A few tips and tricks:
- Solid center conductor coax is a bad idea on a mobile antenna (especially on something that
moves, like a trunk lid). Yes, it seems obvious, but you'd be surprised...
- Punch-mounts are 20 times better than clamp-on-the-edge-of-the-trunk-lid mounts and 50
times better than magnetic mounts. I've yet to see a mag mount that supplies a decent ground
plane. HOWEVER - mag mounts are better than nothing - and are much better if you screw
a 1/2 wave antenna onto it - something intended for a zero-ground-plane installation (like what
would be on a fiberglass ambulance roof or a motorcycle saddle bag or a Corvette rear deck).
There are good oes out there, I've used Larsen and a friend loves Tram (the model 1155 is VHF,
the 1175B is UHF).
- A couple of tips about cutting the rods on mobile antennas.
- Quality mobile antennas use hard stainless steel rod for the elements. You will have a
difficult time cutting the stainless with common wire cutters made for copper wire because
the rod is as hard as the tool (if not harder). If you do succeed in cutting it you will
probably damage the jaws... or the hinge, or both. As a practical alternative the
Ridgid S14 (the smallest size) of bolt cutter will work just fine, then a flat file
to remove any sharp burr.
- One good alternative is to use the corner of a triangular file to groove the rod at the point
where you want to cut it and then:
a) Use a carbide rod saw in a hacksaw frame to actually cut the rod.
b) Groove the rod all the way around about 1/3 of the way through. Then use two pairs of
Vise-Grip pliers (or a single pair of Vise-Grips and a bench vise) with one set of jaws on
one side of the cut and the other set of jaws on the other side of the cut.
You should then be able to bend and snap the rod at the point where you scored it.
Finish the job with a coarse file or a grinding wheel (carefully!) to dress the sharp edge at the break.
- Another alternative is a small power tool (like a Dremel) with a small abrasive cut-off
wheel. Harbor Freight has a chinese clone of a Dremel at a decent price (and they also sell
all of the tools - like the cutoff wheels- for them). If you have a real Dremel you can at
save a few dollars with the HF cutoff wheels.
- If you purchase a black Larsen whip please keep in mind that the black coating they use
is a soft plastic applied by dipping the rod in the liquid color dip. CAREFULLY cut the rod
using the technique above AND be careful to not damage the copper plating under the plastic.
Once you have cut the rod take off about 3/8 of an inch of the black coating in order to insure
that you get good contact with the antenna mount.
Be careful to remove just the black plastic coating and no more (this is the voice of
experience talking!). You will find that just under that plastic coating is a thin copper
plating layer on top of the steel rod. The copper is a much better conductor than the steel
and is the primary radiator! Just take off the plastic and then insert the whip back into the
base and tighten the setscrew into the copper plating.
- Another tip: The US dollar bill is the same length as a 440-450 MHz whip antenna.
- Glass mount antennas were an interesting idea, and somewhat worked, but no longer
work today. All of the car and truck manufacturers now use a metalized glass to cut
down the solar heating inside the vehicle interiors. That metalization does a good job
of shielding and that kills the RF. Operating a HT from inside your car used
to be about -3db to -6db in the plain glass days, now it's like -10db worse.
- Yet another tip: Look at this photo. Notice
anything unusual? Like the thickness of the antenna rod? It's not too well known but the
diameter of the radiating element has a distinct effect on the bandwidth of the antenna.
When you make the element fatter it decreases the resonant frequency, meaning that you
end up making it shorter to compensate. The end result is that it is little more
broadbanded. Due to the thickness of the rod the left antenna presents a SWR that is less
than 2.5 to 1 from 442 MHz up through 467 MHz (i.e. from the repeater section of the UHF
ham band all the way up through GMRS)... and by the way it's a home-built antenna. The
right hand antenna in the photo is a stock 460 MHz whip (i.e. GMRS) from Larsen.
- And one more point: Let's say that you have a two meter 5/8 wave antenna with a
standard flexible whip. That's about four feet (1.2 meters) of radiating
element. At 60 MPH (95 KM/H) the wind bends back the top foot of that whip to almost
horizontal while the bottom foot is vertical. What kind of a pattern does that give you?
I switched my 2m mobile radio to a center-loaded dual band whip with a fairly stiff rod
about a year ago. My signal flutter was reduced by over 50%...
- Check out the The KØBG web site link on the previous page. You can spend hours
there learning about mobile installs, including antennas.
- Mobile disguise antennas are a compromise. One trick that a local police department
did for the undercover cars was to mount the license plate on thicker-than-normal plastic
mounts, and feed it as the raidiating element with the car body as the ground plane.
The UHF Micor was the radio of choice as they had a circulator as part of the transmitter
and that prevented the horrible SWR from destroying the final transistors.
Another trick is to rebuild the existing vertical rod car radio antenna to be a mobile radio
antenna. A company called Sti-Co Industries turned that idea into
a business and is still in operation today in New York at 716-662-2680 or at
sti-co.com. They call them "covert antennas".
A diplexer (UHF or VHF and FM broadcast) or triplexer
(UHF or VHF / FM / AM) allows simultaneous operation
of the 2-way radio and the broadcast radio.
Cars that don't have a vertical rod antenna can use a horizontal dipole inside the front
or rear vinyl bumper shell but the soild metal bumper that is a few inches away on the
other side of the foam block causes SWR and directional problems.
Another trick is to position a whip antenna inside the radiator grille - but some cars
don't have a lot of room there, plus it can be very directional becasue the air conditioning
condenser acts like a reflector.
Contact Information:
The author can be contacted at: his-callsign // at // repeater-builder // dot // com.
The main web page was created and copyright 1995. This section was split from the main page 12-Nov-2011.
Layout and hand coded HTML © Copyright 1995 and date of last update by Mike Morris WA6ILQ
This web page, this web site, the information presented in and on its pages and
in these modifications and conversions is © Copyrighted 1995 and (date of
last update) by Kevin Custer W3KKC and multiple originating authors. All Rights
Reserved, including that of paper and web publication elsewhere.
