BIRD Wattcher 3126A, Инструкция

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The forward wave travels and its power flows from the source to the load. It has an RF voltage
E
f
and current I
f
in phase, with E
f
/ I
f
= Z
o
.
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:
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 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 produced 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 measurements 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 position, 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 measuring 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
CAUTION
The Wattcher, elements and line section have been calibrated together. Replacing any compo-
nent without recalibrating all the components as a unit may affect accuracy.
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 equipment is supplied as a matched and calibrated set and the
parts, particularly the elements, should not be interchanged 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:
W
L
= Load in Watts = W
f
-W
r
Bird Models 3126A, 3127A, 3128A
Wattcher RF Monitor/Alarm
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