T-Match Antenna Tuner for Receiving

Impedance matching between a transmitter and an antenna is essential. You can even kill your PA if the reflection is too high and much of the energy is reflected back. But it can be important to match your antenna with your receiver, too, in order to reach the best possible reception.

So I decided to build my own tuner (instead of buying one) for my receiver experiments. My choice was the ubiquitous T-match, because it can match wide range of antenna impedance, and it is simple to build.

Since there are no significant power requirements against the tuner, I could use variable capacitors with any voltage rating. So I bough two of those they use in pocket radios. The exact value does not matter too much until it is in the 400-600 pF range somewhere.

I built the coil using a big yellow Amidon core I bought a long time ago. The winding can be calculated using toroids.info. In my design I made a tap after each 6th turns, there are 11 terminals in total. The resulting inductance range is adequate for my purposes.

One of the benefits of this construction that the terminals can be soldered directly to the switch, no extra mounting or wiring necessary. It greatly simplifies the overall construction and needs no extra mounting and wiring.

The usual blissful mess…. 🙂

I built the whole thing into those die-cast alloy cases. It is a great EMI shield, does not allow dust to get in, and very sturdy, so the whole unit can be used even outdoors on a field day or a hiking. It is not waterproof, of course, but it was not the goal anyway.

I went for 50 ohm BNC connectors, because most of my cables and receivers use that.

The components are a tight fit, so I had to be careful to put the holes exactly to the right place, which was not always easy. There was no room for much play. There were only millimetres between the components and the box. Also, the axles of the switch and the capacitors made it nontrivial to mark the precise location of the mounting holes.

It cannot be seen, but it stands on those self-adhesive rubber feet. It is a really nice addition, it greatly hepls to keep the unit in place when a bit heavier cables are attached. It also protects the paint on your receiver if you put the ATU on the top of it.

This little gadget works pretty well. I tested it using a random long wire and a RTL SDR receiver. Seeing a wide spectrum is very useful, you can see how the noise floor changes as the resonance sweeps through as you adjust the tuning. The tuner adds quite good signal boost in most cases. It also helps reducing receiver overload, since it acts like a (not too sharp) preselector, suppressing strong signals out of the band.


[1] What does an Antenna Tuner do?