Bird Technologies 4527 - Manual (Page 29)

17
Load Matching
When a Bird 43 is used to tune a load to a transmitter and a good match is
obtained, removing the unit will not change the match quality. A good 50 ohm
load can terminate a 50 ohm transmission line of any length without altering
conditions at the transmitter. The 43 is just an extra length of 50 ohm line in
series with the measurement.
When the load is not well matched (an antenna with a VSWR of 1.5 or 2.0) the
line length between the load and the transmitter will transform the load imped-
ance 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 fre-
quencies where five inches is a small fraction of a wavelength, but at higher fre-
quencies 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
wave-
length, the impedance is unchanged. To have identical match with the wattmeter
in or out of the circuit, insert or remove
1 ⁄
2
wavelength of line (including the unit).
To do this, use a length of cable which, when added to the unit, equals a
1 ⁄
2
wave-
length at the frequency of interest. If multiple frequencies are needed, a sepa-
rate cable length is required for each. See Figure 11 for sample cable lengths.
Note: Cable length is measured from end to end of the connector’s
outer conductor, except for UHF or mini-UHF plugs where the 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 poly-ethylene
(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.