Bird Model 43 Wattmeter
7
Load Power
For loads with a VSWR of 1.2 or less, the power dissipated in a load
(W
l
) is equivalent (with less than one percent error) to the forward
power (W
f
). When appreciable 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. 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).
W
1
WattsIntoLoad
W
f
W
r
–
=
=
Содержание Thruline 43
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