Bird Technologies 4308 - Manual (Page 15)

Chapter 2
Theory Of Operation
Travelling Wave Viewpoint
The operation of the Model 4308 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
back to the transmitter. The closer the line termination, or load, is matched to the line
impedance, the smaller the reflected wave will be. To determine the number of watts
dissipated in the load, it is necessary to determine both the power of the forward wave
and the power of the reflected wave. The difference between the two will indicate
effective power dissipation.
Travelling Wave vs Standing Wave
The interference between the forward and reflected waves, produces a standing wave in
a coaxial system. In the standing wave concept, VSWR (voltage standing wave ratio) is
a widely used tool. There is a simple relationship between forward power, reflected
power, and VSWR.
Where W
f
= forward power; and W
r
= reflected
VSWR
W
W
W
W
r
f
r
f
=
+
−
1
1
!