jhs@mitre-bedford.ARPA (10/22/85)
Hot off the presses (or perhaps just reinvented): We at the MITRE-Bedford Amateur Radio Club (MBARC) wanted to feed a 160-meter half-wave at the end supported by our tower, to avoid draping feed lines across a parking area. We came up with the following scheme, which can be adapted to other bands by the obvious scaling of dimensions. We observed that an end-fed antenna exhibits a high impedance, which our antenna gurus here at MITRE say should be maybe 1200 to 1800 ohms, and of course it would be purely resistive if the antenna is exactly resonant. The exact resistance depends on height above ground and other factors. Anyway, we noted that a quarter wave transformer to match 1800 ohms to 50 ohms needs to have a characteristic impedance of exactly 300 ohms. (In general, the square root of the product of the given impedance and the desired impedance.) So we made up a special cable consisting of a convenient length of RG-8/U soldered to a quarter wavelength of "Radio Shack's Finest" black foam-filled TV twinlead, which incidentally, I have had good luck with even when running the full legal limit through it at high VSWRs. The hot side went to the end of the half-wave antenna and the "ground" side we connected via a water-pipe ground clamp to a cross leg of our Rohn tower. We figured that at 160 meters this was more or less "ground". Purists would want to add a 1:1 balun at the coax-to-twinlead junction, I suppose. We fired the thing up and voila! a match of 1.5 to 1 or better was had over much of the 160-meter band, though it did get up to 3:1 at 2 MHz. Presumably this could be tweaked in to cover the band segment of interest, and of course it would work fine with a tuner even at 3:1. Signal reports have been outstanding. In summary, a quarter wave transformer made of 300 ohm twinlead seems to work well as a matching network for end-feeding a half-wave antenna. A balun between the coax and the twinlead appears to be unnecessary. Soldering the twinlead directly to the coax and covering the joint with a heavy wrap of electrical tape makes a mechanically satisfactory connection, though shrink tubing (I like to call it psychiatrist tubing) would doubtless be more permanent. Take care to connect the twinlead correctly so that the coax shield goes to ground and the coax center lead goes to the antenna. Of course if you use a balun this is not a concern. Hint: Crimp-on "butt" connectors seem to work well for attaching more durable wires to the fragile #20 or whatever is in the twinlead. If you use this idea, take care to keep the twin lead away from metal objects as much as possible. We used rope tied around the twinlead to support the top end, and rope to support it clear of metal objects at the bottom end just before the coax. Twinlead is not as strong as coax and should not be subjected to unnecessary tension. One final note: The antenna ought to work reasonably well also at half the design frequency, with maybe a 2:1 VSWR, easily handled with a tuner. 73, John S., W3IKG
carpenter@nbs-vms.ARPA (CARPENTER, ROBERT) (10/22/85)
If you are old enough (I admit it), you may find a similar feed method which is the J antenna. It is described in ARRL handbooks from the late 30s to perhaps the early 50s - as a VHF antenna. I've seen another way of feeding 160 meter verticals. BAck in the late 50s Ev Dillard, W3BKO, got an AM broadcast license for Wheaton, MD. It just so happened that his FM station, WASH, had its transmitter in Wheaton. He tried all kinds Wheaton. Of course the FM tower was solidly grounded at the bottom, with no leg insulators - it is a self-supporting tower. He tried all the normal shunt-feed systems, connecting a little way up the tower, but the impedance was awful. Finally someone pointed out that the tower was about a half-wave on 1540 kHz and deeper though was necessary. The least-trouble solution turned out to be to drive four posts in the ground, perhaps ten feed outside the tower base and run wires from them to a point a quarter-wave up the tower. they were then firmly attached to the tower up there. The bottoms of the wries were connected together and fed as a high impedance end fed half-wave. The wires formed a quarter-wave stub insulator with the tower. This system was used for perhaps 15 years. Dillard eventually sold WDON (AM), and it now uses another loction, sharing a tower with a station on 1050 kHz. Sorry for rambling....... Bob, W3OTC ------