3-11
ratio is reached. It must be understood, however, that the antenna does have a specific impedance at a
given frequency and that, when necessary, this impedance may be determined by use of an rf impedance
bridge.
A typical rf impedance bridge circuit is shown in figure 3-9. Rf impedance bridge measurements
require an rf signal generator, a detector, and a calibrated rf bridge to determine transmission-line
impedance. The bridge compares the parallel resistive-reactive combination with the series combination
and can typically measure impedance over a frequency range of 500 kHz to 60 MHz.
Figure 3-9.—Typical rf bridge.
Basically, the bridge is balanced with a known capacitance under short-circuit conditions. The
unknown impedance is then inserted in lieu of the short bus, and the bridge is rebalanced. The difference
between the known impedance under short-circuit conditions and the balance measurements obtained
with the unknown impedance inserted in lieu of the short is the value of the unknown impedance.
Q-7.
What is the result of an impedance mismatch between a receiver or transmitter and its
transmission line or antenna?
POWER MEASUREMENTS
It is often necessary to check the input and output signal power levels of electronic equipment. The
determination of dc power is computed by using a derivative of Ohm's law (P = IE = I2R = E
2/R).
However, the presence of a reactive component in ac circuits means that apparent power is being
measured or calculated unless the rms voltage-current value is multiplied by a power factor to obtain true
lower. The measurement of ac power is further complicated by the frequency limitations of various power
meters. If there is no phase difference, ac power may be computed in the same manner as dc power by
determining the average value of the product of the voltage and current. In practical ac circuits, the
apparent power must be multiplied by the cosine of the phase angle between the voltage and current in
order to compute true power.
In the repeated measurement of audio-frequency (af) power, you may use a normal power meter
calibrated directly in watts. However, when reactive components of dissipative impedance introduce a
