Railfan Technology > A question about magnetic antennas

Hello. I have a question. I just got a magnetic scanner antenna to hook up to the car so I can listen to railroad frequencies while I'm out on the road. But I'm concerned that it will fall off when I'm on a 70 MPH interstate highway. What should I do to prevent it from falling off?

Depends on the maker, the magnet in the mount, and the top of your car.  I have had a Larsen Kul-rod or some such for at least 25-30 years, and it hasn;t budged more than an inch or two in high storm winds and at extralegal speeds. The roof of a long-wheelbase Montero makes a great ground plane.

Only times I had mine knocked off was when I was on two-lane highways and a semi would blow past in the opposite direction.
Wasn't every time, but it was more than a few. I'd tend to slow down if possible and hug the right shoulder to get out of the opposing winds and turbulence.

I have had one for 30 or 40 years and never had a problem.  It has a rather strong magnet.  The only time I have a problem is when I back my truck into the garage and forget to remove it.  If the magnet is strong, you should never have a problem with it.

John

Best solution is to buy a quality magnet mount manufactured by a reputable company--Larsen, for example.  Typically, good magnet mounts have an NMO antenna connection, which is the most common mobile antenna thread.  Then one can buy a suitable NMO mobile antenna to screw onto the magnetic mount.  Yes, this costs more than buying a "complete" magnetic antenna, but it is typically those that are cheaply made with a weak magnet.  Also, by buying the antenna separately, one can buy a quality antenna that is tuned to the 160 mHz band.

Speaking about mobile antennas in general, I've used many over the years, but a real favorite of mine is the Sti-co Flexi-whip.  Its performance is about par with most any other good antenna, but where it shines is that it lives up to its name.  It's ultra-flexible and won't bend or break even under the most arduous conditions.  The Flexi-whip is a good choice for a magnet-mount antenna because it is much less likely to get ripped from the vehicle if it hits garage-doors, low-clearance parking garage ceilings, etc.  Its only downer is the price--the Flexi-whip is usually in the $45-$50 range for just the antenna.  Not cheap, but it has been worth every penny for me.

To improve the holding power of the mag mount (or achieve holding power on an aluminum of fiberglass body), put another mag mount on the opposite side of the panel. Those Neodymium magnets will also help.

Arved Grass
Fleming Island, FL

As others have said, spend a few bucks for the Larsen magnetic mount.  Make sure you cut the quarter wave to the proper length.  I've used one for over 10 years and never had a problem with it holding.  I had a professional install on the roof of my new Honda Pilot and use the Stico flex antenna so it will go in the garage without issues.  It works great.

When placing on the roof, put it in the center for the best ground plane.  This improves reception.

railfaneric Wrote:
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> But I'm concerned that it will fall off
> when I'm on a 70 MPH interstate highway. What
> should I do to prevent it from falling off?

Good suggestions above. I was driving "flow of traffic" in Arizona (75 MPH speed limit) while chasing a Z train, and the antenna, an off brand 1/4 wave cut to 160-162 MHz, held just fine. A flying branch on a windy day knocked it off one time, but the antenna stayed on the roof sideways as the combination of the magnet strength and cable held it in place before I could safely stop.

The antenna has caught a plastic bag one time, but it stayed in place.

In other words, whatever that might cause your antenna to fall off might be such a freak accident, that you probably won't be able to do anything about it.

I have knocked mine off on low hanging branches off road lots of times with cheaper antennas but never had one come off at speed, even well into tripple digits.
Lately, the bigger problem is getting the thing to come off when I want it to - last time I had to slip a plastic 3 inch putty knife under it to get it to release!

-JA

 

A friend of mine has a car with one of those huge plexiglas sun roofs.  He is trying to figure out how to mount his magnetic antenna base since there is not enough of a reasonably flat metal service to provide a really secure base.  Has anyone here successfully delt with that?  The idea of mounting another magnet under the plexiglas might be one possibility, although I might worry about scratching the plexiglass.

JBWX

Your friend actually  may have a two-fold problem.  First is the challenge of the roof having no metal surface for the magent to attach two.  Second is that the non-metallis roof also may not provide a good ground plane for the antenna--which can degrade reception.   I've seen people get around the problem by mounting the antenna on the hood or cowl of the vehicle just ahead of the windshield.  The metallic hood of the vehicle then acts as at lease a paritial ground plane for the antenna.  I had a mount like this on one of my personal vehicles sometime back--it was a 4WD vehicle that had to be frequently parked in low clearance parking garages.  The setup worked, but the antenna was somewhate directional--with the signals coming in somewhat stronger when they came across the hood area heading to the antenna. 

As I have also warned before--if you are permenantly mounting a mobile antenna on a modern vehicle, I strongly advise having a professional radio shop do it if you are planning to use the radio to transmit.  A typical amateur or commercial mobile radio will put out 25-50 watts of transmit power (some up to 100W).  That can adversely affect vehicle electronics if the antenna is mounted in a poor location.  As an example, I knew a fellow who installed his own antenna.  What he did not know was that an antenna mounted in the wrong place on his particular model of vehicle would activate the rear window wiper every time he transmitted on the radio.  This was a known issure to many professional radio intallers, who would not locate the antenna in that particular spot on that specific vehicle model.

i never had mine fall off the vehicle, and that was at 110mph for a few miles.. dont think you need t worry

Dave
In Pa

I've had one in the trunk of my car for years with good coverage - and it won't fall off!
Use cable ties to mount it.

Bill Deaver
Tehachapi, CA