Bird Electronic 4314B, Operation Manual

Page: 17 / 48

5
Chapter 3 Theory of Operation
Traveling Wave Viewpoint
The operation of this wattmeter is based on the travelling wave con-
cept of RF transmission. As RF power is applied to a transmission
line, there is a forward wave travelling from the transmitter to the
load, and a reflected wave travelling from the load to the transmitter.
The closer the load is matched to the transmission line, the smaller
the reflected wave will be. To determine the amount of power dissi-
pated in a load resistor, it is necessary to determine the wattage of the
forward wave and the wattage of the reflected wave. The difference
between the two indicates load power.
VSWR (Voltage Standing Wave Ratio) has become a widely used tool
in the travelling wave concept. The standing waves which are pro-
duced by transmission line mismatch cannot be read directly. How-
ever, the VSWR can be easily determined by the use of the VSWR
Conversion Nomographs (fFigure 5 on page 14 and Figure 6 on
page 15), without requiring the use of unwieldy and expensive slotted
line equipment. Many users find that the ratio of is more useful than
VSWR.
Coupling Circuit
When the wattmeter is connected into the system, the RF power is
directed through the unit’s line section. The air line is a short, uniform
section which does not impair the impedance of the RF coaxial line
into which it is inserted. When the Plug-In Element is installed in the
socket of the line section, the RF waves travelling through the line
produce energy in the coupling circuit of the Plug-In Element by both
inductance and capacitance (Figure 2 on page 7). The inductive cur-
rents will flow according to the direction of the travelling waves pro-
ducing them. Therefore, assuming that the Plug-In Element remains
stationary, the coupling currents produced from the waves of one
direction add in phase, while those produced from the waves of the
opposite direction subtract in phase. The additive or “arrow” direction
is assigned to the forward wave. The element is designed so that the
wave components travelling in the opposite direction of the arrow will
nearly cancel each other out, making the element highly insensitive to