3-10
standard resistor, R, is adjusted until the balanced rectifier outputs of Q1 and Q2 are equal (indicated by a
null in the deflection of the voltmeter connected between the emitters of Q3 and Q4). The dial setting of R
gives the value of ZX. For phase angle determination, the circuit is switched to CAL and the input voltage
is adjusted for full-scale voltmeter deflection. The circuit is then switched to PHASE; thus, the paralleled
outputs of Q1 and Q2 are applied to rectifier CR1 only. With S2 in the phase position, there is no input to
the base of Q4. If Z is purely resistive, the outputs of Q1 and Q2 cancel, and the voltmeter indicates zero
deflection. For a complex impedance, the base of Q3 will be unbalanced with respect to the base of Q4;
and the voltmeter deflection, calibrated in degrees, determines the phase angle of the unknown
impedance. Typical commercial impedance angle meters, operating at 2 MHz, are accurate to within 4%
for impedances of from 10 to 500 ohms.
Figure 3-8.—Impedance-angle meter.
Q-6.
What do impedance-angle meters and vector bridges have in common?
IMPEDANCE TESTING OF ANTENNAS AND TRANSMISSION LINES
The amount of current that flows in an antenna is one of the most important factors affecting the
performance of transmitter equipment. As much of the rf energy generated as possible must be efficiently
transferred to the antennas to secure the maximum radiated power from a transmitter. Also, for best
reception, maximum transfer of energy from the antenna to the receiver must occur. Efficient
transmission and reception conditions prevail whenever the transmitter (or receiver) is properly matched
to the transmission line and the transmission line is properly matched to the antenna. Normally,
performance tests concerning impedance match consist primarily of taking standing-wave measurements.
In certain instances, it may be found that a change in antenna impedance has resulted in an undesirably
high standing-wave ratio. This could be the result of a new antenna installation or an interfering structure
near the antenna that influences antenna characteristics.
In practice, the antenna-matching network is varied to match the new antenna characteristics, since
the transmission line is designed to match equipment impedance. This can best be done by making a
series of standing-wave-ratio checks and antenna-matching adjustments until an acceptable standing-wave
