By: Yuri Medinets, UB5UG, Kiev
Line: Radio #1, 1984, p.24
It is a simple solution increase the efficiency of
a vertical antenna. Just to remove all high currents parts of
the antenna from ground. The losses at nearby subjects are decrease
and the overall performance of the antenna increase. So, we come
to a Top Fed Vertical Antenna.
Figure 1 shows one
band Top Fed Vertical Antenna. Upper horizon part of the antenna
stretched with help of a fishing rope by 1.0-mm in diameter.
To the middle of the horizon wire the inner conductor
of the coaxial cable is connected. The outer sheath of the
is unconnected. Coaxial cable is going straight down or with some
angle to the horizon wire. An RF-Choke is installed at the distance
lambda/4 from the top end of the coaxil. The RF-Choke made like
a resonant LC-circuit. The circuit has resonant at the working
frequency of the antenna.
RF- Choke practically made by coiling around of one
side of a ferrite ring (having low losses at HF) with large OD/ID
one or two turns by the antenna coaxial cable. A Resonance Coil
coiled at another side of the ring. The circuit (coil and bridged
to it capacitor) has resonance at the working frequency.
The Choke may be made without a ferrite ring. Choke
looks like two- three large turns of wire in 2- 4- mm in diameter
bridged to capacitor. Coaxial cable in the needed place does one
turn around the coil. The coax and the coil are tight by a Scotch.
Capacitor should have high Q and high working voltage. TX with
100- Wtts output power may induce across the capacitor 400-500-
Data (approximately) for the Choke (made by wire in
2-mm in diameter) are given in Table 1.
The Choke does not influence to the coaxial going below
the Choke. Below the Choke coaxial (any length) may go in any
way- lay on the ground or roof, go near a home wall, etc.
It is very possible to create a multi- band Top Fed
Choke Antenna. Use vertical wires that have resonance for each
band and just place at needed places on to coaxial cable the special
resonance Chokes. Figure 2
shows the schematic for 5-band Top Fed Choke Antenna.