Practical_Antenna_Handbook_0071639586

606 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
An advantage of this type of meter is that we can get an accurate measurement of
VSWR as long as we properly calibrate the device. With the switch in the forward position,
and RF power applied to J 1 (XMTR), potentiometer R 6 is adjusted to produce a
full-Âscale deflection on meter M 1 . When the switch is then set to the reverse position, the
meter will read reverse power relative to the VSWR. An appropriate VSWR scale, with
calibration specific to the circuit configuration employed, is provided. This design was
commercially available from a number of manufacturers in the middle of the twentieth
century, most notably being packaged with some of the E. F. Johnson Viking Matchboxes
of the era.
A significant problem with the bridge of Fig. 27.12 is that it cannot be left in the circuit
while transmitting because it dissipates a considerable amount of RF power in the
internal resistances. Some of these meters were provided with switches that bypassed
the bridge when transmitting so that the bridge was only in the circuit when making a
measurement. Others, however, had to be removed from the line before transmitting.
An improved bridge circuit, shown in Fig. 27.13. is the capacitor/resistor bridge—also
called the micromatch bridge. We can immediately see that the micromatch is improved
over the conventional bridge because it uses only 1 W in series with the line (R i ), thus
dissipating considerably less power than the resistance used in the previous example.
Because of this, we can leave the micromatch in the line while transmitting. Recall that
the ratios of the bridge arms must be equal for the null condition to occur. In this case,
the capacitive reactance ratio of C 1 /C 2 must match the resistance ratio R 1 /R ant . For a
50-ÂW antenna the ratio is 1 ⁄50, and for 75-ÂW antennas it is 1 ⁄75 (or 1 ⁄68 for a single compro-
J 1
XMTR
R 1
1
J 2
ant
C 1
820 pF
S.M.
D 1
IN60
RFC 2
1 mH
C 2
30 pF
R 1
10 k
sensitivity
RFC 1
1 mH
M 1
0–500 A
Figure 27.13 Micromatch wattmeter.