Practical_Antenna_Handbook_0071639586

616 P a r t V I I : T u n i n g , T r o u b l e s h o o t i n g , a n d D e s i g n A i d
close to the ohmmeter so that you can measure the resistance of the shield separate from
that of the center conductor. On the other hand, even if you know which is at fault, it
probably isn’t going to change the ultimate disposition of the cable!
Once you have completed the basic open-Âcircuit and short-Âcircuit ohmmeter measurements
on the cable, your next step should be to measure the loss of the transmission
line at one or more frequencies of interest. Although the author owns numerous lengths
of RG-Â8 that exhibit the same losses at HF as his newer stock does, older cables that
have not been adequately protected from the elements in prior usage or storage can
develop losses that get higher as the cable deterioration progresses, so low loss at the
time of initial installation does not guarantee low loss later on. Moisture is the enemy of
coaxial cables, so existing cable stock should not be stored outdoors or in damp basements
or garages unless all cable ends have been sealed.
Losses in new cables increase with frequency and with the total length of cable used
in any particular link between transmitter and antenna. Depending on the specific type
of flexible coaxial cable employed, losses can run from as low as 0.2 dB/100 ft at 1.8
MHz (RG-Â8 with foam dielectric) to as much as 0.5 dB/100 ft (RG-Â58 and RG-Â59). At 30
MHz the losses are approximately 1.0 dB/100 ft and 2.5 dB/100 ft, respectively. Arguably
the most used type of flexible coaxial cables at MF and HF are members of the
RG-Â8 and RG-Â213 family. Their loss (with standard dielectric) ranges from 0.25 dB/100
ft at 1.8 MHz to 1.3 dB/100 ft at 30 MHz. The reader is encouraged to consult specific
vendors’ online catalogs for the correct loss data for cable types of interest, since the
number of variations in cable dimensions and materials makes presentation of so many
possible choices in a single chart unworkable.
Figure 27.22 shows a test setup for measuring coaxial cable loss. The first step in the
procedure is to determine the length of the piece of cable under test. One end of the line
should then be terminated in its characteristic impedance—usually 50 or 75 W. The termination
must be capable of dissipating the output power of the source, so usually a
100W (or higher) dummy load is employed. The simplest RF source is often your own
transmitter or transceiver. Two identical RF power meters (M 1 and M 2 ) are used to simultaneously
measure the input power to the line (M 1 ) and the power delivered to the
load (M 2 ) while the RF source is activated. The difference in the two power readings is
then the power lost in the cable itself. To express that loss in decibels, we use
Loss
dB
⎛ P
= −10 log
⎝
⎜
P
M2
M1
⎞
⎠
⎟
(27.19)
where Loss = loss of cable, in decibels
P M1 = (cable input) power reading on M 1
P M2 = (cable output) power reading on M 2
Figure 27.22 RF attenuation test setup for coaxial cable.