19
Startup and Operation
c. Determine the volume of dry gas to use for the purge.
Table 4
shows
approximate volumes inside various coax sizes. Add the length of the
antenna to the length of the transmission line to determine the overall length
of the system. You may ignore the volume inside the radiators. We suggest
three volume changes of dry gas for an "average" system.
NOTE
A standard nitrogen cylinder (9 inch diameter by 55 inches tall) contains
about 200 cubic feet (5.7 m
3
) of gas.
CAUTION
Do not raise pressure over 20 psig (~135 kPa), even briefly. Note that it
takes time for the entire system to fill with the new pressure and the
pressure gauge to stabilize.
d. Connect a source of dry gas (cylinder nitrogen or air from a compressor-
dehydrator) to the system as shown in
Figure 11
. Raise the gas pressure to
12 or 13 psig (83 - 90 kPa).
CAUTION
You must blow dry gas
through
the system, not just maintain a pressure.
The gas
volume
accomplishes the purge.
e. If the relief valve has opened, the nitrogen cylinder will slowly drain or the
compressor-dehydrator will not shut down.
Leave the system
pressurized
After completion of the purge, reduce the supply pressure to about 5 to 7 psig
(35 to 48 kPa), allowing the pressure relief valve to close and seal the system.
After the pressure has stabilized, keep careful note of cylinder pressure or
compressor-dehydrator running time, to be sure that no large leaks have been
overlooked. This is especially important immediately after installation or any
subsequent opening and reassembly.
Table 4. Volume of coax per 1000 feet of length
Coax Size
Volume
1-5/8"
13 cu ft. (0.37 m
3
)
3-1/8"
50 cu. ft. (1.4 m
3
)
4-1/16"
90 cu. ft. (2.6 m
3
)
6-1/8"
200 cu. ft. (5.7 m
3
)
9-3/16"
450 cu. ft. (13 m
3
)
