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For each frequency which will not tune correctly you will need to determine whether
the tuning is high or low in frequency. Generally any frequencies which will not tune
will always be out the same way. When the antenna is made most frequencies are
deliberately made on the low frequency side and adjusted upwards by the
placement of "tuning rings". Tuning rings are single short circuit rings of 20 amp
fuse wire placed on the windings of an individual part of the antenna. A tuning ring
inductively raises the frequency of the section of antenna over which it is placed.
It must be understood that the tuning of an antenna on a particular vehicle or
installation may not hold for other vehicles or installations. To determine whether
any particular frequency tap is high or low hold the tune key down on the relevant
frequency and observe the VSWR on a suitable meter. Get an assistant to slowly
move his outstretched arm closer to the antenna tap in use.
If the VSWR gets better then the
antenna is too high in frequency.
This indicates that there is insufficient
antenna to ground capacity. Usually
this happens when the antenna is
mounted too far away from the body
of a vehicle. Either re-site the
antenna closer to the vehicle or
remove any tuning rings which are
already on the antenna.
If the VSWR gets worse when
following the above procedure then
too much capacity is already present,
this is frequently encountered when
mounting the antenna too low on a
vehicle bumper bar or when
mounting close to bodywork as in
cab-over type vehicles. In this case
either re-site the antenna further
away or add extra tuning rings to the frequency sections affected until a suitable
VSWR is obtained.
Note:-Truck cab-over installations usually produce distorted radiation patterns even
when the VSWR looks good.
When tuning is complete any new rings added should be coated with epoxy resin to
secure and protect the ring from damage. Five minute quick setting type epoxy is
suitable. If rings need to be removed they may be cut off using a sharp pair of side
cutters. Take care not to cut into the body of the antenna.
Note:- If the wander lead is damaged or lost and requires replacing, the number on
the first tap indicates the length of the replacement wander lead required.
Example:- Wl-60 indicates the length of the wander lead was 60cm. When making
a replacement wander lead ensure it is made to this length to obtain optimum
performance.
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