave loops instead of half wave dipoles provides about 1 dB additional gain at low elevation angles (there is a great article about building a 6 meter quad turnstile written by L. B. Cebik, W4RNL in the May 2002 QST magazine that goes into further detail about the performance and advantages of this antenna design).

Another advantage, from a construction perspective, is that the spreaders required for a dipole turnstile would have to be 10 feet across. The spreaders for quads only need to be 7 feet across meaning that the use of lightweight pvc is that much more practical. The quad configuration is also perfect for being suspended by a push up fiberglass mast since the antenna is very light weight and virtually all of the loads are directed vertically down the center of the antenna which is also the center of the mast. This results in very little flexing and stress on the light duty fiberglass section at the top of the mast. I built my antenna using the Max Gain Systems MK-6-Standard fiberglass mast which stands 32 feet tall when fully extended.

From previous experiments with 6 meter loops I have found that 20 feet of insulated 14 AWG wire is resonant at the bottom of the 6 meter band (just above 50 MHz) where the SSB activity is concentrated.

The feed point is the most complex component in the entire antenna. It consists of a piece of 1/4″ thick Lexan with three SO-239 connectors (one for the feedline and one for each end of the phasing line) and four #10-32 stainless bolts (one for each loop end) mounted to it. The SO-239s and bolts are then wired together such that the feedline is wired directly to one loop and one end of the phasing line and the other end of the phasing line feeds the second loop. I used red and yellow electrical tape to mark which bolts attach to which loop. I also notched the Lexan sheet to fit around the mast so that I could wire tie it in place.

The phasing line is made using RG-63 coax which has an impedance of 125 Ohms. This is required because the feedpoint impedance of each of the loops is also about 125 Ohms. When fed together via the phasing line the final antenna impedance is approximately 62 Ohms which matches well with the standard 50 Ohm feedline (for this antenna I used a run of 100 feet of LMR400 coax). I purchased my RG-63 coax from The Wireman. The phasing line needs to be 1/4 wavelength long at the bottom of the 6 meter band. To calculate this you can use the formula (246/frequency) = quarter wavelength in feet. Therefore 246/50.5 = 4.871 feet = 58.46 inches. Next we take into account the velocity factor of the RG-63 coax, in this case 84%. Therefore 58.46*0.84 = 49.1 inches which is the length that the phasing light should be including the connectors on each end.

The PVC spreaders are made using 3/4″ PVC conduit glued into a 4-way junction box. By drilling a 3/4″ hole in the center of the junction box it allows the spreader assembly to slide down the top mast section and rest on top of the 1″ section of the mast. This is left to float in place and is not attached to the mast in any other way.

The key to the construction of my version of a quad turnstile is that the entire antenna hangs from the top of the mast. To accomplish this I used a 1/2″ PVC cross with a nylon bolt running through the center. The bolt slides in the end of the 3/4″ fiberglass mast section and prevents the PVC cross from sliding off the top of the mast. I then fed the antenna wires through the cross such that the wires intersect at 90 degree angles. This method serves to secure the wires to the peak of the mast as well as providing some strain relief for the antenna wires. I then splayed out the wires and ran them through notches cut at the end of the PVC spreaders. I then centered the wires relative to the top of the mast and taped the wires to the end of the spreaders to keep them in place until they are attached to the feed point. The wires were then attached to the feedpoint using ring terminals that were soldered to the ends of the wires. The ring terminals make it easy to connect the wires to the bolts mounted on the feedpoint.

To erect the antenna I first raised the top 3/4″ section of the mast until the top wires became taught. This only requires about 3 feet of the top section to be extended such that the spreader assembly is not lifted off of the lower mast section. I then locked the top mast section in place. Next I raised the 1″ section of mast until the lower wires were taught and secured the feedpoint in place with a wire tie and taped the feed line and phasing line to the mast for strain relief. Then I continued raising each mast section until the mast was completely raised, resulting in a peak height of nearly 30 feet. I then finished guying the mast. For this antenna I only guyed the mast at 3 intervals, the bottom, the middle, and the top since the antenna is not heavy and is very evenly loaded on the mast. Three guy lines per interval were used made from UV resistant rope anchored using 10″ spiral ground anchors. This resulted in a very stable mast and the antenna that has held up well, even on breezy days. A tautline hitch is a great knot to know for tensioning guy ropes for masts as well as tents when camping.

