Bird Technologies 4331 - Manual (Page 40)

conditions, since a good 50 ohm load can be placed at the end of a 50 ohm
transmission line of any length without altering conditions at the transmitter.
Using the VSWR Nomograph, determine the intersection of the forward and
reflected power values. The slanted line passing closest to this point indicates
the VSWR.
What happens when the load is not well matched, like an antenna with a
VSWR of 1.5 or 2.0? Since the length of line between a mismatched load and
the source transforms the impedance of the load as seen at the source, line
length now becomes critical. If the adjustments for maximum power transfer
were made with the Model 43 in place, removing it shortens the line by four
inches, plus two connectors. This still is no cause for concern at low frequen-
cies where four to five inches is a small fraction of a wavelength. At higher
frequencies; e.g., above 100 MHz, power output and frequency of the source
may be affected.
It is a principle of transmission line theory that the impedance is identical
on either side of a 1/2 wavelength. In order to duplicate the conditions in
your transmission line with the Model 43 either in or out of the line, it is
only necessary to insert or remove a 1/2 wavelength.
This is easily done by making up a length of cable which, when added to
the Thruline Wattmeter, equals 1/2 wavelength at the frequency of measure-
ment. If more than one frequency is involved, a separate cable is needed for
each frequency.
Bird Model 43 Wattmeter
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