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Author Topic: 4 ft cb antenna to 2 meters  (Read 1946 times)

Posts: 2

« on: April 03, 2006, 11:18:49 PM »

  I am trying to find info on taking my old fiberglass  cb antennas that are wire wound and taking them down to 2 meters and or 6 meters. and still be 4 foot long!! I drive semi truck and 4 footers work well up on top of the mirrors 73 jim aj9j

Posts: 10248


« Reply #1 on: April 04, 2006, 07:00:04 AM »

I just have to ask why? The biggest problem with any antenna on a big rig is the lack of ground plane. Almost without exception, the bodies are made of fiberglass. Adding insult, the (widely used) mirror brackets are inadequate ground planes even for 440 MHz.

One way to counter this is to use two antennas in a dipole fashion, or perhaps a 1/2 wave with a little decoupling. Either one will work much better than a 1/4 wave utilizing the mirror bracket.

HF antennas suffer even more, and few truckers install them correctly. When I say correctly, I'm speaking of maximizing the available ground plane.

Alan, KØBG


Posts: 17483

« Reply #2 on: April 18, 2006, 02:18:54 PM »

Basically the antenna is a fiberglass rod with wire wound
around it.  The amount of wire/number of turns depends on
how long the antenna is relative to a wavelength, and how
the turns are distributed on the rod.  As a general rule,
the lower the frequency for a given length, the more turns
of wire you need.  Also, the higher up the rod you put the
turns, the more turns you need, but the higher the antenna
impedance is (which improves matching and efficiency.)

The problem is that on 6m a 4' antenna is very close to
a quarter wavelength, and on 2m it is longer than a quarter
wavelength, so it takes a bit of a differnt approach.

For 6m a quarter wavelength is around 54", so it very
little shortening is needed.  I'd suggest starting with
about 60" of wire distributed evenly along the rod and
check the SWR.  Shifting the turns towards the bottom
will lower the resonant frequency, towards the top will
raise it.  Or you can adjust the wire length as needed.
Probably one of the simplest approaches is to wind just
a few turns up the rod, then close-wind the remainder of
the wire near the tip.  Try this until you get the antenna
resonant a bit low in frequency, then remove turns for
final adjustment.

For 2m, a 48" whip is a 5/8 wavelength, and theoretically
could be matched with a base loading coil wound on the
fiberglass rod.  A starting point would be about 50nH
near the base (but not too close to the metal base, which
will act as a shorted turn) then a straight wire running
up the rod.  You'll have to experiment to find a combination
that works.  I would also suggest that you try out using
a straight 19" wire as a quarter wavelength - it would
be far easier to adjust, and will work nearly as well
(sometimes better) as the 5/8 wave antenna.  You can,
of course, keep the 4" fiberglass rod if you prefer, even
if the wire runs less than half way up.

One problem you may have is that the antenna mounts
and whip assemblies aren't designed for VHF antennas.
A mirror mount makes a less-than-ideal groundplane for
VHF antennas, and some mounts may add some reactance
or lead length that affects the antenna tuning (and may
make it difficult to get a low SWR.)  So you'll just have
to experiment and see how it works.
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