After erecting the antenna I checked the SWR and found it to be 1:1 at the bottom of the 6 meter band. I also found that the 2:1 SWR bandwidth was quite large and easily covered the SSB portion of the band. The following day after getting the antenna on the air there was sporadic E band opening in North America and I was able to hear several stations. During this opening I worked my first 6 meter contacts, receiving good reports from stations in Oklahoma, Arkansas, Alabama, Tennessee, and Manitoba. Not too bad for 100W into an omnidirectional antenna in western Pennsylvania.

I also worked about four hours of the June 2016 ARRL VHF Contest and made 20 contacts with stations in 14 grid squares. Like a lot of antenna systems, this one allowed me to contact most of the stations that I was able to hear. I have, however, gained an appreciation for why most people use directional antennas for VHF work. While the extra gain directional antennas provide would definitely be a positive, I can see now how favoring one direction over another can be especially advantageous on 6 meters. More than once I could hear multiple strong stations on the same frequency and with a directional antenna I could have significantly nulled out one of those stations in order to make it easier to hear and work one station at a time. Another advantage would be the ability to focus your signal in the direction in which the band is open, instead of broadcasting in all directions.

With all of that said I am very pleased with how this antenna project worked out. This design is an inexpensive and effective way to get on 6 meters and make contacts which was my goal in the first place. It is also a good all around lesson in antenna design, construction, and phasing lines.

Slim Jim Antennas - April 14, 2016

I am a big fan of the J-Pole antenna style and have built a few of them in the past. Electrically the J-Pole is a half wavelength antenna that is end-fed by a quarter wavelength stub to achieve a feed point impedance of 50 Ohms. The Slim Jim is very similar except that there are two half wavelength segments with the second folded next to the first. This arrangement makes them ideal to be constructed from 450 Ohm ladder line. This also allows them to be incredibly compact and portable since they can be rolled up very easily.

I found a very useful calculator that gives you all of the dimensions needed to build a Slim Jim or a J-Pole from ladder line (you will have to convert from metric to imperial units yourself). Using this calculator set for 146 MHz (the middle of the 2 meter band), I assumed my 450 Ohm ladder line had a velocity factor of 0.91 and cut it to a length of 56 inches (not including the 0.5 inch on each end to be stripped and soldered together). I then cut the half wave radiator 36.75 inches from the top and the quarter wave matching section 18.375 inches from the bottom. This left a gap of 0.8 inches between the two. The 50 Ohm feed point was calculated to be about 1.8 inches from the bottom, however, using my antenna analyzer I found the the feed point should be 2.1875 inches from the bottom. With a piece of RG-8X feedline soldered at the feed point this antenna has an SWR under 2:1 across the entire 2 meter band. To complete the antenna I drilled a 0.25 inch hole at the top so that the antenna can more easily be hung from a mast or a tree or clipped to a wall if used as an indoor antenna.

This type of antenna is a very economical way to build a VHF or UHF antenna. It also has a considerable amount of gain compared to a standard quarter wavelength vertical and is just as easy to build and transport.

Baluns & Ununs - April 14, 2016

Anyone building antennas will come across designs that either recommend or require the use of a balun or unun. The design and construction of these components can get quite complex and are beyond the scope of this blog and my own knowledge. In short these devices act as impedance transformers from balanced loads to unbalanced loads (balun) or from unbalanced loads to unbalanced loads (unun). They can also be used as a common mode choke to eliminate any RF on a coax feed line’s shield. That said, it is actually quite easy to construct your own baluns and ununs and to learn something in the process. You can also save a considerable amount of money.

Amidon sells a good starter kit for building baluns and ununs. It includes everything you need including a book with dozens of designs with various impedance transforming characteristics. I used this kit to make a 1:1 balun. Since the kit uses 14AWG wire, it should be capable of handling 2KW of power continuously. This is overkill for me since I will never be putting more than 100W through the balun. The 1:1 balun is essentially a choke that blocks current flow on the shield of the coax feed line. It is constructed using 10 bifilar wraps on the toroid core using approximately 4 feet of wire.

