ANTENTOP- 02- 2004, # 006

Multirange Trap Antennas

Figure 6 A coaxial cable capacitor

antenna need resonance (a quarter wave) counterpoises for each operation range. Use not less than two counterpoises. If the antenna is placed at a small altitude above a metal roof and braiding of feeding coaxial cable have good electrical contact with the metal roof, the antenna can be used without any counterpoises.

Antenna feeding: Quite possible to use a 50- Ohm coaxial cable for vertical trap antenna feeding. Also it is possible to use a two wire line for dipole and vertical trap antenna feeding. In this case it need ATU between the line and the transceiver.

A trap capacitor: A high voltage is at a trap capacitor when the antenna works to transmission. So it need a high voltage capacitor trap to be used at a trap. Such capacitor is costly and rather rare. Hams often use a length of a coaxial cable instead of a high-voltage capacitor. A 50- 0hm coax has near 100-pF/meter, a 75- 0hm coax has near 70-pF/meter. Coaxial cable capacity can be find off from a data sheet for the coaxial cable or is metered practically. Figure 6 shows a coaxial cable capacitor. For a capacitor with a small capacity (up to 30-pF) it is possible to use whole coaxial cable length, see Figure 6A. For a capacitor with a high capacity cut the coaxial cable on to several lengths, as it shown in Figure 6B.As a high-voltage capacitor in trap it is possible to use a bilateral PC-board by width of 1 to 3 millimeters. In this case capacitor get more bulky the made on coaxial cable basis. It is possible to tune the PC- capacitor on necessary capacity by slitting a foil on one of two sides of this capacitor. Do not forget about atmospheric protection of the trap capacitor.

Other way for trap design: It seems to me in the end of 70s in different radio amateur literature were appeared articles about using "coaxial cable trap" for W3DZZ. There is very simple method for trap making. Figure 7 shows the trap. It is wise way for trap design, but radio amateur should have an experience using the method. In different radio amateur literature there are a lot of data for design of the trap, but classical methods are described at reference [6].

References:

1. (8) By Alois Krischke : Rothammels Antennenbuch.- Franckh- Kosmos, Verlags- GmbH@Co., Stuttgart, 1995, 11 edition.

2. Jay Rusgrove, WA1LNQ: The Cheapie GP // QST, 1976, February, p31.

3. (9) Morgan h. K. : Multifrequency Tuned Antenna System. - Electronics, vol. 13, August 1940, pp. 42-50.

4. Buchman C. L., W3DZZ : The multimatch Antenna System. // QST, March 1955, pp.22-23, 130.

5. The Radio Amateur's Handbook, 1970, by ARRL publication.

6. The ARRRL Antenna Book, 19 Editions // ARRL Amateur Radio, 2000.

Figure 7

A coaxial cable trap

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