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51
Startup
9 Startup
We strongly recommend initial characterization as the best way to identify
both initial problems and possible future system damage.
Initial characterization
Before beginning initial
characterization:
Important
In the days before the hazards of intense RF power were realized, it was
common practice to have a technician climb the tower and adjust the
impedance match using the transmitter as a signal source and reading
the VSWR or return power on the transmitter. This practice MUST NOT
be used, as few transmitters can be operated at a low enough power
level to avoid exposing the rigger to an unsafe RF level. For reference,
see 29 CFR, Section 1910.97, the OSHA standard for exposure to non-
ionizing radiation.
To test and adjust VSWR safely, use low-power test equipment, such as
a network analyzer. If you don't have access to low-power test equip-
ment, please call Shively Labs before proceeding.
WARNING
Whenever a rigger is on the tower in the area of the antenna, shut off
the transmitter signal and lock and tag it out so that it cannot be turned
on accidentally.
Low-power test equipment should be used to prevent excessive radia-
tion exposure to the person doing the adjusting.
A high transmission line VSWR may indicate damaged transmission line and is
likely to cause problems in the future, including serious damage to your equip-
ment.
Read transmission line
VSWR.
The first step is to characterize the transmission line by itself.
a. Briefly disconnect the transmission line from the antenna system input. Seal
the antenna system input to prevent the entry of moisture.
b. Terminate the coax transmission line in an instrument-quality 50-ohm load.
c. Measure and record the voltage standing wave ratio (VSWR) across the
operating band. File this information with this manual for future reference.
d. The VSWR of the transmission line should be within the manufacturer's
specifications. If it is, proceed. If not, you should call the manufacturer
before connecting the antenna. Problems must be worked out with the
design engineer on a case-by-case basis.
Read transmission line
TDR.
With the transmission line still terminated in 50 ohms, make a time domain
reflectometer (TDR) plot. Label and file the plot with this manual.
Read system VSWR.
You tested the VSWR of the transmission line alone. Now test the VSWR of the
system as a whole.
a. Remove the load and connect the transmission line to the antenna system
input.
b. Measure VSWR. VSWR at this point should be around 1.2 : 1 or better. If it
is not, check to be sure all the radiators are functioning (below).
c. Record the reading and file it with this manual.