Fall 2002 Issue

View of the front of the 2-meter antenna tuner showing the junk-box selection of knobs. Note: The shaft of one of the capacitors must be insulated if you use a metal box.

Here is a simple project that you yourself can build, or you can make into an evening-long club project. W2OQI describes a straightforward antenna tuner for 2 meters† constructed from easily obtainable parts.

By Van R. Field, W2OQI
 

†To take a look at the 6-meter antenna tuner I built, see the “Project Corner” column (p. 63) in the September 1997 issue of CQ VHF.

Why build a VHF antenna tuner? After all, you buy a rig, an antenna, some coax, put it all together, and people talk to you! However, would more QSOs result from more power to the antenna?
The serious VHF-UHF operator, whether on FM or SSB, needs to take a look at the SWR at the rig. If you use a beam cut for the SSB end of the band, what happens on 147 MHz? Get a reasonable SWR bridge and look. Some of the newer rigs have them built in.

I decided to tackle the problem when I put up a beam for SSB. I forgot that when I built it, I was thinking FM and 147 MHz. On the low end, the SWR was high enough to fold back the power on my rig. Rig manufacturers protect your equipment by building the equipment so that the power automatically cuts back when the SWR rises. If your antenna doesn’t take the power, then the transistors in your rig heat up, and with no fold-back circuit they will overheat and burn out!
To solve my problem I found two old tuning capacitors about 25 pF each and some #16 wire for the coil. The ground wire from a piece of Romex electrical cable will do. I wound 5 turns on the shank of a 3/8-inch drill bit, and then the hunt was on for a box. RadioShack came to the rescue with a 51/4" ¥ 3" ¥ 21/8" aluminum box.

A look at figure 1 reveals that one tuning capacitor is in series and needs to be insulated from ground. A plastic knob will be necessary here. A plastic box might be a better choice to make it easy to insulate the shaft. I used a metal box because I had it. If a plastic box is used, be sure to run a heavy ground bus from the ground side of both connectors. A #12 wire would be appropriate, if you have a soldering iron to handle it.

I used BNC connectors because years ago I standardized my shack to use them. Most hams use UHF connectors. Be careful of cheap ones. The silver-plated variety found advertised in ham magazines is much easier to solder and generally much better built.
It would be handy to include an SWR bridge in the box. However, as simple as the circuits are, they are not that easy to get working. After trying out several circuits, I concluded that an inexpensive RadioShack unit (part No. 21-533) will do fine. It’s marked 3–30 MHz, but it seems to work all right through 2 meters. A dummy load can check it for you. A 50-ohm dummy load is a handy piece of shack equipment. If one is available, the tuner may be checked.

Simply adjust each capacitor for the lowest SWR. If the capacitor is all the way open or closed, the coil can be pulled apart or squeezed together to adjust the final inductance. The tuner should adjust to 1:1. It should adjust the same way on your antenna.
 

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A Simple-to-Build 2-Meter Antenna Tuner

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