Theory of Operation
7
capacitive current, while that produced by the wave travelling in the
opposite direction will subtract. The additive or “arrow” direction is
assigned to the forward wave.
The electrical characteristics of the element are carefully adjusted so
that, for the reverse travelling wave, the inductive current will
completely cancel the capacitive current. The result is directivity
greater than 25 dB. Thus, the element is sensitive at either of its
settings, but to only one of the two travelling waves. Thruline
Wattmeter measurements are also independent of position along the
transmission line.
Like similar diode devices, the Bird 43 indicates the carrier component
of amplitude modulation, with very little response to side band
components added by modulation.
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:
W
l
= Watts into Load = W
f
– W
r
Good load resistors, such as Bird Termaline loads, will give negligible
reflected power.
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