Bird Technologies APM-16 - Manual (Page 16)

4
Current is produced in the coupling circuit by the travelling waves in the line
section. Both inductive and capacitive coupling contribute to this. Inductive cur-
rent flows in the direction of the travelling wave, while the capacitive current is
independent of the direction of the travelling wave. Therefore, the inductive
current produced by one travelling wave will add in phase with the correspond-
ing capacitive current, while that produced by the wave travelling in the oppo-
site 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 APM-16 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 equal
(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, given by:
W
l
= Watts into Load = W
f
– W
r
Good load resistors, such as Bird Termaline Loads, will give negligible reflected
power.
Standing vs. Travelling Waves
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 except for the
phase angle of the reflection coefficient (distance, load to minimum).