Bird Technologies APM-16 - Manual (Page 17)

5

vs.

The simple relationships:
can be used to convert between the standing wave ratio ( and the reflected/
forward power ratio , which can be directly read from the Thruline Wattme-
ter. The relationship between  and  is graphed in Figure 3 and Figure 4.
Note: Attenuation, measured in dB, can be derived from the power
ratio by the equation
N
db
= 10 log .
VSWR scales and their attendant controls for setting the reference point have
been intentionally omitted from the Bird APM-16. Experience using the Thruline
Wattmeter for transmitter tune-up, antenna matching, etc. will show that the
power ratio measurement is as useful as the standing wave ratio.
A trial is suggested – forget about VSWR for a few days and think in terms of  =
W
r
/ W
f
. The 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 mini-
mize W
r
. Anything done experimentally to this end will be seen when the ele-
ment is in the reflected power position.
and
Where

= VSWR
and

= W
r
/ W
f

1+ 
1

–
---------------- =

 1 –
 1 +
------------ =
2