Kenwood TS-900 Operating Manual page 7

The SWR bridge is a useful instrument, but unfortunately
it is quite often misunderstood, and overrated in importance.
Basically, the SWR bridge will indicate how closely the
antenna load impedance matches the transmission line.
With long transmission lines, such as will be used in many
fixed station installations, it is desirable to keep the im-
pedance match fairly close in order to limit power loss.
This is particularly true at the higher frequencies. The
longer the line, and the higher the frequency, the more
important SWR becomes. However, in mobile installations
the transmission line seldom exceeds 20 feet in length, and
a SWR of even 4 to 1 adds very little to power loss. The
only time SWR will indicate a low figure is when the anten-
na presents a load close to 50 ohms, but many mobile
antennas will have a base impedance as low as 15 or 20
ohms at their resonant frequency. In such a case, SWR
will indicate 3 or 4 to 1, and yet the system will be radiat-
ing efficiently.
The really important factor in your mobile antenna is that
it should be carefully tuned to resonance at the desired
frequency. The fallacy in using an SWR bridge lies in the
fact that it is sometimes possible to reduce the SWR reading
by detuning the antenna. Field strength may actually be
reduced in an effort to bring SWR down. Since field
strength is the primary goal, we recommend a field strength
meter for antenna tuning.
For antenna adjustments, the transceiver may be loaded
lightly, using the TUN position instead of operating at full
power output. This will limit tube dissipation during adjust-
ments, and will also help to reduce interference on the
frequency. In any case, do not leave the transmitter on
for very long at one time. Turn it on just long enough
to tune and load, and get a field strength reading. Start out
with the antenna whip at about the center of its adjust-
ment range. Set the VFO to the desired operating fre-
quency and then adjust the PLATE control for a dip, and
then the LOAD control. Then observe the field strength
reading. The Field Strength Meter may be set on top of
the dash, on the hood, or at an elevated location some
distance from the car.
Change the whip length a half inch, or so at a time, retune
the finals each time, and again check the field strength at
the antenna. Continue this procedure until the point of
maximum field strength is found. This adjustment will be
most critical on 75 meters, somewhat less critical on 40,
until on 10 meters the adjustment will be quite broad.
After tuning the antenna to resonance, the finals can be
loaded to full power.
MICROPHONE