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 607
mise value covering both impedances). The small-Âvalue trimmer capacitor (C 2 ) must be
adjusted for a reactance ratio with C 1 of 1 ⁄50, 1 ⁄75, or 1 ⁄68, depending upon how the bridge
is set up.
The sensitivity control can be used to calibrate the meter. In one version of the micromatch,
there are three power ranges (10, 100, and 1000 W). Each range has its own
sensitivity control, and these are switched in and out of the circuit as needed.
The monomatch bridge circuit in Fig. 27.14 is the instrument of choice for HF and low-Â
VHF applications. In the monomatch design, first seen by the author in Collins accessories,
the transmission line is segment B, while RF sampling elements are formed by
segments A and C. Although the original designs were based on a coaxial-Âcable sensor,
later versions used printed circuit foil transmission line segments or parallel brass rods
for A, B, and C.
The sensor unit is basically a directional coupler with a detector element for both
forward and reverse directions. For best accuracy, diodes CR 1 and CR 2 should be matched,
as should R 1 and R 2 . The resistance of R 1 and R 2 should match the transmission line surge
impedance, although in many instruments a 68-ÂW compromise resistance is used.
The particular circuit shown in Fig. 27.14 uses a single dc meter movement to monitor
the output power. Many recent implementations employ two separate meters (for
simultaneous viewing of forward and reverse power, as in the Kenwood wattmeters) or
a single meter with crossed-Âneedle pointers (as in certain Daiwa units).
R 1
100 k
CR 2
IN60
C 2
0.01 F
J 1
XMTR
B
C
J 2
ant
R 4
15 k
A
CR 1
IN60
R 2
100
R 3
15 k
FWD REV
S 1
M 1
C 1
0.01 F
R 5
10 k
0-500 A
C 3
0.05 F
Figure 27.14 Monomatch wattmeter.