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 603
Measuring RF Power
Measuring RF power has traditionally been notoriously difficult, except perhaps in the
singular case of continuous-Âwave (CW) sources that produce pure sine waves. Even
in that limited case, however, some measurement methods are distinctly better than
others.
Suppose the peak voltage of a waveform is 100 V (i.e., from negative peak to positive
peak is 200 V). Since a CW waveform is a pure sine wave, we know that the root
mean square (RMS) voltage is 0.707 × peak voltage. Power dissipated in a resistive load
is related to the RMS voltage across the load by
P V Z 0
RMS
= ( )2 (27.9)
where P = power, in watts, delivered to antenna
V RMS = RMS voltage, in volts, measured at feedpoint
Z 0 = feedpoint impedance, in ohms
Assuming a load impedance of 50 W, the power in our hypothetical illustration
waveform is 100 W.
We can measure power on unmodulated sinusoidal waveforms by measuring either
the RMS or peak values of either voltage or current, assuming that a constant resistance
load is present. But accurate measurement becomes more difficult in the presence
of complex waveforms such as modulated signals. For instance, on a Bird model 4311
peak power meter the various power readings—peak(PEP) versus average—vary
markedly with modulation type.
One of the earliest practical RF power-Âmeasuring devices was the thermocouple RF
ammeter (see Fig. 27.10). This instrument works by dissipating a small amount of
power in a small resistance inside the meter and
measuring the resulting heat generated with a
Thermocouple
Input
terminals
Resistor
element
thermocouple. A dc ammeter reports thermocouple
current. Because it works on the basis of
the power dissipated in heating a resistance, a
thermocouple RF ammeter is inherently an
RMS-Âreading device. It is thus very useful for
making average power measurements. If we
know the RMS current into the antenna feedpoint
and the resistive component of the load
impedance, then we can determine RF power
from the familiar expression
P= I 2 × R L
(27.10)
provided the reactive component of the load impedance
is zero or very low.
dc voltmeter
Figure 27.10 Circuit for thermocouple RF ammeter.