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.

Internal view shows that the chassis was reused from an older piece of equipment.

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.