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 605
The average power is then found by multiplying the peak power by 0.707. Some
meter circuits include voltage dividers that precede the meter and thereby convert the
reading to RMS, and thus convert the power to average power. Again, it must be
stressed that terms like RMS, average, and peak have meaning only when the input RF
signal is both unmodulated and a pure sinusoid. Otherwise, the readings are meaningless
unless calibrated against some other source.
A family of RF power meters is based on various bridge methods. Figure 27.12
shows a bridge set up to measure both forward and reverse power. This circuit was
once popular for VSWR meters. There are four elements in this quasi-ÂWheatstone
bridge circuit: R 1 , R 2 , R 3 , and the antenna impedance (connected to the bridge at J 2 ). If
R ant is the antenna resistance, then we know that the bridge is in balance (i.e., the null
condition) when the ratios R 1 /R 2 and R 3 /R ant are equal. In an ideal situation, resistor R 3
will have a resistance equal to R ant , but that might overly limit the usefulness of the
bridge. In some cases, therefore, the bridge will use a compromise value such as 67 W
for R 3 . Such a resistor will be useable on both 50-Â and 75-ÂW antenna systems with only
small errors. Typically, these meters are designed to read relative power level, rather
than the actual power.
J 1
XMTR
R 3
68
J2
P 2
R 1
47 k
C 2
0.01 F
D 1
IN60
or
IN34
R ANT
R 2
47 k
R 4
47 k
R 7
47 k
C 1
0.01 F
R 5
1 k
FWD REV
S 1
M 1
0-100 A
R 6
50 k
Figure 27.12 Bridge arrangement to measure both forward and reverse power.