Bird Technologies 4314B - Manual (Page 17)

5
Chapter 3 Theory of Operation
Traveling Wave Viewpoint
The operation of this wattmeter is based on the travelling wave concept 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 transmis-
sion line, the smaller the reflected wave will be. To determine the amount of
power dissipated 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 travel-
ling wave concept. The standing waves which are produced by transmission line
mismatch cannot be read directly. However, 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 currents will
flow according to the direction of the travelling waves producing them. Therefore,
assuming that the Plug-In Element remains stationary, the coupling currents pro-
duced 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 can-
cel each other out, making the element highly insensitive to the reverse wave
direction. Because of the highly directional characteristics of the element, the
resultant direct current sensed by the microammeter indicates the power level of
only the RF waves travelling in the arrow direction.
The Plug-In Element is designed so that it can be rotated 180 degrees in its
socket in the line section. When it is rotated, the meter indicates the power in a
direction opposite that of the initial reading. If the forward direction power is
read first, the reflected direction power will be read after the Plug-In Element is
rotated. The energy resulting from the inductively coupled component of the
forward wave will bring about cancellation as described above.