Theory of Operation
9
NOTE: Attenuation, measured in dB, can be derived from the
power ratio by the equation N
db
= 10 log φ.
VSWR scales and their attendant controls for setting the reference
point have been intentionally omitted from the Bird 43. Experience
using the Thruline Wattmeter for transmitter tune-up, antenna
matching, etc. will show that the power ratio measurement is as
useful in practice as the standing wave ratio.
A trial is suggested – forget about VSWR for a few days and think
in terms of
φ = W
r
/ W
f
. The two meter readings, W
r
and W
f
, give
a useful, approximate picture of the results without bothering to
calculate the power ratio exactly. Consider that, for an antenna
matching problem, the main objective usually is to minimize W
r
.
Anything done experimentally to this end will be seen when the
element is turned to the reflected power position.
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