Tuners & Amplifiers
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QRP Antenna Tuner - July 5, 2020
The design for this tuner is nothing revolutionary, it is a basic L-match tuner. My version was inspired by this one, except I used a tapped inductor (10uH air core inductor wound on a piece of 1/2" PVC pipe) instead of the variable type. I also used a 15-384pF variable capacitor that I have had laying around for years and never found a permanent use for. The goal for this tuner was to a have a flexible impedance matching device for use with my various QRP tranceivers that can tune almost anything. Even though I already have a very capable QRP autotuner (the Elecraft T1), manual tuners come in handy from time to time.
To wind the coil, I used the K7MEM coil calculator. For 10uH on a 1/2" PVC pipe form using 24 AWG enamel wire I used 40 turns and tapped the coil 12 times. This allows the inductance to be varied by clicking through the various rotary switch positions. An important thing to note is the piece of plastic under the tuning capacitor and the space between the capacitor frame and the front of the enclosure. This isolates the stator from the enclosure since the rotor is grounded to the enclosure and you don't want to short two halves of the capacitor. A clever feature of this tuner design is that the maximum capacitance can be increased by switching additional capacitors (270pF & 510pF) in parallel with the main variable capacitor using a DPDT center off toggle switch. This allows the tuner to match a wider range of impedances when additional capacitance is required.
The tuner was constructed inside of a 5" x 4" x 3" aluminum enclosure. I used BNC connections instead of SO-239 since this is the standard for QRP operations and matches my QRP transceivers. To allow for easy connection of a wire antenna directly to the back of the tuner use a BNC to binding post adapter.
HobbyPCB Hardrock 50 Amplifier & Auto Tuner Kits - December 28, 2017
Now that I have a couple of QRP radios, especially the very capable mcHF, I became more interested in amplifier options to boost my power output when the need arises. I also wanted to build the amp myself to add to my collection of kit built equipment. After looking around, I decided to go with the HobbyPCB HARDROCK-50 Amplifier and ATU.
At $300 for the amplifier and another $180 for the tuner this is not the most economical kit, however, this is a very full featured amplifier and the antenna tuner adds a lot of additional functionality. The amplifier can be driven by a PTT signal or a carrier and also features rig interfacing with some QRP radios for automatic band switching. Both the amplifier and tuner can be inline or bypassed. This is a great feature since the tuner can be used with your radio at QRP levels when you don’t need to use the amp. It is also a very wide range tuner, covering roughly a 10:1 SWR range. The tuner board also adds 60 meter band and standalone Watt/SWR meter functionality.
The kits themselves are very well put together with well made boards (pre-populated with surface mount components) and good quality components. The case is super rugged and all of the machining and fit and finish were very well done. I really like that the amp is completely air cooled and should only require a fan under extreme circumstances. The instructions (HR-50, ATU) are also very well put together with lots of pictures and a ton of detail to step you through the assembly, calibration, and operation of the amplifier.
The amplifier section took about 7 hours to build and calibrate. Most of this involves soldering all the components (including all 15 relays) as well as winding and mounting the 14 inductors. The auto-tuner board took another 3 hours to assemble. This board is almost entirely relays (17) and inductors (9). Integrating the two boards together is very straightforward. It’s a testament to the design of these kits that you can build the amplifier as a standalone kit and then add the auto-tuner later and only have to modify one coax connection.
These kits have been around for a few years and various corrections and bugs have been ironed out, so my kits didn’t require any hardware mods and came pre-loaded with the current firmware revisions. The amp’s screen displays the hardware, amp firmware, and ATU firmware versions on boot-up.
Once everything is running the screen displays the current keying mode, the band selected, heatsink temperature, and power supply voltage. When the amp is keyed the LED turns red and the display shows a bargraph of power output as well as the SWR and output power in PEP. To initiate the auto-tuner you press the Key Mode button after keying the amplifier.
In order to integrate the amp with my mcHF I adjusted the full power setting of the radio from 10 watts output to 2-4 watts output depending on the band. This was done to provide the appropriate drive level for 50 watts out of the amplifier. Using this setup I tuned around the bands and found a Belgian station calling CQ on 80 meters and received a good signal report when I responded to him. Not a bad first contact. I look forward to using this amp and tuner more in the future especially with my mcHF since I am no longer limited to QRP operating.
