Bird Technologies 4314C - 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 trav-
elling wave concept. The standing waves which are produced by transmission
line mismatch cannot be read directly. However, the VSWR can be easily deter-
mined by the use of the VSWR Conversion Nomographs (Figure 4 on page 12
and Figure 5 on page 13), without requiring the use of unwieldy and expensive
slotted line equipment. Many users find that the ratio of forward to reflected
power 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 cou-
pling 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
the reverse wave direction. Because of the highly directional characteristics of
the element, the resultant direct current sensed by the meter 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 wattmeter 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.