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Some people believe that mag mount magnets lose their effectiveness over time, eventually causing a dangerous situation as the antenna could rip loose from the vehicle and become a projectile or distraction at an unfortunate time -- e.g. on a highway due to air forces or perhaps at a stoplight under heavy braking.

Mitigating these concerns are the fact that the antenna is still attached by coax and that I personally have never seen it mentioned in the news nor have I heard of any traffic laws prohibiting mag mount antennas on a vehicle.

Still, with safety in mind, I use the screw in trunk lip mounts.

Should I keep telling my friends to stay away from the mag mounts or are these supposed dangers merely paranoia and urban legend?

Paul
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  • I can't really answer this, but I suspect not. They use a very strong magnet, and as you said, even if the magnet lets go (IMHO unlikely) it is still attached by the coax. When I was using a mag mount I had a cloth between it and the car for a while to prevent scratches to the paint. Not the best idea, I know. Even like that it never fell over at highway speeds. – Rod MacPherson Oct 30 '13 at 04:29
  • The cloth would make it lose it's effectiveness as an antenna, the bottom of mag mounts needs to be connected to a conductor, so... – PearsonArtPhoto Oct 30 '13 at 10:07
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    No, that's not quite true. Car paint isn't very conductive, but mag mounts still work. Most mag mounts actually have a plastic or rubber bottom to reduce scratches. The ground connection between the mount and car surface is **capacitive**. There's some ground loss there, of course, and it gets worse if you increase the distance by adding stuff in between. Performance improvements by adding a better ground connection (wire to car chassis) have been reported, which is not surprising. – oh7lzb Oct 30 '13 at 10:22
  • http://www.cosjwt.com/index.php?a=3 has some math regarding this. Ground loss goes down when you go to higher frequencies, and up when you go lower. – oh7lzb Oct 30 '13 at 10:48
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    Another consideration is what happens to the antenna in an accident. The high G forces could fling the antenna off in the direction of travel. If the coax holds, all is good, it won't go anywhere. If not however, it's a projectile for which YOU are liable regardless of who is at fault for the collision. – WPrecht Oct 30 '13 at 13:50
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    Liabilities vary by jurisdiction, I wouldn't make strong statements in an international forum. Over here I'd guess whoever is at fault pays if the accident causes parts of other cars to come off and cause further damage. After all, hams are not the only ones attaching stuff to their cars (think ski boxes, mag-mount yellow alert lights, whatever you tie on top of a trailer). – oh7lzb Oct 30 '13 at 13:54
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    However, in most cases if **you** attach something to your car that is obviously not going to hold up to an accident, then you are just as liable as Ford would be if their vehicles had a tendency to launch projectiles any time they were involved in an accident. In modifying your vehicle, if you fail to anticipate and account for the fact that you may be involved in an accident, you assume liability for the consequences. – Dan KD2EE Oct 30 '13 at 14:50
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    I will say that my non-mag-mounted antenna fell off once because it lost a screw. – Bill - K5WL Oct 31 '13 at 15:28

4 Answers4

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Magnet mounts can come off, and they do fall occasionally. I can't quote any statistics, but it has happened to me twice in the past - tall HF antenna combined with too small magnet mount and high speed. Luckily the cable held the thing on the roof. Short VHF/UHF sticks (think 1/4 wave for 2 meters) would never fall with the same mount - the wind load is just so much smaller. For my irregular HF use, I've switched to a larger magnet mount and thinner vertical, and the constantly used VHF/UHF sticks are drilled-through.

There are also stories of shattered glass (antenna comes off and hits your own back window) and scratches on roof.

If the antenna fell down and stopped in the middle of a highway, it would be dangerous to a motorcycle, but less of a hazard to a car. It might break a tire, which in turn could lead into a more serious accident. A thick HF antenna could puncture a car somehow. Large magnet mounts are heavy, and one could bounce higher into air and penetrate a wind shield. In an accident it would probably get loose and fly away, but it would have the same velocity as the much heavier car, and the damage caused by the car would probably be dominant.

If you loose one of these on a road, do go back and pick it up immediately. If you loose one on a highway, in many places it's both dangerous and illegal to stop and go pick it up yourself, so you might have to call the police for assistance.

