In this post we’re going to build an L-Match tuner for your QRP wire antenna. But you might ask: Why do I need a box between a bit of wire and my transmitter? That’s a good question!
Amateur radios are constructed in such a way that they have an impedance of 50 ohms. For portable use an end fed antenna is desirable. The problem is that an end fed antenna produces a feed point impedance that can be up to several thousand ohms. That’s very far from the 50 ohms of your radio. If you try to transmit directly into the end fed antenna, very little RF will make it to the antenna. The RF that doesn’t make it will be reflected back to the radio, perhaps even damaging it.
This is where the L-Match tuner comes in. An L-Match tuner is a device that can add either inductance (L) or capacitance (C) to the antenna, bridging that gap between 5000 ohms and 50 ohms, thus matching it to the radio. The L-Match tuner is an extremely useful device that every QRP operator will want to have.
Gathering the Parts for the L-Match Tuner
For your L-Match tuner, you are going to need a variable capacitor and some inductors. I originally purchased a 20-200pf variable capacitor on ebay. If I were to do it again I’d buy a 365pf variable capacitor for its greater range.
The next thing you’re going to need is inductors. I used two T50-6’s and 3 T37-6’s because it’s what I had on hand. I would recommend that you use all T50-6’s. Amazon has 10 T50-6 Toroids from Micrometals that is convenient, but if you really want to stock up, buy them on kitsandparts.com where you can get 25 of them for $6. That’s a lot of toroids! You need enameled magnet wire too. 24awg will do, and you can buy a huge roll of that but I recommend the 10 feet of 26awg (solder heat strippable) wire for $1 at KitsAndParts.com.
Next you’ll need 5 switches. I used these DPDT switches because I already had them. You can use SPDT switches too. Note the wiring diagram below for details.
The Radio Shack project box on the left is the one I used and it was a fairly tight fit, but just right for this project. You’ll want a chassis that has at least one metal side to use as a common ground.
A panel mount SO-239 or BNC connector and binding post terminals will be used to connect the radio, antenna wire, and counterpoise. Be sure to check the QRP Parts Catalog for some of the stuff you’ll need for this build.
Building The L-Match Tuner
This project was inspired by Peter Parker, VK3YE. One of his videos presented a similar tuner, and I went ahead and used his inductance values: 4uH, 2uH, 1uH, 0.5uH. I also added another 2uH. Peter uses just the four inductors but I wanted to have the flexibility of just a bit more inductance.
Using T50-6’s, these inductors should have 32 (4uH), 22 (2uH), 16 (1uH), and 11 (.5 uH) turns. That means that each T50-6 should have a wire going through the middle once for each turn. If you lose count, don’t worry. Just count the turns on the inside of the toroid and add enough til you have them all. I highly recommend reading up on how to wind toroids by looking at the QRP Labs instructions for the Band Pass Filters as the process is pretty much the same.
Next you need to wire your switches. To the left is the basic diagram for the L-Match tuner. There’s not a lot to it. On the left is the coax connector. The center conductor runs through the switched inductors and on to the antenna connector. The variable capacitor is wired between the antenna connector and the coax ground. The diagram shows just a couple of inductors to show you how to wire them. DPDT and SPDT switches are shown from behind. I put the smallest inductor on the right side when viewed from the operating position. All 5 inductors (4, 2, 2, 1, and .5) give tons of combinations between .5uH and 9.5uH.
You’ll notice that in both wiring diagrams the inductor is either used or bypassed depending on switch position. The capacitor is positioned near the output in order to keep the leads as short as possible. Use a metal chassis as a common ground.
The red terminal connector (that the antenna wire connects to) should be insulated from the chassis, while the black one should touch the chassis. I ran a wire from the coax ground to the chassis using the terminal connector, and the capacitor chassis also bolted to the tuner chassis, so there was no need for a ground wire.
Check Your Work
Check continuity from the coax center connector to the red antenna connector, and make sure that each switch has continuity in both positions. I just switched mine all up and then all down and so I only needed two continuity checks. If there’s a break in continuity, just follow the signal path until you find the break. You might have to warm up an inductor connection to make sure the enamel is fully removed.
Now that it’s done, grab your SWR meter and tune up your antenna- you’re good to go!
Check the pictures of my build below and you’ll see that I started off with a prototype in an old PC PSU case and moved it over to the Radio Shack chassis I mentioned before. Interestingly, the tuner works much better in its new case. I think the grid mesh of the old case did funny things with the RF. The antenna is now easier to tune and a stray RF issue that I was trying to track down is now solved. What a difference a case made.
A drill punch, bits, and a step drill bit really made things easier, although I admit that getting it all to fit wasn’t that easy. I had to creatively widen several holes to have enough wiggle room for everything. That big knob hides a lot of mistakes! I also used tweezers extensively for the small soldering and wiring of the switches.
I hope this inspires you to give your own L-Match tuner build a go. It’s a lot of fun. Using equipment you’ve constructed yourself is also extremely rewarding.
Let me know what you think in the comments below, and thanks for reading. 73!