Practical_Antenna_Handbook_0071639586

C h a p t e r 2 7 : T e s t i n g a n d T r o u b l e s h o o t i n g 619
Dummy Loads
A dummy load is a nonradiating substitute for an antenna used when measuring or testing
any and all parts of the antenna system from the transmitter out, exclusive of the
antenna. In fact, British radio engineers often refer to dummy loads as “artificial aerials”.
There are several uses for these devices. Amateur radio operators should use
dummy loads to tune up their transmitters before switching to an actual antenna for
final adjustments. Good operating practices require operators to check the frequency
for activity by other stations before completing their tune-Âup procedures on an antenna.
Another use for dummy loads is to aid in troubleshooting antenna systems. Suppose
we have a system in which the VSWR has suddenly become very high—high
enough to “kick” our transmitter off the air. Starting at the transmitter, we can disconnect
successive elements of the RF transmission system and connect the dummy load at
the disconnect point. If the VSWR suddenly drops to a normal value, then the difficulty
is downstream (i.e., toward the antenna). By repeating this process, we will eventually
find the bad element (which is often as likely to be a coaxial cable, a connector, or a
switch as it is to be the antenna itself).
Figure 27.24 shows the most elementary form of dummy load, which consists of one
or more resistors connected in parallel, series, or series-Âparallel so that the total resistance
is equal to the desired load impedance. The power dissipation is the sum of the
individual power dissipations.
Note It is essential that noninductive resistors be used for this application. For this reason,
carbon-Âcomposition or metal-Âfilm resistors are used, although the former are becoming
harder and harder to find. For very low frequency (VLF) work, it is permissible to use special
low-Âinductance counterwound wire resistors. These resistors, however, cannot be used for
frequencies greater than a few hundred kilohertz. In general, homemade dummy loads should
use many resistors in parallel, rather than in series, since it is easier to keep the stray
inductance down in the former configuration.
Several commercial dummy loads are shown in Figs. 27.25A through 27.25C. The
load in Fig. 27.25A is a 5W model, typically used for servicing citizens band equipment.
The resistor is mounted directly on a PL-Â259 coaxial connector. These loads typically
work to about 300 MHz, although many are not really useful over about 150 MHz. A
PL-259
coax
connector
Figure 27.24 A very basic dummy load.