Bird Technologies 3128A - Manual (Page 19)

7
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 con-
trol potentiometers 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 tuneup, antenna matching etc., shows that the power ratio
φ is
no mean competitor, in practical usefulness, to the standing wave ratio ρ =
VSWR.
A trial is suggested for a few days - forget VSWR and try thinking in terms of f = Wr / Wf when the Thruline Wattme-
ter is used. It will be noted that even without bothering to calculate the ratio exactly the two meter readings, Wr and
Wf, give an automatic mental impression which pictures the situation. Thus, in an antenna matching problem the
main objective usually is to minimize Wr, 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 evalu-
ated, 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/forward 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 Watt-
meter it can be readily shown that reducing
φ below 10 percent produces little in the way of increased W
L . TV trans-
mitter 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 subitable 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.
Φ
ρ 1 –
ρ 1 +
------------
2
= ρ
1 Φ +
1 Φ –
------------------ = and
where ρ VSWR =
and
Φ
W
r
W
f
------- =