Tuning the antenna (part 2)
Before you start.
Before you start tuning the antenna,
you should make sure that everything is set up correctly for the job. Poorly
setting up the system may lead to you cutting it too short.
How low?
Although lower is usually better, most antennas will not tune down to 1:1.
You should be able to get the SWR
down to 1.5:1 or better on your middle channel, although less than 2:1 should be
OK. The SWR at the top
and bottom channels will be higher than in the middle.
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* Connections: Poor connections and joins can cause problems.
* Mounting: On a car, low mountings such as a bumper mount can cause the
SWR to read higher (and
reduce performance).
For base systems, objects such as walls, trees, poles, etc can affect the SWR.
Where possible, the antenna should
be fitted well clear of such obstructions.
* Coax: Do not skimp on the coax. If you are limited by cost, buy a
cheaper radio and get good coax. Second-hand coax may look like a bargain,
but water could have got in or the coax could be damaged in some other way.
* Antenna: As with the coax, try to buy your antenna new. If limited by
money, by a cheaper radio and spend more on the antenna. Second-hand antennas may
look OK, but could have water or corrosion inside.
IMPORTANT!
Before cutting an antenna to size, be
sure to check that it is absolutely necessary, and then only cut small amounts
off at a time. You can always cut more off, but you cannot stick it back on.
Cut too much off?
Strictly speaking, the antenna is now,
to use a technical term, knackered.
However, there may be a cheap and dirty trick. Some people have been known
to use electrical connecting blocks to add pieces, but there are other
possibilities. Antenna lights and
capacity hats which connect to the tip become part of that antenna, so may reduce
the required length of the main section.
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STEP ONE - measure on lowest channel.
(a) Set the radio to the lowest channel. For a single band radio, this
is simply channel 1. For a dual band CB, also select the EU (CEPT) band.
(b) Set the FWD/REF switch to FWD.
(c) Set the radio to transmit.
(d) Adjust the CAL knob so that the meter reads at the end of the
scale, where you may see the word SET or CAL.
(e) Switch the
SWR
meter to REF or
SWR
(whichever name it uses).
(f) While transmitting, take the
SWR reading and make a note of it.
STEP TWO - measure on highest channel.
(a) Set the radio to the highest channel. For a single band radio,
this is simply channel 40. For a dual band CB, also select the
UK band.
(b) Follow steps 1(b) to 1(f) above.
STEP THREE - work out whether to lengthen or shorten
the antenna.
Where was the
SWR reading
lower?
Channel 1: Shorten the antenna.
Channel 40: Lengthen the antenna.
Repeat as required.
Steps One to Three should be repeated until the
SWR readings are about the same on the highest
and lowest channels. The SWR will usually be lowest on the middle channel.
