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Author Topic: Magnet Mounts....Just how safe are they????  (Read 14994 times)
K3NRX
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« on: November 13, 2011, 02:09:54 PM »


....when using a Workman Hamstick that is pretty long???....I was on the highway today and the damn thing blew off while doing about 60-65 mph.....fortuneately no one was behind me......had to pull over and dismantle the whole thing and throw it in the trunk....I am now hesitant to use this operation at relatively high speeds, and it's quite unnervng to me.....anyone have any ideas as to how I can make this safer and still use the magnet mount???.....Should I get a trunk lip mount or something for a hamstick this long???.....

V
KA3NRX

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K0BG
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« Reply #1 on: November 13, 2011, 02:38:59 PM »

Ask 100 people who have them, and half will say they've come off, the other half say will say no. And, it doesn't make too much difference how many magnets they have. They just get heavier, and do more damage when they do come off.

According to the NHTSA, there has been one fatality, and less than 50 reported injuries caused by flying mag mounts. Note the work reported! I have a friend over in Texas, who has lost one 2 meter mag mount, and two coils off his old Hustler HF antenna, and he doesn't know when they came off. There were there when he left, but not there when he got to where he was going. Makes you wonder just how often this happens.
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K3GM
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« Reply #2 on: November 13, 2011, 06:31:31 PM »

A couple of years ago, I put a temporary 6m Hamstick on the roof of my new Tahoe to work some weak signal springtime Es on while I was doing some traveling.  My two permanent NMO roof mounts were both filled with FM antennas, so I put the Hamstick on a Larsen mag mount.  The damn thing blew off the roof of the Tahoe at 70mph, startled the crap out of me, and ended up stuck to the side of the rear quarter panel poking out sideways into the adjacent lane of the New Jersey Turnpike. I got the EL58 I was looking for, but ended up with a nice scuff on the side of the  truck for my effort. No more mag mounts.....
« Last Edit: November 13, 2011, 06:47:06 PM by K3GM » Logged
K5LXP
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« Reply #3 on: November 13, 2011, 06:55:41 PM »

Yeah, unless you happen to be working the rig you may not notice it when it departs.  I lost one off my service van once, when I got there all I had was a frayed coax dragging behind.  Odds are they'll just bounce and skid on the road for a ways and that's the end of it, but you always wonder. 

I use a mag mount for my portable APRS and have personally witnessed it blow off from the gust of a truck going the other way on the highway.  Fortunately it was on a "short leash" of coax coming out of the trunk and it didn't get far, but it showed me that even a 2M quarter wave is enough profile to get plucked off in the right circumstances.

Mark K5LXP
Albuquerque, NM
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KD5TXX
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« Reply #4 on: November 13, 2011, 07:20:55 PM »

I have used Mag mounts (5" type) for 10 years and never lost one.  And I drive 50-60K a year.  With a small 3" mag, I have had them fall.  I now only use a 5" mag mount and use 40m-10m whips.  I use the mini ones, so that may be why they don't fall.
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W8JX
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« Reply #5 on: November 13, 2011, 07:40:21 PM »

Ask 100 people who have them, and half will say they've come off, the other half say will say no. And, it doesn't make too much difference how many magnets they have. They just get heavier, and do more damage when they do come off.

Simply not true. Yes as stated above with proper magnet size there will be no problem. In 25 plus years of using them I have only lost one and that was doing 70 mph west bound up Bozeman pass with 60+mph head winds. (a 4 inch one) The only weakness of one is size of magnet for antenna in use. Some vendors cut corners here to cut costs.
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K1CJS
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« Reply #6 on: November 14, 2011, 05:38:36 AM »

It really doesn't make much difference what kind of magmount you have, a single magnet base, a three magnet base, or what guarantees there are about that particular magmount.  Magmounts should not be used at highway speeds, period.  And especially not for heavier antennas, no matter how much the manufacturer says that they'll hold up.  If you don't have the thing fly off and cause damage elsewhere, you're going to have damage done to your vehicle.

A friend used a three magnet base magmount antenna--because the guarantee was that it would not come off.  Well, it didn't come off--but it did leave some beautiful scrapes along the roof of his SUV as it slid backwards during a long road trip.

Use what you want to use, but just remember, you'll pay one way or the other--either for a good, quality mount or for the damages that a cheap mount will cause.
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W8JX
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« Reply #7 on: November 14, 2011, 05:56:53 AM »

It really doesn't make much difference what kind of magmount you have, a single magnet base, a three magnet base, or what guarantees there are about that particular magmount.  Magmounts should not be used at highway speeds, period.  And especially not for heavier antennas, no matter how much the manufacturer says that they'll hold up.  If you don't have the thing fly off and cause damage elsewhere, you're going to have damage done to your vehicle.

