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.
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.
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
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.
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 .
(8) By Alois Krischke :
Rothammels Antennenbuch.- Franckh – Kosmos, Verlags – GmbH@Co.,
Stuttgart, 1995, 11 edition.
Jay Rusgrove, WA1LNQ: The Cheapie
GP // QST, 1976, February,
(9) Morgan h. K. : Multifrequency Tuned Antenna System. - Electronics,
vol. 13, August 1940, pp. 42-50.
Buchman C. L., W3DZZ : The multimatch Antenna System. // QST,
March 1955, pp.22-23, 130.
The Radio Amateur’s Handbook, 1970, by ARRL publication.
The ARRRL Antenna Book, 19 Editions // ARRL Amateur Radio, 2000.