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Figure 24.18 — Variations of an antenna system with different losses.
The examples are discussed in the text.
24-20 Chapter 24
antenna tuner between the transmitter and the transmission line going
to the antenna. This is particularly
true for a single-wire antenna used
on multiple amateur bands.
The tuner is usually located
near the transmitter in order to
adjust it for different bands or antennas. If a tuner is in use for one
particular band and does not need
to be adjusted once set up for minimum VSWR, it can be placed in
a weatherproof container near the
antenna. Some automatic tuners
are designed to be installed at the
antenna, for example. For some
situations, placing the tuner at the
base of a tower can be particularly
effective and eliminates having to
climb the tower to perform maintenance on the tuner.
It is useful to consider the performance of the entire antenna system when deciding where to install
the antenna tuner and what types of
feed line to use in order to minimize
system losses. Here is an example,
using the program TLW. Let’s assume a flattop antenna 50 feet high
and 100 feet long and not resonant
on any amateur band. As extreme
examples, we will use 3.8 and
28.4 MHz with 200 feet of transmission line. There are many ways to
configure this system, but three examples are shown in Figure 24.18.
Example 1 in Figure 24.18A
shows a 200-foot run of RG-213 going to a 1:1 balun that feeds the antenna. A tuner in the shack reduces
the VSWR for proper matching in
the transmitter. Example 2 (Figure
24.18B) shows a similar arrangement
using 300-W transmitting twin lead.
Example 3 (Figure 24.18C) shows a
50-foot run of 300-W line dropping
straight down to a remote tune near
the ground and 150 feet of RG-213
going to the shack. Table 24-8 summarizes the losses and the L-network
component values required.
Some interesting conclusions
can be drawn. First, direct feeding
this antenna with coax through a
balun is very lossy — a poor solution. If the flattop were l/2 long — a
resonant half-wave dipole — direct