Elecraft T1 QRP Autotuner Kit - October 28, 2016
In the months since I completed my mcHF SDR transceiver kit, I have thought about building a QRP antenna tuner to go along with it. After some investigating I came across the Elecraft T1 which is available assembled or as a kit and can handle 10W of continuous power. While I have never owned any Elecraft gear, they have a very good reputation and several people in my ham radio club swear by their equipment. The kit looked like a fun project and a perfect match for my QRP gear so I decided to order one.
The kit took about a 4.5 hours to complete. The included instructions are very detailed and do a good job of emphasizing critical parts of the build. The biggest issues arise in regard to several components that need to be mounted in a very specific way in order for the case to fit properly. The circuit boards are fairly tightly packed, but anyone with good soldering experience should have no problem assembling this kit.
The finished product is very compact and incredibly simple to operate. I really appreciate that the instructions are printed on the front label in case you forget. So far I have used it to tune a couple different antennas of various designs and it performs very well. It finds matches quickly and the relays aren’t annoyingly loud like some autotuners. I look forward to getting a lot of use out of this and my mcHF.
Manual Antenna Tuner - July 19, 2016
Automatic antenna tuners are incredibly convenient devices. They also require power, interface cables, and many have a limited impedance matching range. Manual tuners, on the other hand, feature a simple design and can have a very wide matching range.
After deciding that I wanted to build a T-match type of tuner, I needed to find some high voltage air variable capacitors. I looked around at what was available and found this great pre-made assembly that features two 22-360pF variable capacitors (rated for 1kV) and a 12 position rotary switch (rated for 5A). This should be able to handle 100W when properly matched. After deciding on this component I selected an 8″ x 6″ x 3.5″ aluminum enclosure for the tuner.
The next part of the tuner is the inductor. In order to make use of the 12 position rotary switch I needed to make a coil with taps. I also had to keep the coil relatively small so that it would fit in the enclosure I was using. After doing some research I found that an inductance of 30-40uH is typical for a T match antenna tuner and should be able to match a good range of impedances from 160M – 10M. K7MEM has a great single layer air core inductor calculator that uses common PVC pipe as a coil form. For the coil I decided to use 18AWG teflon insulated wire since it should be large enough to handle 100W and the teflon insulation is both easy to work with and very heat resistant. Using the calculator I found that 46 turns of wire on a 1.25 inch PVC pipe resulted in a 35uH inductor that would fit nicely in the enclosure.
To wind the coil I used a ring terminal to secure the starting end and started winding. At every tap point I separated the insulation and soldered a jumper to the exposed wire. Then I continued winding until the next tap point and so on until I finished the coil with another ring terminal. I tapped the coil at turns 2, 4, 6, 8, 12, 16, 20, 24, 30, 36, and 42. The finished coil was then bolted on top of nylon spacers to the enclosure.
To finish the tuner I wired the taps in the order they were wound to the rotary switch. The start of the coil should be wired to the point where the variable capacitor rotors are connected together. The end of the coil should be grounded to the enclosure along with the common point on the rotary switch. Each variable capacitor stator is wired directly to the center of a SO-239 connector, one to the input and one to the output of the tuner. Finally, I added a ground stud to the enclosure.
When using a tuner of this design it is good to keep in mind that the most efficient match occurs when the capacitance is at a maximum and the inductance is at a minimum. Therefore when adjusting the tuner I always start with the capacitors at close to their maximum setting (fully meshed) and the inductor on the first tap. I then click through the inductor taps until I see a dip on the SWR meter and adjust the capacitors to achieve the lowest SWR I can.
I have used this tuner with a couple different antenna designs and it performs fairly well. When used with various dipoles and other wire antennas I have been able to achieve matches the majority of the time. However, I may need to adjust the design of the coil since it seems that I never make use of the later taps and may not require such a large coil.
Update – October 2019
After using the tuner more regularly in my station recently I came to the conclusion that it would be very useful to add a bypass switch. This was done using a DPDT toggle switch with one side wired to the tuner and the other side shorted. Since I was reworking the tuner a bit, I also redid all the labels on the tuner to make it a little more pleasing to the eye.