Homemade T-network tuner
last updated 2 January 2022
This is a homemade T-network tuner that I probably picked up at a hamfest some years ago – I don’t remember the details. It is very well built.
It has an 18 uH roller inductor with a turns counter, and the two 310 pF variable capacitors have reduction drives and nice large knobs. The only other control is a bypass switch, which is convenient.
The internal construction shows that the chassis was original from some other equipment, possibly a receiver. I found markings for two 12AX7 tubes / valves, but most prior markings have been polished off.
Overall, this tuner has a nice feel to it, and a handle on the side is convenient, given its size.
In the tuner efficiency test, this was the best of the T-network tuners on 17m and 15m, and the worst on 40m and 12m (where the roller inductor was at minimum). I was unable to find a match for the low impedance on 20m. The large capacitors make it more suitable for lower frequencies than higher ones: while the 18uH coil is often recommended for 80m, I’ve used it successfully on 160m on occasion, although the matching range will be more limited.
One drawback that I want to investigate is that there may be parasitic capacitance in the wires running from the bypass switch to the tuner that changes the matching on higher frequencies. That’s just a guess, however, as I haven’t disassembled it far enough yet to see what is under the chassis.