For other projects I decided to use the same FT-240-K core with 18AWG wire covered in 16AWG PTFE insulation. While the 18AWG has less power handling capability, it should be more than adequate for 100W usage as well as being cheaper and easier to work with.

The first balun I made using these materials was a 4:1 current balun. This balun is intended to transform a 200 Ohm load for use with a 50 Ohm coax feed line. This type of balun is commonly used in Off-Center-Fed dipole antennas because the feed-point is placed at the location on the antenna where the impedance is approximately 200 Ohms on multiple bands. The balun is constructed using two sets of 8 bifilar wraps on the toroid using approximately 8 feet of wire. Each pair of windings is then wired in series with the other pair. This design can be thought of as two 1:1 baluns wired in series and in fact an alternate design of this balun uses two separate 1:1 balun cores wired in series to achieve the same affect.

Next I made a 9:1 unun for use with an end-fed antenna I am building. End-fed antennas exhibit very large impedances and consequently require considerable impedance transformation to get the feed point within the range of an antenna tuner. Unlike a dipole, an end-fed antenna is unbalanced and therefore an unun is used instead of a balun. This design uses 8 trifilar wraps on the toroid using approximately 6 feet of wire. Each wrap is then wired in series to create the desired impedance transformation.

I also made another 1:1 balun. The main difference here is that I constructed it using two SO-239 connectors because I intend it to act as a coaxial feed line isolator. My plan is to use this in conjunction with the 9:1 unun as part of my end-fed antenna project. The idea here is that a section of coax from the 9:1 unun acts as the counterpoise for the end-fed antenna and the line isolator is used to choke the RF current in the shield of the coax and allow the remainder of the feed line to continue to the antenna tuner without risk of radiating RF.

For all of these projects you can see that I used colored electrical tape to mark the various windings. This is essential to keeping track of the wiring and assuring that the windings are wired together correctly. For all of these I also used standard NEMA 4X 4″x4″x2″ plastic electrical boxes which are cheap and commonly available. I also used 10-32 stainless steel hardware for the antenna connections and silver-teflon SO-239 connectors.

Loop Skywire Antenna - October 26, 2012

Design

When I bought my house this spring, I immediately started planning an HF antenna for my ham radio station. It’s been several years since I last had a permanent base station set up and I wanted to get on the air. After evaluating my options, I decided that a loop antenna would be my best option. The biggest advantage for me is that a large loop antenna fed with ladder line allows for good performance on a wide range of frequencies using a single antenna. This design also allows me to maximize the amount of antenna that I can fit on my 1/2 acre lot (an 80 meter loop is only 72ft on a side vs the 135ft overall length of an 80 meter dipole) without having to put up masts or towers.

The general idea of a loop skywire is to put up as much wire as possible, without worrying about cutting it to resonance, and feed it with ladder line. Since ladder line exhibits very low loss compared to coaxial cable, even with a large impedance mismatch, the total amount of signal loss in the ladder line will be minimal. With a good antenna tuner between the ladder line and the radio, all of the HF ham bands should be available.

Components

For wire I purchased Wireman #531, which is insulated wire made up of stranded 13 AWG copper-clad steel. The steel core makes it strong (400 lb breaking strength) and helps minimize stretch, while the copper cladding gives it good electrical conductivity. The insulation helps to protect the wire from the weather.

The feedline I chose is 300 ohm ladder line, which is a little harder to find than its 450 ohm cousin. Some ladder line is cheaply made, but this type from DX Engineering uses 18 AWG copper-clad steel and works very well. They also make a great antenna feed point kit with built in strain relief slots for use with their 300 ohm ladder line. It is well worth the money.

While I could have used a balanced tuner, or some other type of manual antenna tuner, I decided to go with an automatic antenna tuner for my station due to their ease of use and their ability to store impedance matches to memory. The memories allow the tuner to pull up previous tuning settings without having to rematch the radio to the antenna, saving a lot of time. I use a LDG AT-200ProII in my station and it has worked great so far with my loop antenna. I chose this model for its wide impedance matching range, its ability to store 4000 frequency & impedance combinations, and its 200 Watt power rating. Although I don’t plan on using more than 100 Watts in my station, the 200 Watt model is only slightly more expensive and because of its higher power rating it will hopefully be even better equipped to withstand the high impedance mismatches that this antenna presents.

