User guide

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:
• 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:
Note: Z
o
is the characteristic impedance of a uniform line section. For
useful lines it is usually a pure resistance of 50Ω. The RF circuit of the
Bird 43 is a length of uniform air line with Z
o
= 50Ω.
W
f
WattsForward E
f
2
Z
o
⁄ I
f
2
Z
o
E
f
I
f
====
W
r
WatssReflected E
r
2
Z
o
⁄ I
r
2
Z
o
E
r
I
r
====