Bird Technologies APM-16 - Manual (Page 28)

16
Load Matching
When a Bird APM-16 is used to tune a load to a transmitter and a good match is
obtained, removing it will not change the match quality. A good 50 ohm load
can terminate a 50 ohm transmission line of any length without altering condi-
tions at the transmitter. The wattmeter is just an additional length of 50 ohm
line in series with the measurement.
When the load is not well matched, e.g. an antenna with a VSWR of 1.5 or 2.0,
the line length between the load and the transmitter will transform the load
impedance as seen at the transmitter. Removing the wattmeter shortens the
total line length by four inches plus two connectors. This is still not significant at
low frequencies where four to five inches is a small fraction of a wavelength, but
at higher frequencies the frequency or power output of the transmitter may be
affected.
Transmission line theory shows that if the line length changes by exactly
1 
2
wavelength, the impedance at the transmitter is unchanged. To have identical
match quality with the Bird APM-16 in or out of the circuit, it is necessary to
insert or remove
1 
2
wavelength worth of line (including the wattmeter). To do
this, use a length of cable which, when added to the wattmeter, equals a
1 
2
wavelength at the frequency of interest. If more than one frequency is involved,
a separate cable length is required for each. See Figure 10 for sample cable
lengths.
Note: Cable length shown (in inches) is measured from end to end of
the outer conductor of the connectors, except for UHF or mini-UHF
plugs where the cable length is measured from tip to tip of the center
pins.
Note: Dimensions shown are for solid polyethylene cable like
RG-58C/U or RG-8/U, which have a velocity of propagation 66% of
that of air. If RG-58 or RG-8 type cables containing foam polyethylene
(velocity of propagation of 79%) are used, the dimensions in the graph
must be multiplied by the ratio of the relative velocities; 79% ÷ 66% =
1.2 in this case.