Bird Technologies APM-16 - Manual (Page 15)

3
Chapter 2 Theory of Operation
Traveling Wave Viewpoint
The easiest way to visualize Thruline operation is from a travelling wave view-
point. In transmission lines the voltages, currents, standing waves, etc., on any
uniform line section result from the interaction of two travelling waves:

The forward wave (and its power) travels from the source to the load.
It has RF voltage E
f
and current I
f
in phase, with E
f
/ I
f
= Z
o
.

The reflected wave (and its power) originates by reflection at the load
and travels from the load back to the source. It has an RF voltage E
r
and current I
r
in phase, with E
r
/ I
r
= Z
o
.
Each wave is mathematically simple and has a constant power:
W
f
= Watts Forward = E
f
2
/ Z
o
= I
f
2
Z
o
= E
f
I
f
W
r
= Watts Reflected = E
r
2
/ Z
o
= I
r
2
Z
o
= E
r
I
r
Z
o
is the characteristic impedance of a uniform line section. For useful lines it is
usually a pure resistance of 50 ohms. The RF circuit of the Bird APM-16 is a
length of uniform air line with Z
o
= 50 ohms.
Coupling Circuit
The Bird Plug-In Element contains a coupling circuit that samples the travelling
waves. The element circuitry and its relationship to the rest of the Bird APM-16
are illustrated in Figure 5.
Figure 2 Schematic