5-30
or isolators are required. However, by the use of automatic-level control, the power output of the sweep
generator can be maintained relatively constant at the point of measurement. The source impedance may
thus be maintained very close to the nominal value. With this arrangement, any impedance variation in
the connecting cables, connectors, and adapters is effectively cancelled since these components are within
the leveling loop. The attenuation of a device under test will be displayed on the associated crt as a
continuous response curve as it is scanned. This will result in an attenuation versus frequency plot of the
device under test only.
IMPEDANCE
Circuit impedance is measured conveniently by using the reflectometer principle. The individual
values of the incident and reflected signals (swr) in a transmission line feeding an unknown impedance
are measured. The ratio between these signals indicates how closely the load impedance matches that of
the transmission line.
Another method is the use of an auto-mechanical load control to hold the forward power at a
constant level while the return load of a specific load is measured. A short is then placed in the circuit,
and 100% reflected power is measured. The loss detected is then calculated to obtain swr figures.
NOISE FIGURE
By using a frequency-sweeping receiver and an automatic noise-figure meter, you can make noise-
figure measurements on broadband microwave devices, such as a traveling-wave-tube amplifier. To
conduct such a test properly, you must first check the receiver noise figure.
Q-11.
In swept-frequency testing the impedance of a transmission line, what electrical characteristic
is actually being measured?
SWEEPING ANTENNAS
Antenna system testing is one of the more common and useful applications for using the swept-
frequency technique. The main parameters that an antenna system is tested for are vswr, frequency
response, and impedance. Figure 5-31 shows a typical test setup for testing a transmitting antenna for
vswr. Remember that any transmitting antenna can also act as a receiving antenna and send induced
power from adjacent antennas back to the test equipment. You should make an initial power check on the
antenna to prevent damage to your test equipment. Figure 5-32 shows a typical hf transmitting antenna
vswr display as measured using the swept-frequency technique. The setup for testing a receiving antenna
vswr, shown in figure 5-33, is similar, with the exception of the attenuators. The measured vswr (within
the operating frequency range) of any broadband antenna should not exceed a vswr of 2.5 to 1. The vswr
for any single-tuned antenna should not exceed 1.5 to 1 at the tuned frequency.
