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 615
add a notch filter between the analyzer and the device under test because the filter can
alter the impedances reported by the instrument.
In addition to antenna measurements, SWR analyzers are equipped to measure
other parts of the RF path in a typical installation. In particular, they can measure the
velocity factor of a transmission line, help in tuning or adjusting matching stubs or
matching networks, measure capacitance or inductance, and determine the resonant
frequency of LC networks.
Doping Out Coaxial Cable
When you install an antenna, or do a bit of preventive maintenance, or find the antenna
is not working properly, one system component it is important to check is the transmission
line. Typically, a two-Âstep process is employed. The first step consists of ohmic
checks at dc, as shown in Fig. 27.21. This is particularly useful with coaxial cable, where
neither the inner conductor and the outer conductor (shield) is available for direct visual
inspection. With terminals A and B open there should not be any resistive path
across the input terminals (as shown). If a high resistance is seen, there might be some
contamination in the system or the dielectric insulation has failed, permitting a leakage
current to flow. (Or you may have your fingers on the conductive tips of the test leads!)
Alternatively, a low resistance indicates a short circuit somewhere along the cable’s
length. If the cable has been cut, or an object passed through it, or the connector is
messed up (commonly because of poor soldering), a short can result.
If terminals A-ÂB are shorted together, a low resistance should be noted. If not, then
it is likely that the center conductor is open—typically as the result of some prior excessive
stress (such as extreme bending) on the cable. Of course, to determine whether the
shield or the center conductor is at fault, you will need to bring both ends of the cable
Figure 27.21 Ohmmeter test of coaxial cable.