Bird Technologies 4331 - Manual (Page 24)

1. Why make something similar to a hypothetical dc volt-
ohm-meter with control pots for the voltmeter multipli-
ers? Even more complications arise when diodes at RF
are involved.
2. Experience using the Thruline Wattmeter for transmitter
tune-up, antenna matching, etc. (i.e., on operating prob-
lems), shows that the power ratio φ is as useful in prac-
tice as the standing wave ratio p = 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, for an antenna matching problem, the main objective usually is to
minimize W
r
. Anything done experimentally to this end is noted directly
when the Model 43 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 p and the reflected/forward power ratio φ ,
indicated by the Thruline Wattmeter, the latter may be conveniently used
to measure VSWR. The relationship is given in figures 4 and 5.
Note that when
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 p
= 2, below which improved antenna match becomes less and less worthwhile
in many systems. Experimentally, by using the Thruline Wattmeter, it can
p and
p
p
=
+
−
=
−
+
L
NM
O
QP
1
1
1
1
2 φ
φ
φ
Where p VSWR
and
W
W
r
f
=
= φ
Bird Model 43 Wattmeter
10