Wikipedia has some information on how magnets can be demagnetized. Permanent magnets are made from "hard" ferromagnetic material which are very hard to demagnetize. Magnets on the roof are unlikely to get hot enough to degauss (140 °C / 280 °F for neodymium, 300 °C / 570 °F for ferrite and SmCo), and they're not subject to strong external magnetic fields, either. Hammering or jarring - some vibration is going to be present, and after a very long time that might have some effect, but that's just my guess.

Typically a mag mount will be decommissioned quick enough that demagnetizing should not be a concern. Water tends to get into the connectors and cables, and the cables break from stress under doors.

Magnet mounts are very popular. I don't remember any documented serious accidents with them. If accidents would happen, there would be articles on the amateur magazines, blog posts, and pictures on the Internet. I would claim they're pretty safe, if the relation between antenna size, mag mount strength and vehicle velocity are in a good balance.

oh7lzb
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  • It's also pretty easy to test how well the magmount is holding on to the car roof: simply try to take it off. Of course that isn't perfect, but I'd say it gives a pretty good *indication* of whether there's any real risk of issues. That said, for HF, I'd probably prefer a rear bumper mount, but that's also because it gives on the order of one more meter antenna height with the same total height (given that vertical clearance is an important factor for tall car installations). – user Oct 30 '13 at 10:34
  • This answer is informative and I'll accept it until someone can find a news report or similar evidence showing a man mount was the cause of an accident. – Paul Oct 30 '13 at 21:58
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They can absolutely come off. Here is a forum thread with numerous posters reporting antennas falling off from the wind of passing a semi - it's a storm chaser forum, but the storms weren't even mentioned as a factor in many of these incidents. Here, several QRZ posters advise that mag mounts may fall off, and this poster reassures us that having the right size magnet is key: his has only fallen of "a couple times"!

It's not a function of not having a big enough magnet, it's the fact that even the best mag mount on the market will be the "weakest link" in your vehicle, and the first thing to give in to wind. Even when it's not falling off, it can move around and scuff your paint, especially if you move it between vehicles on a regular basis. In many ways, an NMO mount is less damaging to a vehicle than a magnet mount - the damage is confined to a very small area compared to a 3-5" diameter magnet, the hole can be somewhat easily patched or even more easily plugged, and there's no damage to the surrounding paint.

Dan KD2EE
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  • Looking at the forum, two antennas that came off were very large, a Comet SB15 (58") and a Comet SBB7 (55"). Comet makes two antenna mounts. One is recommended for antennas 45" or less and the other for antennas 50" and less. So these antennas were probably too big for a mag mount. I have a Comet SBB5NMO (38") with a Larson mag mount and have had no problems. – Walter Underwood K6WRU Nov 05 '13 at 16:15
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Check with the antenna manufacturer. They frequently have mounting recommendations. For example, the following is straight from Diamond Antenna's website.

Use of magnet mounts is not recommended with tall antennas, i.e SG7900A, SG7900ANMO, CR627B, CR627BNMO & HV7A, NR22L, NR7900A.

Source as of 30 October 2013: http://www.diamondantenna.net/Product_Catalog/techno.html

N3QEH
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  • I used the SG7900 some years back. When I called RF parts and asked about a mount, they said "we don't recommend that antenna for mobile use". I was tempted to ask what they DID recommend it for... In the end I used an NMO mount, but put a ryobi weedeater blade under the roof to stiffen up the mount. Worked great, went through a lot of miles and heavy storms (nine years very active in Skywarn) and outlasted the car. Fortunately the 7900 will fold over. As to whether a bad mag mount has EVER caused an accident, probably so though I doubt you'd find documentation unless it was unusual. – user103218 Dec 19 '19 at 15:12
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I had a mag mount attached to the fender of my van (fiberglass roof and hood) and only about 75% of the base was able to fit on the top of the fender. Well when I got on the highway, up to about 60 mph and hit a bump, off went the antenna, hanging about 2 feet off the van, guy next to me beeped and braked, I pulled right over and drug it back in the window. No accident, but learned my lesson. That being said I have the same mag mount antenna on top of my Jeep and it stays on through the bumps and craters of I-95 at 80mph. So I'd say, from experience, if 100% of the base is attached to metal, you should be good. That being said, there's an exception for every rule...

Justin Johnson
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