Bird Technologies 4527 - Manual (Page 15)

3
Chapter 2 Theory of Operation
Travelling 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:
•
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
.
•
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 cur-
rent I
r
in phase, with E
r
/ I
r
= Z
o
.
Formulas
Each wave is mathematically simple and has a constant power:
Forward
Reflected
Note: 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 43 is a length of uniform air line with Z
o
= 50 ohms.
Coupling Circuit
The coupling circuit that samples the travelling waves is in the Bird Plug-In Ele-
ment. The element circuitry and its relationship to the rest of the Bird 43 are
illustrated in Figure 3.
W
f
WattsForward E
f
2
Z
o
⁄ I
f
2
Z
o
E
f
L
f
= = = =
W
r
WattsReflected E
r
2
Z
o
⁄ I
r
2
Z
o
E
r
L
r
= = = =