Where appreciable power is reflected, as with an antenna, it is necessary to subtract the reflected
power from the forward power to get the effective power. This correction is negligible, less than
1 percent, if the loading device has a VSWR of 1.2 to 1 or less.
VSWR scales, and their attendant controls for setting the reference point, have been intentionally
omitted from the Thruline Wattmeter for two reasons.
a. Why make something similar to a hypothetical dc volt ohmmeter with control potentiome-
ters for the voltmeter multipliers? Even more complications arise when diodes at RF are
involved.
b. Experience using the Thruline Wattmeter on operating problems, such as transmitter tune-
up, antenna matching etc., shows that the power ratio
φ
is no mean competitor, in practi-
cal usefulness, to the standing wave ratio
ρ
= VSWR.
A trial is suggested for a few days - forget VSWR and try thinking in terms of
φ
= W
r
/ W
f
when the Thruline Wattmeter is used. It will be noted that even without bothering to calculate
the ratio exactly the two meter readings, W
r
and W
f
, give an automatic mental impression which
pictures the situation. Thus, in an antenna matching problem the main objective usually is to
minimize W
r
, and anything done experimentally to this end is noted directly when the Thruline
Element is turned to the reflected position. Furthermore, the ratio of readings, even if only mentally
evaluated, is a reliable guide to the significance of the remaining reflected power.
ρ
vs.
φ
and its
Significance
Since there are definite simple relationships between standing wave ratio
ρ
and the reflected/for-
ward power ratio
φ
indicated by the Thruline Wattmeter, the latter may be conveniently used to
measure VSWR.
Note that around
φ
= 10 percent, below which W
r
will appear insignificant and may be hard to
read, you are close to the commonly accepted lower limit
ρ
= 2. Trying to adjust to an even
lower value of
φ,
in order to improve antenna match still further, becomes less and less worthwhile
in many systems. Experimentally by using the Thruline Wattmeter it can be readily shown that
reducing
φ
below 10 percent produces little in the way of increased W
L
. TV transmitter antenna
lines and VHF omnirange transmitters are among those systems that require much lower levels
of reflected power but for reasons other than simple power transmission. A very small level of
reflected power, e.g.,
φ
= .06 percent, corresponds to
ρ
= 1.05. With just a single element suitable
for measuring W
f
, detection of reflected power is possible down to about
φ
= 1 percent (
ρ
=
1.2), providing W
f
approaches full scale. However, measurement is possible only down to about
φ
= 5 percent (
ρ
= 1.5).
Measurement and
Monitoring of
Transmitter Power
Little more need be said about this in view of the preceding load power discussion. The Thruline
Wattmeter is useful for the continuous monitoring of transmitter output and is also helpful for
the continuous monitoring of reflected power, for instance in checking intermittent antenna or
line faults.
Like diode devices in general, the Thruline meter indicates the carrier component on amplitude
modulation, with very little response to side band components added by modulation.
Chapter 2 - Theory of Operation
13
