I like to experiment with antenna tuners. Actually, "tuner" is not a correct term but we won't quibble about that. Nothing "tunes" your antenna. Those devices sold as tuners are Antenna Couplers and they are used to match the impedance at the end of your feed line to something your transceiver likes to see - say, for instance, 50 ohms. The conventional and commercial tuners are designed for 50 ohm coaxial cable and, sort of as an afterthought, the manufacturer runs a wire over to a 4/1 ratio "balun" for balanced feed-lines.
I have always used open wire (now "window line") feeders (except for beams) because for all practical purposes they are lossless. Oh, sure you can run your antenna off its resonant frequency with a 50 ohm coax feed line but the SWR goes way up and the radio "folds back" or reduces power to keep from hurting itself. With open wire feeders the SWR might be very high, but since the feed line is nominally lossless you don't have that problem and your tuner will match the feeders to the radio with no sweat.
I, however, do not like baluns. A balun is a chunk of iron (or ferrite) and as far as I am concerned iron and RF do not get along well together. For that reason I have always favored link-coupled antenna tuners and still one of the best is the old Johnson KW "Matchbox" which I used for many years. Manufacturers are beginning to market some link coupled tuners but they are, generally, low power jobs and they are expensive. So, if I have to have a low power tuner and I want link coupling, why don't we just build one?
I call this one the "Priceless" low-power tuner because it cost almost nothing. A dollar or two for a piece of plywood and some parts from the junk box. Parts which you can easily find and buy for practically nothing at the next hamfest. Here are some pix of building this tuner.After you check out the pix we will talk about the components.
The coil form is from an old Army surplus tuning unit and has a diameter of 2 1/2". A piece of PVC of comparable size would serve just as well. There are 14 turns of number 16 silver-coated copper wire on it. (There is nothing sacred about that - I just used the wire that it was originally wrapped with. You can use any copper wire of a similar size.) The coil is tapped in the center at turn number seven (7) and again at turn ten (10) from the grounded end. The link is four turns of covered stranded hookup #16 wire (the red wire) with two turns on either side of the center tap. The link turns go to the screw-on connectors on the rear panel. Let's take a look at the capacitors.
The loading capacitor (on the left) is two sections, 475pf per section. The frame is grounded as is one end of the coil. (the schematic shows this clearly) One SECTION of it goes to the ungrounded coil end, the other section goes to the center tap. The TUNING capacitor is two sections, 385pf and 120pf. The frame goes to the SO-239 input on the rear panel. Using the CLIP either section can be connected to coil tap at turn #10. AN IMPORTANT NOTE - THE CAPACITORS MUST HAVE INSULATED TUNING SHAFTS AND KNOBS AS THERE IS RF ON THE SHAFTS.
So how does it work? This is a low-power tuner but I expect it would handle 100 to 150 watts or more with no arcing. I have used it extensively on PSK running up to 50 watts of key-down carrier. And now for a close look at those SWR readings which are pretty impressive for a cheap tuner. Incidentally, in each of these pictures the tuner was connected directly through about 70 feet of window-line to my 80 meter full-wave loop.
So there you have it, my friend. A nice low-power (great for QRP) tuner for practically no cost except for a piece of plywood. And you'll have some fun putting it all together. I like getting something for nothing, don't you? Experiment by using the clip on different sections of the tuning capacitor. Normally the larger section works on lower bands, the small section on the higher frequencies. Have fun, and save some dough! I'll look for you on the bands and 73 from W4BTX!
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