The final piece of this arrangement is the balun. In this case I used a high power current balun between the ladder line and the antenna tuner. This device blocks the current on one side of the ladder line from continuing on to the shield of the coax on the other side. In this way it transforms the balanced load of the antenna and feedline into an unbalanced load for use with the antenna tuner and radio. I could have made my own balun, but I decided to buy a DX Engineering BAL050-H10-AT. This is heavy-duty (rated for 10KW) balun designed for exactly this type of application and is much better constructed than anything I could have made on my own.

Construction

Putting up the loop was a relatively straightforward process. The first step was to pick which trees to put the support ropes in. I don’t have a ton of options on my small lot, but four trees were spaced appropriately for me to make a trapezoidal shaped loop. To get the ropes (I used 3/16″ Dacron) into the trees I used some light nylon cord tied to a wrench and tossed the wrench over the highest branch I could reach. I then pulled the heavier rope up over the branch. Next I attached the insulators that I had made using 1 inch 45 degree PVC elbows (painted black for stealth) and bungie cords. The bungies act as a stress relief between the trees and the antenna, thereby allowing the trees to move in the wind without jerking the antenna too hard. I used bungies on three of the four corners, leaving only the corner nearest the feed point without one.

I then ran the antenna wire through the insulators until I had both ends at the location of the feed point. By taking the slack out of the ropes I was able to start trimming the antenna wire such that when the insulators were lifted as high as I could get them the antenna wire was tight. After a few adjustments, and some branch trimming, I was able to get the antenna in the air. I then set the antenna back down and attached the ladder line to the feed point and raised the antenna to its final position.

Finally I mounted the balun to the side of the house and ran the ladder line to the balun. To support the ladder line I attached some rope to the feedline with zip ties and hoisted it using an eyebolt screwed into the eave of the house. I also made a spacer/strain relief for the ladder line to keep it away from the aluminum siding on my house. This is necessary because if ladder line is too close to anything conductive it can unbalance the feedline, thereby causing it to radiate like the antenna.

Performance

After trimming, my loop ended up being 215 feet in circumference and uses 47 feet of ladder line. I lucked out on the length of ladder line that I needed; you have to be careful not to use a length that is harmonically resonant on any of the frequencies you wish to operate, otherwise the feedline could radiate and cause interference. While this antenna is technically a little short for use on the 80 meter band, it will tune on that band along with all of the remaining HF ham bands (except 160 meters).

Considering the limitations of my property in terms of the size and height (around 30 feet) of the antenna, I couldn’t be happier with it so far. I love the ability to operate from 3.5 to 30 MHz without having to switch antennas. Overall performance has been great. In my limited time using my new station I have been able to contact stations in Europe and throughout the US, as well as have a lot of fun in the Pennsylvania QSO Party (my home state) where I was able to contact pretty much every station that I could hear.

Update - October 21, 2014

After reevaluating the trees in my yard I realized that I could rework the layout of my loop skywire antenna. This would allow me increase the size of the loop, improve the feed point arrangement, and increase the height of the antenna.

After adding a fifth anchor point the loop is now a distorted pentagon instead of trapezoidal in shape (see the sketch for the rough layouts). The new arrangement is not only larger, but also higher than before, which should help its performance. With a new circumference of approximately 244 feet of wire the loop is much closer to resonance on the 80 meter band than it was previously.

Due to the repositioning of the feed point I had to shorten the feedline. I decided on 37 feet of 300 Ohm ladder line as an acceptable non-resonant length for the feedline. This is a good length since it keeps it well off the ground while still providing some slack for movement. So far the performance has been at least as good as the previous version.

Update - July 15, 2015

Now that I’ve had this antenna setup for a few years now I have come to recognize the drawbacks. The biggest of these is a lack of tuning bandwidth. This resulted in having to re-tune the system even when changing frequency by only a few kilohertz, very annoying. After playing with a MFJ 926B remote automatic antenna tuner at Field Day this year I decided to modify my antenna to make use of one of these units and see if it improved my operation. The 926B is actually pretty similar in design to my LDG tuner except it is mounted in an enclosure suitable for use outside and can be powered via coaxial cable using a BiasTee power injector so no extra cables are needed. It automatically initiates tuning when a mismatch over 2:1 SWR is detected while transmitting and it saves the match settings to memory.

