Startup
26
a. Determine how wet the system is. If a system of rigid line carefully protected
from weather and assembled in dry weather is average, a system exposed
to moisture during storage or installation will be relatively wet. New semi-
flex transmission line, delivered pressurized with dry gas, will be relatively
dry; used semi-flex will be extremely wet.
Important
Never apply transmitter power while the antenna is under vacuum.
b. If you have any liquid water in your transformer or your transmission line,
use a vacuum pump to dry the transmission line and transformer. Apply as
much vacuum as you can to the system and hold the vacuum for 8 hours.
This should remove any liquid water. [A vacuum pump can be rented or
borrowed from a refrigeration contractor.]
c. Determine the volume of dry gas to use for the purge.
d.
Table 5
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.
NOTE
A standard nitrogen cylinder (9 inch diameter by 55 inches tall) contains
about 200 cubic feet (2.6 m
3
) of gas.
e. Shively Labs Models 1235 and 2577 compressor-dehydrators will provide
about 12 cubic feet (0.34 m
3
) per hour; the Model 1234 about 78 cu ft (2.2
m
3
) per hour.
f. Connect a source of dry gas (cylinder nitrogen or air from a compressor-
dehydrator) to the system as shown in
Figure 14
.
g. Raise the gas pressure to 12 or 13 psig (83 - 90 kPa).
If the relief valve has opened, the nitrogen cylinder will slowly drain or the
compressor-dehydrator will not shut down.
Leaving the System
Pressurized
After completion of the purge, reduce the supply pressure to about 5 to 7
psig, 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 5. 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
)
