Bird Technologies 8255 - Manual (Page 25)

DC Resistance Measurement
Preparation
and Tools
Required
Be sure that the vent plug is installed.
Preparation:
s Tools: Common hand tools
s Ohmmeter with an accuracy of ± 1% at 50
ohms.
s Temperature of the load between 20°C to 25°C
(68°F to 77°F)
VSWR and RF Impedance are the true benchmark of a
quality dummy load. Accurate measurement of the dc re-
sistance between the inner and outer conductors of the
RF input connector will provide a good check of the con-
dition of the load resistor. Checking the dc resistance is
simply used to measure a change in the condition of the
resistor over time. The tracking of the dc resistance must
start before the resistor is first put into service. Perform
the following steps and record the value for future com-
parison. Check and record the resistance of the load peri-
odically according to use.
WARNING
Never attempt to connect or disconnect RF equipment
from the transmission line while RF power is being
applied. Leaking RF energy is a potential health
hazard.
Checking the
DC
Resistance
Measure the dc resistance. Follow the instructions below.
1. Turn off RF power and interlock circuitry before
any electrical disconnection’s are made.
2. Disconnect RF coaxial line.
3. Connect the multimeter test leads across the cen-
ter and outer conductor of the load resistor. Refer
to figure 3.
4. Record the value of the resistance before the load
is put into service. Compare subsequent values
with the latest reading. If the values vary more
than two ohms this could be an indication of a
failing resistive element.
Maintenance
13