Bird Technologies 4412A - Manual (Page 13)

3
Theory Of Operation
On any uniform RF line section there are voltages, currents and standing waves
present when RF power is applied. These are the results of two travelling waves,
forward and reflected. The characteristic impedance of these lines is a pure resis-
tance, usually 50 ohms for useful lines. The main RF circuit of the wattmeter is a
short, precision type line section whose characteristic impedance is 50 ohms.
Coupling Circuit
The coupling circuit which samples the travelling waves is in the plug-in element.
The circuitry of the element and its relationship to the other components of the
wattmeter are illustrated in figure 1. Energy is absorbed in the coupling circuit of
the element both by mutual inductance and by capacitance from the travelling
RF waves of the line section. These inductive currents flow according to the
direction of the travelling waves producing them, whereas the capacitive por-
tion of these currents is independent of the direction. It is, therefore, apparent
that the two currents produced from the waves travelling in one direction add in
phase, while the currents produced from the waves travelling in the opposite
direction subtract in phase. The arrow on the Plug-In Element indicates the addi-
tive direction of wave travel. The element is so designed that the currents from
the wave components travelling in the opposite direction of the arrow cancel
each other out almost completely, making the element highly insensitive to the
reverse wave direction. The additive direction signal, which is to be measured, is
detected from which an accurate power reading is obtained. Because of the
highly directional characteristics of the element, the resultant direct current
which is sensed by the microammeter indicates the power level of only the RF
waves travelling in the arrow direction.
Figure 1 Element Schematic Diagram
RF Coaxial Line
XMTR
or
LOAD LOAD
or
XMTR
Directional Coupling
Diode
Detector Element
Meter
DC Contact
Circuit Board