Practical_Antenna_Handbook_0071639586

254 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
Random length
( ) 4
Antenna
tuner
VSWR
meter
Transmitter
Figure 10.1 Random-length antenna.
that a l/4 monopole radiates well only with the help of the ground system beneath it,
any antenna with a single-wire feedline depends on the nearby ground to pass the return
currents back to the transmitter circuits. If the transmitter is trying to push electrons
into a longwire on each half-cycle of RF, it must necessarily be receiving or
pulling an equal number of electrons from a return circuit of some kind. In the worst
case, where no attention at all is paid to the ground return paths, the transmitter typically
gets its return path from a mix of anything and everything that’s connected to the
chassis and the ground of the power amplifier: ac power line, mic cable shield, audio
cable shields, etc. Lack of attention to an appropriate ground return path is a major
reason why equipment in a radio transmitting environment becomes “hot” whenever
the transmitter is operated. The issue is especially relevant to the installation of a longwire
antenna, since, in many cases, the fed end of the antenna actually comes right into
the radio room.
Failure to include an adequate ground return for a longwire raises the apparent
feedpoint impedance and causes a large portion of the transmitter output power to be
dissipated in the resistive losses of other ground return paths rather than in the radiation
resistance of the antenna. Ideally, many radials of lengths greater than or equal to
l/4 at the lowest frequency of operation, as shown in Fig. 10.2, should be included as
part of any longwire antenna project.
The kind of ground we’re talking about is not at all like the power company’s
ground, which is a safety ground. The longwire requires an RF ground, which is discussed
in detail in Chap. 30. At the very least, a l/4 radial can be connected to the