Practical_Antenna_Handbook_0071639586

604 P a r t V I I : T u n i n g , T r o u b l e s h o o t i n g , a n d D e s i g n A i d
There is, however, a significant problem that keeps thermocouple RF ammeters
from being universally used in RF power measurement: The instruments are highly
frequency dependent. Some meters are advertised as operating into the low-ÂVHF region,
but the results will have meaning only if a copy of the calibrated frequency response
curve for that specific meter is available so that a correction factor can be added
to (or subtracted from) the reading. As a general precaution, at 10 MHz and higher, the
readings of a thermocouple RF ammeter must be viewed with a certain amount of skepticism
unless the original calibration chart is available.
RF power can also be measured by knowing the voltage across the load resistance.
In the circuit of Fig. 27.11 the RF voltage appearing across the load is scaled down to a
level compatible with the voltmeter by the resistor voltage divider (R 2 /R 3 ). The output
of this divider is rectified by CR 1 and filtered to dc by capacitor C 2 .
Obtaining the voltage measurement from a simple diode voltmeter is valid only if
the RF signal is unmodulated and has a pure sinusoidal waveform. While these criteria
are almost always met with CW transmissions, they are not valid for other waveforms,
such as those characteristic of voice analog modes. If the voltmeter circuit is peak reading,
as in Fig. 27.11, then the peak power is
P V 2
=
R0
L
(27.11)
J 1
RF
input
R 2
3300
R 1
50-
dummy
load
R 3
1000
C 1
0.001 F
CR 1
IN34
or
IN60
C 2
0.001 F
M 1
0 – 1 mA
Figure 27.11 RF voltmeter “wattmeter”.