Mobile Antennas

A few tips and tricks:

1. Don't use solid center conductor coax on a mobile antenna (especially on something that moves, like a trunk lid). Yes, it seems obvious, but you'd be surprised...
2. 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, unless you screw a 1/2 wave antenna onto it - something intended for a zero-ground-plane installation (like what would be on a motorcycle saddle bag or a Corvette rear deck). Larsen makes some good ones, as does Tram (the model 1155 is VHF, the 1175B is UHF and both work much better than any 1/4 wave on a mag mount).
3. 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.
   • One good alternative is to use the corner of a small triangular file to groove the rod at the point where you want to cut it and then use a carbide rod saw in a hacksaw frame to actually cut the rod.
   • 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 least get the wheels from HF.
   • Another method is to use the corner of a small triangular file to groove the rod at the point where you want to cut it. Groove the rod all the way around. Then use two pairs of Vise-Grip style slip-joint pliers (or a bench vise and a single pair of Vise-Grips) 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 wire at the point where you scored it. Use a grinding wheel (carefully!) or a coarse file to dress the sharp edge at the break.
   • 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 dip. You will need to use a strip of fine sandpaper to take off the black coating (about 3/8 of an inch) at the new end of the whip in order to insure that you get good contact with the antenna mount. Be careful to just sand off 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 the copper plating layer on top of the steel rod. Don't damage that plating with the sand paper. Just take off the plastic and then insert the whip back into the base and tighten the setscrew into the copper plating.
4. Another tip: The US one dollar bill is the same length as a 440-450 MHz whip antenna.
5. 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. Due to the thickness of the rod the left antenna presents a decent SWR 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.
6. And one more point: Let's say that you have a two meter 5/8 wave antenna with a standard flexible whip. The whip is a radiating element that is just about four feet (1.2 meters) long. At 60 MPH (95 KM/H) the wind bends back the top foot of that whip to almost horizontal. What kind of a pattern does that give you? I switched my 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%...

This web page split from the main page 12-Nov-2011.