Performance notes – Bird Technologies 4314B User Manual
Page 31
19
Performance Notes
Although the Model 4314B Thruline Wattmeter is equipped with Quick-Change
connectors, it must be remembered that the power rating and insertion loss
may be affected if other than “N” type connectors are used. Power limits must
be governed by the type of connector or transmission line used. For other types
of Quick-Change connectors, see Chapter 5 for the replacement parts list.
When a Model 4314B is used to match a load to a transmitter and a good match
is obtained, removing the instrument will not cause any change in the condi-
tions. A well matched 50 ohm load can be placed at the end of a 50 ohm trans-
mission line of any length without altering conditions at the transmitter.
When the load is not well matched, like an antenna with a VSWR of 1.5 or 2.0,
line length becomes critical since the length of line between a mismatched load
and the source transforms the impedance of the load as seen at the source. If
the adjustments for maximum power transfer were made with the Model 4314B
in place, removing it shortens the line by four inches plus two connectors. This
still is no cause for concern at low frequencies where four to five inches is a
small fraction of a wavelength. At higher frequencies; e.g., above 100 MHz, the
power output and frequency of the source may be affected.
To insure that the impedance is identical on either side of ½ wavelength in your
transmission line with the Model 4314B either in or out of the line, it is only nec-
essary to insert or remove a ½ wavelength.
Make up a length of cable which, when added to the Thruline Wattmeter, equals
½ wavelength at the frequency of measurement. If more than one frequency is
involved, then a separate cable is needed for each frequency. Use figure 8 to
determine the cable length required.
Note:
When using UHF Plug PL-259, the cable length is measured
from tip to tip of center pin of plugs.
Note:
When using other connectors, the cable length is measured
from the ends of the outer conductor
of the connectors.