Bird Technologies 4304A - Manual (Page 15)

4
Coupling Circuit
Figure 2 Thruline Wattmeter Schematic
The element, which contains the coupling circuit, and its relationship to the rest of the
Bird 4304A is shown in Figure 2. Current is produced in the coupling circuit by travelling
waves in the line section, through both inductive and capacitive coupling. The inductive
current flows in the direction of the travelling wave, while the capacitive current is inde-
pendent of the direction of the wave. Therefore, the inductive current produced by one
travelling wave 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” direction is assigned to the forward wave.
The electrical characteristics of the element are carefully adjusted so that, for the
reverse wave, the inductive current will completely cancel the capacitive current, giving
a directivity greater than 20 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 4304A indicates the carrier component of
amplitude modulation, with very little response to side band components added
by modulation.
RF Coaxial Line
XMTR
or
LOAD LOAD
or
XMTR
Directional Coupling
Detector Element
Diode
Meter
DC Contact
Circuit Board