The idea of a remote antenna tuner is that it allows the tuner to be located at the feed point of the antenna. This means that the tuner is matching the impedance mismatch of the antenna only; not the antenna, plus feedline combination that it was dealing with previously. This allows the tuner to find a match much more easily and also results in a much better tuned bandwidth because the only variable changing is the impedance of the antenna not the impedance of the antenna and feedline combined.

A side benefit of this new setup is that in order to reach the tuner as it is mounted on the side of my house I had to add some wire to my antenna which now contains approximately 270 feet of wire. The antenna is now solidly resonant in the 80 meter band. In addition to adding wire I separated the feed point in the air by about 10 feet. This allows the wires to drop to the tuner at an angle in order to keep the shape of the loop as intact as possible and prevents the two ends from contacting or crossing one another. I also installed an Alpha Delta TT3G50 surge protector in the coax line from the tuner.

In the short time that I’ve had this setup on the air I have been very pleased with its operation. To tune the system I switch to CW mode on my Icom 7200 and transmit a tone (with the power turned down to 10W). The tuner then initiates its tuning cycle and matches the antenna. This generally only takes 2-3 seconds, much faster than before. It also tends to find much better matches and regularly achieves close to 1:1 SWR. As hoped, the tuning bandwidth is greatly improved as well and I am no longer required to retune every time I move around a band. This new setup is also much cleaner looking on the house with no ladder line or balun, just wires going to the low profile tuner and a coax run along the brick.

2 Meter J-Pole Antenna v2 - June 3, 2012

I recently purchased a house and decided to build another J-Pole for use as the 2 Meter antenna for my new station. My old one is still mounted on my parent’s house and in good shape after several years of exposure to the elements. My old design’s performance was fairly good, but for this new build I decided to tweak the dimensions somewhat (with the help of my MFJ-259B Antenna Analyzer) to get the SWR as low as possible across the whole 2 Meter band. By adjusting the feed point and lengthening the driven element I was able to get the following results:

Frequency SWR

144MHz 1.8

145MHz 1.6

146MHz 1.4

147MHz 1.3

148MHz 1.5

I really like the design of this antenna due to its simple and cheap construction, ideal feedpoint placement for low stress on the coax, and the fact that the whole antenna can be easily grounded since at DC it is essentially a short circuit.

Carolina Windom Antenna - July 7, 2009

I have wanted to build a multi-band wire antenna for some time now and this past Field Day I had an opportunity to use a very good one. The Carolina Windom is essentially an off-center fed dipole (OCFD) that uses a portion of its feed line as a vertical radiator. I used one to make about a 100 PSK31 contacts on 20, 40, and 80 meters during Field Day.

The key to an OCFD’s operation is the fact that there is a point on the antenna where the input impedance is approximately 200 Ohms at multiple frequencies. When fed with a 4:1 balun this provides a reasonable match to the standard 50 Ohm load that my coaxial cable and radio like to see. Even if the antenna doesn’t provide a perfect 1:1 SWR over all bands, it keeps it low enough that a simple antenna tuner can compensate for any mismatch. The problem with OCFDs is that since the two legs of the antenna are of different lengths, the currents in each leg are out of phase. This means that in order to avoid feed line radiation, you should use a 4:1 current balun to compensate for this current imbalance. These are not readily available for sale, but can be constructed from kits. Here’s a great article which describes how to wind your own current balun using this kit from Amidon. You can also buy an equivalent balun from DX Engineering or Balun Designs.

The Carolina Windom, however, wants the feed line to radiate (at least a portion of it) in order to gain the vertical radiation and some performance. To achieve this the Windom uses a 4:1 voltage balun, which matches the antenna’s leg voltages and then uses a separate 1:1 choke balun to isolate the feed line from the vertical radiator. Both of these baluns are readily available and relatively inexpensive.

