Elecraft T1 QRP Autotuner Kit

img_0841In 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.

img_0842The 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.

img_0844img_0843The 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

Manual Tuner (10)Manual Tuner (9) 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.

Manual Tuner (6)Manual Tuner (4)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.

Manual Tuner (2)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.

Manual Tuner (5)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.

Manual Tuner (3)Manual Tuner (1)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.

Manual Tuner (8)Manual Tuner (7)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.