When ALL else fails.....

This will match your Wire Antenna 

Back in the “olden days” we didn’t have SWR meters and we fed our wire antennas with open-wire feeders. We used open-wire feeders because coax was expensive and we could make our own feeders if necessary. We didn’t need an SWR meter because (although few of us knew it) our feeders were virtually lossless and hooked to a link in the coil of the final amplifier, our antenna worked just fine on all the bands the FCC allowed us to use.

Times have changed. But open (or window) feeder line is still practically lossless. The problem is that all our nice radios like to see an unbalanced 50 ohms and that lossless feed line is balanced and will have a wide range of impedance.

What to do?

Today we use antenna tuners to match our feed line to the transceiver. This is usually accomplished by using a commercial tuner that is made for 50 ohm coax, and as an after-thought the manufacturer runs a wire over to a 4 to 1 ratio balun and hooks that to a couple of feed-through insulators for open-wire transmission line. The 4 to 1 balun is not efficient for a feed-line with an impedance that can range from perhaps 5 ohms up to 5,000 ohms. Somewhere in the bands the balun may match your antenna to the transceiver, but everywhere it is going to be a compromise. Immediately after a QSO sometime, open your tuner and place your hand on its balun - if it is warm (sometimes HOT) you were losing power).

The "problem" with coax.

Amateurs in the early ‘50s using new-fangled coax often had trouble getting a good match between their transmission line and their transmitters. But they found that by cutting the transmission line, in small increments, they would find a point where the transmitter loaded fairly well at a certain frequency. Without getting technical, what they were unknowingly doing was finding a point on the coax which was a multiple of a quarter wave where the standing waves traveling up and down the line were canceling each other and the impedance at that point was close to the coax’s nominal 50 ohms.

Do what the "old-timers" did.

We can use the same technique with our balun-output antenna tuners and enjoy the advantage of an all-band antenna and low loss feeders and still keep the SWR meter happily sitting there on 1 to 1. Don’t worry about the length of feed line from the antenna just run a convenient length into the shack. Then, all we have to do is find an easy method of changing the length of that feed line. Of course you could do this by having various lengths of window line which you manually inserted or removed each time you changed frequency, but that’s way too much work.

What if we had a set of relays that would do the job? What if we used a system that could add one foot, two feet, four feet, eight feet or sixteen feet? That would add a variety of lengths to the feed line - from one foot up to thirty-one feet. And some combination of those lengths will give us a line length that will allow the tuner to match the antenna to the transceiver’s input requirement Okay - how do we do that?

CONSTRUCTION (Click on the small pix for larger Pictures)

In the photos you will notice some black “lumps” on the 12 volt control lines for the relays. Those are large ferrite beads. Are they necessary? I don’t know, but I had them and put them on the lines to possibly keep any RF out of the power lines. They may not be serving any good purpose. You could try this setup without ferrite beads and add them later if you do have any RF getting into the power lines. (Incidentally, the tuner pictured above is my "Tucker," my favorite all-time tuner. Yes, it looks like a Vectronics, but it's not. MFJ bought Tucker, gave it to their subsidiary Vectronics and replaced the heavy-duty roller with their POS roller and now sells it for lotsa bucks. Don't be fooled by the faceplate. A Vectronics "looks" like a Tucker but it is not.NOT a Tucker. If you can f ind a Tucker - BUY IT!)

Ten DPDT relays will do the job. The schematic shows how these relays are controlled by five switches. By manipulating the switches we can lengthen the feed line and find some point where our tuner will like the impedance it sees. The relays I used are Allied Stock#: 850-3112 which are more expensive now than when I bought them. Expensive? A bit. But these are 40 amp contact relays, which may be overkill, but they will handle all the power your can ever run through them and can even stand up to “hot switches.” And consider how much you paid for those commercial antennas that won’t load on several bands? Why not make a one-time investment using a single wire antenna which works on all HF bands with a near-perfect match? Or you can find smaller and cheaper relays at the next hamfest. Mine run on 12 VDC but you could use other voltage coils. Get whatever you can, as cheap as you can, and try this system. If your relays are too small for the power you run, you'll know it the first time you accidentally "hot switch." So reduce power before you switch. Make do with whatever you can assemble, but try this system. It really works and works really well. I have used it for several years with several wire antennas and it never fails to give me a match.

The relay board needs to be just big enough to conveniently hold the relays. After they are mounted, the lengths of window line are connected to each relay. In my case it was handy to mount the relay board high in the ceiling of my workshop which adjoins my shack. When the board is in place the longer pieces of window line are stretched out and secured to rafters with pieces of rope. The only caution is to keep any bends in the attached window line as “loose” as possible – no sharp bends.

The 12 volt relays are connected in pairs to five switches. These could be SPST (single-pole single-throw), but I used DPST (double-pole single-throw) so that one switch side of each switch could be used to light up an LED to keep me aware of what lengths I have added to the feed line.(NOTE: The two meters shown in my installation are two thermo-coupled RF ammeters from my junk box but they are NOT a requirement so ignore them.)

HOW TO USE THE VARIABLE FEED LINE SWITCHER.

With all relays out of the circuit use a few watts and an SWR meter and tune your antenna to the lowest SWR reading possible. If it is not flat, start by switching in a foot of extra feed line. Keep experimenting with different combinations of switches until the SWR falls. At some point one or more switches will prove to be the right length of feed line to give the lowest possible SWR and make your tuner and transceiver live happily ever after. It is handy to make a note of your tuner settings and which switches are necessary for your favorite frequencies so you can get back to them quickly.

Use virtually lossless open wire feeders and this method with a good wire antenna and operate on all bands and SWR will never worry you again. Any questions? Email me at john@w4btx.com .

ALL HF BAND MATCHING SYSTEM

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