Bird Technologies 3128A - Manual (Page 18)

6
The reflected wave originates by reflection at the load, travels, and its power flows 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 .
Note that each component wave is mathematically simple and is completely described by a single figure for power,
for instance:
Z o is the characteristic impedance of the uniform line, and simplifies matters by being a pure resistance, usually 50
ohms for useful lines. The main RF line circuit of the Thruline Wattmeter is a short piece of uniform air line section,
whose Z
o is 50 ohms, in which accurate measurements may be made.
The electrical values of the element circuits are carefully balanced and so designed that the inductive current pro-
duced from the reverse direction wave will cancel its portion of the capacitive current almost completely. The result
is a directivity always higher than 25 dB, which means that the element is highly insensitive (nulled) to the reverse
direction wave. By being highly directional, the Thruline Wattmeter is sensitive at either one of its settings, but to
only one of the two traveling waves which produce standing waves by interference. Thruline Wattmeter measure-
ments are also independent of their position along the standing waves.
Element Socket
An accurately positioned socket for inserting a radio frequency coupling device, called a Thruline Plug-In Element, is
mounted on the outer conductor. The socket is precision bored to hold the rotatable element in its calibrated posi-
tion, with a spring-loaded clasp to keep the element firmly seated. The machined step on the top face of the socket
engages a stop-pin on the element. Rotary movement of the element is thereby restricted to 180 degrees and is
stopped on the axial center line.
The measuring socket has a hole bored through the wall through which an insulated phosphor bronze contact finger
projects. The Plug-In Element has terminals on diametrically opposite sides of its body, so that pickup can be made
from either side. A small silvered button tip can contact the element only in the precise forward and reflected mea-
suring positions, against the end stops. A specially designed jack mounted on the side of the socket mates with the
plug on the DC cable furnished with the read-out meter. The DC jack assembly has a built-in filter capacitor shunted
across the meter circuit. This more fully protects meter readings against the adverse effects of any stray RF energy
generated in the Plug-In Element. The line section is bright silver plated over practically all of its metal parts.
Matching
The scale on the read-out meter reads full scale for the power rating stamped on the cap of the Plug-In Element. The
Wattcher, the Plug-In Element, and the Thruline Section are all stamped with matching serial numbers. The equip-
ment is supplied as a matched and calibrated set and the parts, particularly the elements, should not be inter-
changed with any other like equipment. Such an interchange of the measuring elements could produce readings
with an error greater than the stated 5 percent of full scale accuracy.
Load Power
Power delivered to and dissipated in a load is given by:
CAUTION
The Wattcher, elements and line section have been calibrated together. Replacing any
component without recalibrating all the components as a unit may affect accuracy.
W
f
WattsForward E
f 2
Z
o
I
f 2
= ⁄ Z
o
E
f
I
f
= = =
W
f
WattsReflected E
r 2
Z
o
I
r 2
= ⁄ Z
o
E
r
I
r
= = =
W
L
LoadinWatts W
f
W
r
– = =