Bird Technologies 4331 - Manual (Page 21)

Chapter 2
Theory of Operation
Travelling Wave Viewpoint
The best way to visualize the Thruline Wattmeter principle is from the
Travelling Wave viewpoint. In transmission lines, the voltages, currents,
standing waves, etc., on any uniform line section are the result of the
interaction of two Travelling waves:
s The forward wave travels (and its power flows) 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.
s The reflected wave originates by reflection at the load, travels
(and its power flows) from the load back to the source, and also
has an RF voltage E
r
and current I
r
in phase, with E
r
/ I
r
= Z
o
.
s Note that each component wave is mathematically simple, and is
completely described by a single figure for power, for instance:
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 the uniform line, and simplifies matters
by being nearly a pure resistance, usually 50 ohms, for useful lines. The
main RF circuit of the Model 43 is a short piece of uniform air type line
section, whose Z
o
is 50 ohms, in which accurate measurements may be made.
Coupling Circuit
The coupling circuit which samples the travelling waves is in the Plug-In
Element. The circuitry of the element and its relationship to the other
7