By: Boris Popov (UN7CI)
Credit Line: www.cqham.ru
The antenna was designed for amateurís bandsí 7, 14
and 21-MHz. Antenna fed throw 50-Ohm coax. The antenna has SWR
less the 1.5:1.0 at any band. Antenna has good efficiency and
low angle of radiation at vertical plane (that is good for DX-ing).
Only one power relay is used to switch a working band.†
The antenna made of on a base a vertical
tube that has diameter of 22-mm and length 8.5-meters. With help
of matching networks this vertical is tuned to each used band.† At the 7-MHz band the vertical (with the lengthen
inductor) is a lambda/4 one, at 14-MHz the vertical is a 5lambda/8
one, at 21-MHz band the vertical (with the lengthen inductor)
is a lambda/2 one.
Figure 1 shows the antenna with matching circuits.
Band 7 and 21-MHz:† When D.C. voltage of 24-V is across the relay
its contacts switch on the vertical through the lengthen inductor to 50-Ohm coaxial cable.
Band 14-MHz:† If the 24-V cut
off from the relay the vertical is working at 14-MHz. In the situation
one relayís contact is grounded the vertical. Second relayís contact
turns on the omega- matching network to 50-Ohm coaxial.
Vertical with matching circuits
Vertical may be made from aluminum tube in diameter
22- 30 millimeters. The vertical is installed onto a home-made
insulator. The omega matching may be made from aluminum wire in
diameter 4.5- 8- millimeters. Lengthen
inductor has 5 coils of silvered copper wire in diameter of 2.5-mm
(10-AWG). The inductor has length of 30-mm and coiled diameter
45-mm (air-wound inductor). It was used a Russian power relay
REN-33. However, it is possible to use any power relay that will
work at RF-power going to the antenna.