Impedance Matching

Also known as antenna tuning, impedance matching has taken on new importance in my shack. For years I was using a ricebox that contained an automatic antenna tuner. I got spoiled. I could load nearly anything and, most importantly, I could drive all my RF amps to full output without having to twiddle any input knobs.

Recently I began testing and then using some older rice boxes that did not have built in antenna tuners. It took a while to figure out why the TS-120 was only driving the amp to an output of 200 watts.

Took a look at the SWR present between the TS-120 and the amplifier. It was 2:1. Not bad at all but after reading the manual on the TS-120 I discovered that into a 2:1 SWR the TS-120 would output less than 50 watts. My amp needs 100 watts or so to drive it to full output.

The old Z-match to the rescue. At first I was contemplating on throwing together a pi-network. Then decided on a Z-match. I took a look at offerings regarding a Z-match on the internet. Nothing earth shaking. Someone had figured out that the two coils used in the Z-match could be made as one coil and the common tap between them made into a conventional coil tap. They also discovered that a toroid core could be used. I am not sure why I would want to go to the expense of a toroid core and wind on more turns that I need for the air core coils. Take a look at the original Z-match design on this web site. You need to call up http:\\ Then find the listing for the Z-match project.

So, I decided to use the last two air variable capacitors in the junk box to build a 100watt plus Z-match of diminutive proportions. These were broadcast style variables. Plate spacing probably good to 300volts or so. One with three sections, two of them at 365pf for the dual section part of the tuner. The other cap was the typical 5 tube AC/DC wonder, both sections were paralled and used as the input capacitor.

The single coil is wound on a surplus ceramic form approximately one inch in diameter. Roughly 40 turns of #12 center tapped. Only one output link was used. Five turns of #20 hookup wire with teflon insulation wound on one side of the center-tapped coil. One side of the link to ground. The other side of the link to coax output connector. No need for balanced output in this application but I do intend to provide for that once I get the new Z-match in a proper box with proper connectors.

Breadboard style construction with copper clad board for ground plane where needed.

After careful adjustment this brought the SWR to the amp down to 1.3:1 and improved the output power from the amp to 450 watts. That is pretty close to optimum for a pair of 572Bs. If I really push them, I can get 550 to 600 watts but you can’t tell any difference in the signal at the receive end from 450 watts, so why bother. Now, you CAN tell a difference at the receive end between 200 watts and 450 watts.

Anyone who has used a Z-match (or even an antenna tuner) knows that you can’t just crank one adjustment and then crank the other to get a match. Matching is a two handed operation where you crank both controls as quickly as possible at as low a power as possible until you see a drop in SWR. These adjustments are easier to make if the capacitors are tied to a reduction drive. In my case for this project all I had was some large knobs. No reduction drive. So I was pretty much at the mercy of the standard 180 degree rotation of the caps. Not good.

Trouble is I do not have suitable reduction drives available and no money to buy some. So, we will just have to homebrew them too. I figure I can use some salvaged potentiometer bushings to work with some short 1/4 inch shafts that can be made to drive a large disk ( like maybe a defunct CD ). A rim driven disk run from a 1/4 inch shaft should get me a 10 to 1 drive reduction easily.

Once I get this thing built and de-bugged, I will probably add it as a low power Z-match project.

Yeah, it works great for matching the TS-120 to the amp input but it probably should not be in the circuit on receive. With it in there on receive, I probably loose a whole S-unit in signal strength. Now I have to figure out a way around that one too.

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