ANTENTOP- 01- 2008, # 010

Vertical UN7CI for 7, 14 and 21- MHz

By: Boris Popov (UN7CI)

Petropavlovsk, Kazakhstan

Credit Line: www.cqham.ru

The antenna was designed for amateur's bands 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.

Operation:

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 relay's contact is grounded the vertical. Second relay's contact turns on the omega- matching network to 50-Ohm coaxial.

Figure 1 Vertical with matching circuits

Design

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.

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