Bird Technologies 4304A - Manual (Page 16)

5
Load Power
For loads with a VSWR of 1.2 or less, the power dissipated in a load (W
l
) is equiv-
alent (with less than one percent error) to the forward power (W
f
). When appre-
ciable power is reflected, as with an antenna, it is necessary to use the exact
load power which is given by:
Good load resistors, such as Bird Termaline loads, will give negligible reflected power.
Standing Wave vs. Traveling Wave Viewpoint
As mentioned previously, the Thruline Wattmeter reacts to forward and reverse
travelling waves to measure power in a transmission line. The standing wave
viewpoint, also widely used, is highly developed both in theory and in practice.
This viewpoint can be traced to the early use of slotted transmission lines.
The slotted line measures the standing wave ratio by mechanically positioning a
voltage detector at peaks and nulls along a length of line section. Its drawbacks
are that it is usually too long, too expensive for good accuracy, not portable, and
too slow. These problems grow rapidly as the measurement frequency drops
below 1000 MHz. The Thruline Wattmeter by comparison is fast, convenient, and
accurate. It provides the same information as a slotted line with the exception of
the phase angle of the reflection coefficient (distance, load to minimum).
ρ
vs.
φ
The simple relationships:
can be used to convert between the standing wave ratio (
ρ) and the reflected/
forward power ratio
(φ), which can be directly read from the Thruline Wattme-
ter. The relationship between
ρ and φ 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
φ.
VSWR scales and their attendant controls for setting the reference point have
been intentionally omitted from the Bird 4304A. 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.
and
Where
ρ = VSWR
and
φ = W
r
/ W
f
W
1
WattsIntoLoad W
f
W
r
– = =
ρ
1+ φ
1 φ –
---------------- = φ
ρ 1 –
ρ 1 +
------------ =
2