Bird Technologies 4527 - Manual (Page 16)

4
Figure 3 Thruline Wattmeter Schematic
Current is produced in the coupling circuit by the travelling waves in the line
section. Both inductive and capacitive coupling contribute to this. The induc-
tive current flows in the direction of the travelling wave. The capacitive cur-
rent is independent of the direction of the travelling wave. Therefore, the
inductive current produced by one of the travelling waves will add in phase
with the corresponding capacitive current, while that produced by the wave
travelling in the opposite direction will subtract. The additive or “arrow” direc-
tion 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 Watt-
meter measurements are also independent of position along the transmission
line.
Like similar diode devices, the Bird 43 indicates the carrier component of ampli-
tude 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:
Good load resistors, such as Bird Termaline loads, will give negligible reflected
power.
W
1
WattsIntoLoad W
f
W
r
– = =