Date: Fri, 14 May 1999 14:50:30 -0400
Reply-To: David Beierl <synergx@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <synergx@IBM.NET>
Subject: Re: CB antenna
In-Reply-To: <013201be9e33$11b444a0$6f6f6f6f@aptiva>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 10:55 5/14/99 -0700, Brent Christensen wrote:
>Here's another question for the list (I have a lot lately, since I am
>readying the Syncro for a long trip up the coast next month):
>
>I need to buy an antenna for my K40 CB radio; Any suggestions on placement
>and mounting techniques? (or choice of antenna for that matter). I know
>very little about CB radio technology, but I do have an SWR meter for
>testing purposes.
>
>Brent Christensen
>'89 Syncro Westy "Klaus"
>'95 Cherokee Sport
The biggest issue arises b/c the Westy has no steel roof except over the
front seats. Most antennas (unless designed for marine use) expect to be
located on top of a metal "ground plane" and will perform poorly if they
are not. There's also the question of height and of what you expect from
the radio. If you're using it to talk to local traffic on the road, almost
anything will probably serve all right. If you (for what reason I don't
know) want to seriously optimize performance, a trip to an expert or a copy
of the ARRL antenna book might be in order.
I use a Radio Shack *folding* gutter-mount antenna, located so that I can
reach out the window and fold it while driving. (Unfortunately they don't
make it any more, but I imagine that Antenna Specialist or someone else
does.) For mechanical stability I cut a section out of the gutter
edge-trim, and eventually after repeated problems with grounding I soldered
a braid strap from the antenna to the gutter. I've also found that with
this particular antenna the loading coil contacts deteriorate
eventually. The loading coil is in the middle of this antenna, and is made
with cast fittings attached by setscrew to the upper and lower shaft, then
screwed bodily into the plastic loading-coil housing. They have O-ring
seals and make a pressure connection to the ends of the loading
coil. Occasionally (when the SWR goes nuts) I have to open this up and
scrape the connections a little. I've been extremely satisfied with this
rig; it's convenient, fairly unobtrusive, lets me get into garages and
such, and works quite well. Because of the distribution of the roof vs the
antenna, it probably radiates most strongly to the right-front of the
vehicle. But it works well in practice and in general I find that my comms
are limited by electrical noise (atmospherics, internal van noise, and the
dull background roar from 100,000 CBs on channel 19). In my experience
with a four-watt (output) rig you can talk pretty well with other vehicles
a mile away or more if there is a reasonable line of sight. A figure often
quoted is "a mile a watt" but that relates more to base-to-mobile comms
than mobile-to-mobile. Atmospherics are often very strong, requiring a
very high squelch setting; and Ch 19 is often much noisier than other
adjacent channels b/c of all the road warriors using it.
I also have a scanner antenna with a magnet-mount on the front slope
(passenger side). I bent the shaft just under the first trap so that the
antenna is vertical after the first 8" or so. Not ideal electrically, but
it doesn't poke out pedestrians' eyes. It stays on pretty well unless
whipped by a branch or such. I'd be less inclined to mount a transmitting
antenna there b/c the roof does give some protection from RF, but OTOH you
aren't going to be talking all that much and anyway the passenger will take
more of the heat <g>. Any magnet mount, good idea to run the cable such
that the antenna will not drag on the ground if it comes off (all my
antennas are ground to points on the ends, oh well). Especially make sure
it can't get under a wheel, that experience can be a bit exciting.
Big whips with bumper mounts or rear-quarter-panel mounts are common on
full-size vans. They suffer b/c of all the nearby metal as the antenna
rises up the side, but prob. work well overall as long as you let them
stand up. The plastic roof could be a problem here.
For *best* performance, build a ground plane on the roof (see the ARRL
antenna book) and mount a 102-inch whip in the geometric center of the
roof, a little aft if you want better coverage ahead than behind. But give
up the idea of ever driving under a structure, and learn how to dodge
traffic lights unless you want to be *very* unpopular with the local law.
For even *better* performance mount a beam and a rotor on top of your
ground-plane -- but now we're getting silly. Not that it hasn't been done,
cf the DF crowd and radio "fox-hunting."
Now if you want serious communicating distance, look into getting a ham
license and using a 2-meter rig. They work at 144 Mhz and are essentially
line-of-sight (+10%) but there are repeaters on practically every hill in
Christendom so that fifty miles is probably not uncommon range. In my
experience with marine VHF using 25-watt (output) rigs and no repeaters,
fifty miles is often attainable when talking to Coast Guard or other
outfits with antennas 150 feet up (or whatever it is).
david
David Beierl - dbeierl@ibm.net
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