The first step in constructing my Carolina Windom was to cut the antenna wire to length for an 80 meter version. I used these measurements from Radioworks, who sell pre-assembled Windoms, but other sites also show measurements and formulas for cutting a Windom for any band. My Windom has the same 133ft overall length of a traditional dipole, but is divided into 50ft and 83ft legs instead of equal lengths. Since this antenna will be used for temporary setups such as Field Day and the PA QSO Party, I didn’t make it out of heavy duty wire, instead I used insulated #14 stranded copper wire. This wire is a good compromise between strength and weight. Construction of the Windom is very straightforward; simply solder one end of each wire to opposite sides of the 4:1 balun and attach insulators to the other ends. For the 4:1 voltage balun I used the Unadilla W2AU 4:1 and for the 1:1 choke balun I used the Unadilla W2DU inline-HF. These are well constructed commercial baluns that work well for these purposes. In the picture I do not have the choke balun connected since I did not have it at the time, so in its place I created a poor man’s choke balun by coiling about 10 turns of my RG-58 feed line.

In order to test the antenna I set up my new fiberglass mast to about 25 feet with the Windom on top. While this isn’t an ideal height, it was fine for a test. I connected the feed line to my radio and tested the SWR. I was able to get a match using my Icom IC-703’s internal tuner on all of the HF bands. Since the 703’s internal tuner can only deal with SWRs of less than 3:1, this means the antenna is performing as expected, providing a decent match to the radio on all the HF bands. For a further test I tuned around the 20 meter band. Hearing a special event station in Maine, I gave him a call and he came right back to me. Not a bad first test, getting into Maine with 10 watts. This is exactly what I was hoping for, a solid performing multi-band antenna that I can use for temporary operations. Wire antennas are very simple and cheap to build, and this one is a great project for any type of station.

Here’s a picture of my finished antenna rolled up for storage. In between the 4:1 balun and the Unadilla W2DU inline-HF 1:1 choke balun is 22 feet of RG-8X coax that functions as the vertical radiating section. You could use any type of 50 Ohm coax, but I had some laying around from other projects and it is a good compromise between size/weight and signal loss for a portable antenna such as this.

20 Meter Groundplane - June 12, 2007

Most hams are familiar with the quarter wavelength ground-plane antenna design. It is often the first antenna they buy or build for use on 2 meters after receiving their technician license. It is a design that performs well and exhibits low input impedance, making it ideal for use with ham equipment without the need for special matching techniques. The antenna is easy to construct and due to this simplicity is also highly economical. When considering the type of antenna to build for field day to use with my PSK31 setup this design was the obvious choice. It provides both low take-off angle and omni-directional radiation, allowing me to maximize my operating capability from a simple station. The antenna is made up of a single pole which supports the radiating vertical element and is guyed in place with nylon rope. The two radial elements are spread out and held in place by ropes.

The base of the support pole is made of a ten foot piece of 1.25 inch schedule 40 PVC pipe that was cut in half for easier transport in my car. It is joined in the center by a 1.25 inch PVC coupler. Mounted on top of the PVC pipe is a 13 foot extendable fiberglass fishing rod which I purchased at Gander Mountain (I had originally planned on using a 16 foot rod but none were available when I went to the store, with additional height the radials can be lowered at a steeper angle which in turn raises the input impedance closer to the desired 50 ohms). I joined the fishing rod to the pipe by first removing some of the plastic at the base of the fishing rod so that it could slip inside the pipe. Next I drilled a single hole through the pipe and rod so that I could secure the two pieces together using a small bolt and nut to prevent the rod from sliding further into the pipe. I also wrapped the fishing rod with some electrical tape to compensate for the difference in diameter of the rod itself and its plastic base (this allows the rod to fit snugly inside the PVC pipe thereby stiffening the rod and pipe connection).

The radiating element and both radials are 16.5 foot long 14 AWG insulated stranded copper wire. For ease of assembly I soldered the radiating element to the center of a SO-239 connector and attached solder lugs to the radials. This allows me to attach the radials to the SO-239 with two small bolts passed through the holes on the connector, simplifying construction in the field. I taped the radiating element to the pole prior to raising the antenna. The radials were attached after the antenna was erected and securely guyed since the feed point is only 5 feet off the ground providing easy access for mounting. From start to finish assembling the antenna and guying it in place took about 30 minutes to do by myself (with more people it could easily be erected in 5-10 minutes).

The performance of this antenna was better than expected. It matched perfectly on the lower end of 20 meters despite being cut for the center of the band (this is due to my use of insulated wire which adds capacitive loading to the antenna, electrically lengthening it). Whether you are looking for a solid performing base antenna or a light, compact, portable antenna this may be the project for you.