Ron, Very pretty job! Do you recall the (approx) capacitance range of the split-stator and link capacitors? Also, your pdf file says 3" acrylic tubing for main coil. Is it 3" or 2" ? How many turns on your 10 meter and 160 meter coil and link? (I assume the pictures show the 80-10 version.)
73, Oliver W6ODJ ------------------------------------------- Tnx Oliver. You caught me with a bad memory: the PDF is right, it's 3". The split-stator capacitor is about 200 pf per section, and the link capacitor is about 200 pf total. The values are not critical. The voltage on the small capacitor is low, since it's in a 50-ohm circuit. A receiving capacitor will work well at most power levels at least into several hundred watts. The critical cap is the split stator: it needs to be a good transmitting type with good plate spacing. There can be huge RF voltages across it. I had a QRP version using a small variable with about 1/32" plate spacing. Under some conditions it'd arc at 5 or 10 watts of RF. The 80/10 meter coil has 28 turns total on the main coil with 7 turns for the link. The 160 meter coil is on a 5" form with 32 turns total on the main with a 10 turn link. (Actually, it wouldn't hurt to make the 160 meter coil a bit larger for more matching range.) The turns are spaced 3/16" of an inch, with notches filed in the tubes where the wires rest to hold them in place. I built it for versatility and efficiency, not sleek, modern rig push-button ergonomics. Indeed, I call it the "Frankentuner" since to me it looks like something Dr. Frankenstein might have adjusted in his Gothic castle laboratory as he waited for strokes of lightning to bring his monster to life. It looks to me like Steve, W2MY, built his with similar flexibility in mind. It'll handle a huge range of impedances, and it handles balanced and unbalanced loads equally well. Since the circuit is balanced, the ends of the coil are at high potential (and out of phase) while the center of the coil is at RF "ground". Normally, you'd tap the open wire feeders out equidistant from the center. To load up a single wire, just connect whatever you're using for ground - counterpoise, etc. - to the center of the large coil and tap the antenna out on either side to get a good match. If you want to reduce the SWR presented by a coaxial line, connect the coax shield to the center of the large coil and tap the coax center conductor out on either side as needed for a match. Many tap points will produce a low SWR at the transmitter. The best taps points are as far toward the ends of the coil as possible. That will vary widely with the impedance presented by the feed line, of course. Placing the taps as far from the center as possible produces the lowest "Q", hence the lowest circulating currents and the lowest ohmic losses while providing the greatest bandwidth to minimize the need to "touch up" the tuning when QSYing around the band. With the tap and capacitor settings noted on a small card attached to the front, I can change bands in about 15 seconds. Actually, I briefly considered a Faraday shield for the link, but opted for the mechanical simplicity of a few turns separated from the main coil instead. I've noted no sign of capacitive signal coupling across the windings. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
