Bird Technologies 8833-300 - Manual (Page 8)

Note: Be sure that the knurled ring nut [A] is in place over the base plug [D] with the knurled end
outward (towards face).
Screw on the cover ring [C].
Fasten the cable clamp [E], and tighten the two yoke
screws [H] on the cable.
Push the plug back on the thermoswitch, and tighten
captive knurled connecting ring. Do not attempt to
operate the equipment without interlock attached.
2.6 Checking the DC Resistance
VSWR and RF Impedance are the true benchmark
of a quality dummy load. Checking the dc resistance
is simply used to measure a change in the condition
of the resistor over a period of time. If the application
requires such measurements, follow the steps below
to obtain a benchmark value before the unit is put
into service.
Preparation and Tools Required
Be sure the vent plug is installed, see paragraph 2.4.
Tools: Resistance Bridge or Ohmmeter with accuracy
of ±1% @50 ohms.
Temperature of the load is 20° to 25°C (68° to 70°F).
Procedure
Using the resistance bridge or ohmmeter, check the
dc resistance of the load before it is put into service,
see Figure-5.
Record the value and keep for future use.
Normally the dc resistance of the load will be a
nominal 50 ohms. Stabilization of the resistive film or
temperature can cause a change in the dc resis-
tance.
Check the resistance of the load periodically, accord-
ing to use, and compare this resistance with the
original reading. If the values vary more than 2 ohms
this could be an indication of a failing resistive ele-
ment.
3. Theory of Operation
3.1 General
The Model 8833-300 RF Load consists of a film-on-
ceramic resistor immersed in a dielectric coolant. The
resistor, which is individually selected for its accu-
racy, is enclosed in an exponentially tapered housing,
providing a reduction in surge impedance directly
proportional to the distance along the resistor. When
the resistor unit is immersed in the dielectric liquid,
the characteristic impedance will be 50 ohms at the
connector end and graduate to zero ohms at the rear,
where the resistor joins the housing forming the
return conductor of the coaxial circuit. This produces
a uniform, practically reflectionless, line termination
over the stated frequencies of the load resistor.
3.2 Dielectric Coolant
The dielectric coolant, a low volatility, high flash point
synthetic oil, is chosen for its desirable thermal char-
acteristics and dielectric properties, to which the
diameters of the resistor housing are matched. A
synthetic rubber o-ring around the outside of the
r
e
s
Figure - 4
Thermoswitch Assembly
Figure - 5
Checking DC Resistance
TO-nnn-nn-nnn-nn
Page 8