Friday, July 22, 2011

DX Secret Weapon

DX Secret Weapon

Although it was prominently featured in September 1995 QST Magazine (p.33-41), I believe this antenna has been overlooked by a lot of low band DXers. Perhaps it seems a little too complicated, but I found it well worth the effort it took to design and build one. It is a terrific performer and has greatly exceeded my expectations. This is a low, medium size, horizontal loop with some very interesting characteristics. It has an omnidirectional, fairly low angle pattern with a deep overhead null like a vertical, yet it rejects vertically polarized RF energy and responds only to horizontally polarized energy. It offers a 25 to 30dB improvement in signal-to-noise ratio over a vertical for man-made noise. This is often the predominant noise type in densely populated urban and suburban areas. Results often seem magical as signals that are covered by noise on a vertical antenna become perfectly readable upon switching to the loop. A typical antenna is about 21 feet (6.5m) on a side and about 10 feet (3m) high. (Mine has 23ft (7.0m) sides) Height is not important as with Beverages- the antenna works just as well at 5 feet or 50 feet (1.5-15m). It is double-fed with ladder line in diagonally opposite corners to maintain good balance. The feedpoint is at the center of the ladder line. Coax from the receiver is connected through a matching network with a transformer and series resonating capacitor in the secondary, built in a weatherproof enclosure. For a 21 foot loop, there is a small impedance stepdown on 80m and a larger ratio on 160m. The antenna can be built with remote band switching, as I did with mine. It works well on 80 and has been surprisingly good on 160 (I had not expected much.) It is often my best performing low noise receive antenna.

Low band operators who have already used Beverage antennas are nearly always disappointed with the K6STI loop. Beverages will 'spoil' you for any other low noise receive antennas, but many hams do not have room for them. If you want to reduce atmospheric noise and you do have some room for Beverages or phased verticals, both of which are directive, then those antennas would be a much better choice. Likewise, the WA2WVL cardoid loop array provides good directivity over a narrow bandwidth if you have the room- about 80 feet on 80 meters. (QST, August 1993, p.31) The Ewe, K9AY, and Flag and Pennantantennas also provide good directivity in a fairly small space, but with lower receive signal levels. They might benefit from the use of a receive preamp, and so might the STI loop, to a lesser degree. (I do not use a preamp with mine, however). It is a good idea to add a highly selective tuning network ahead of any preamp to avoid receiver front-end overload. This loop has an omnidirectional pattern and, in theory, it should provide no improvement in S/N for atmospheric noise. K6STI and W6KUT, who built the first loop, noticed an apparent improvement of 1 to 2 S-units even when there was no detectable power line noise present. I have also seen this behavior in my antenna. STI reasons that in populated areas the noise background may consist of many individual man-made noise sources which, in composite, are characterless. Anyone interested in this antenna is encouraged to get hold of the QST article and carefully read and reread it for full understanding. Topband operators might want to consider building a double size loop around 50 feet (15m) on a side if it will be used only on 160m. Matching network component values would have to be adjusted accordingly. Because I do not have room for low band Beverages, the K6STI loop has proven to be a most valuable low noise receive antenna for me.
(Revised and updated 8 Apr 2000)
(Second update 13 Feb 2002)

W7LR tried a 50ft version of this antenna on 160m. He used a "pre-war" Terman handbook (p. 52) to determine that the loop inductance on 160m is about 110uH. He calculated the resonating capacitance at 1830KHz to be 60pF. His transformer was 4 turns, antenna side, and 20 turns for 50 ohm impedance coax. The core was an FB77-1024 ferrite bead. He said it "tuned nicely" (actual resonating capacitance was close to the predicted value). There were no comments as to the actual antenna performance, only that he has tried a lot more receiving antennas since this September 1995 design.

No comments: