Bird Technologies 4527 - Manual (Page 17)

5
Standing Wave vs. Travelling Wave Viewpoint (
ρ
vs.
φ)
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).
The simple relationships:
and
Note: Where r = VSWR and f = Wr / Wf
These 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 Thrul-
ine Wattmeter. The relationship between
ρ and φ is graphed in Figure 4 and
Figure 5.
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 pic-
ture of the results without bothering to calculate the power ratio exactly. Con-
sider 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.
ρ
1 φ +
1 φ –
---------------- =
φ
ρ 1 –
ρ 1 +
------------
2
=