Again simply not true. With PROPERLY side magnet it will not come off at highway speed. Tee only thing I will use a 3 inch mag for is a 1/4 wave 2m vertical. It has NEVER failed at this is 10 of thousands of miles. For a 5/8 wave 2 meter I use a 4 inch and it has never failed except for bozemen pass once which was my fault not mounts. (place is well know for wind gusts well exceeding 100mph often with frontal passages) Package deals that include a antenna rarely have a big enough magnet to securely hold it. I always get magnet separately to insure it is properly sized.  I have held a 8 foot hamstick on roof of a suburban at highways speed many times using  custom mag mount with two 5 inch magnets.

Again simply put, use correctly sized magnet and you will have no problems

A friend used a three magnet base magmount antenna--because the guarantee was that it would not come off.  Well, it didn't come off--but it did leave some beautiful scrapes along the roof of his SUV as it slid backwards during a long road trip.

Some guarantees are not worth paper they are written on. If mag was sized properly, it would not have walked down roof. Once again it is ALL about the size of the magnet vs antenna in use.
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K0BG
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« Reply #8 on: November 14, 2011, 06:05:06 AM »

I just love how positively certain you are John. The reason you're so adamant is because you use one, and never had a problem. Consider yourself lucky, and let's hope you didn't speak to soon!

As I alluded to, there is a lot about mag mounts to love, and hate. But ignoring one, in favor of another isn't the tact to take. Just ask Joe Paterno.
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W8JX
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« Reply #9 on: November 14, 2011, 07:23:52 AM »

I just love how positively certain you are John. The reason you're so adamant is because you use one, and never had a problem. Consider yourself lucky, and let's hope you didn't speak to soon!


Alan I speak from many many years of experience with this. A mag mount is only as good as the magnet it uses. There is flux density (pull/holding power per square inch) and its diameter. The two play a critical role is countering the leverage applied to magnetic base but antenna load for momentum and wind loading. The large diameter magnet not only increases raw holding power (ie more square inches of surface area) but it also increases the amount of "leverage" needed to unseat it due to not only increased holding power but also due to extended contact radius relative to antenna center. Actually Alan even if you took two magnet mounts made of same material and made one 5 inches and other 4 inches outer diameter but deliberately kept square surface area the same by making center hole larger.  The 5in one will have same dead weight lifting/holding power but will better resist be unseated by torque loads on mount placed there by antenna due to increased radius relative to antenna center.

I do not consider myself "lucky" at all but rather it is because of knowledge and simple logic that I have had no problems. I worked in flight test R&D for many years and learn that everything that fails does so for a reason and you modify design to get desired results. In this case bigger magnets will work when PROPERLY sized for antenna in use. Package deals rarely have correct mag size as they are looking to make max possible profit and minimize shipping costs in large volumes by keeping weight light

Now if you cannot wrap you head around this concept then it is you relying on luck not me.
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K6JRS
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« Reply #10 on: November 14, 2011, 09:08:07 AM »

I guess I've been one of the lucky ones. I recently installed a permanent NMO mount for VHF/UHF operations. Before that I put over 250,000 miles, well over 50% at freeway speeds, on a 3-4" mag mount. I think ti was a Diamond mount because I was using one of their longer VHF/UHF whips. This mag mount was used on two different vehicles over it's lifetime.

I should buy a lottery ticket...

73,
Jay  K6JRS
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M6GOM
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« Reply #11 on: November 14, 2011, 09:20:01 AM »

I have never used anything smaller than a 5" magmount so have never had anything fall off. I used a 5" magmount with CB antennas without incidence for nearly 20 years, covering in excess of 100,000 miles per year with several years in excess of 150,000 miles a year in all conditions. I used a Little Tarheel II with 60" whip with a triple magmount and did 80+MPH into headwinds again with no incident.

Nowadays I drill holes but sometimes when vehicle hopping, I'll still use a 5" magmount with a dualbander. I wouldn't use a hamstick with a single magmount and if I were to ever use a 3" mag, I'd not want anything more than a 1/4 wave 2m whip on it.
« Last Edit: November 14, 2011, 09:22:26 AM by M6GOM » Logged
W2RI
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« Reply #12 on: November 14, 2011, 10:07:24 AM »

They can and do come off. When I first started mobiling, I had a triple magnet mount for a 20M hamstick. The thing came off on a parkway; I couldn't have been doing more than 70mph. Now I won't use one unless it's bolted down.
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W8JX
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« Reply #13 on: November 14, 2011, 04:58:53 PM »

They can and do come off. When I first started mobiling, I had a triple magnet mount for a 20M hamstick. The thing came off on a parkway; I couldn't have been doing more than 70mph. Now I won't use one unless it's bolted down.

There is no way a triple 5 inch magnet should come loose unless surface was either not flat or metal was so thin that it oil canned and flexed under load and broke contact area. I have a custom made mount with two 5 inch ones that has held a 20 meter ham stick at 70+ and has also held a full sized hustler 2m collinear under same conditions. (once with 20m stick I went under a lower overhang and took out some lights with it by accident and antenna did not come loose)  It is on a nice flat part of a sturdy roof with stamped stiffeners in it too. You can not place magnets on a weak or curved roof and expect it to hold under high torque load.







« Last Edit: November 14, 2011, 05:00:53 PM by W8JX » Logged

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WB2WIK
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« Reply #14 on: November 14, 2011, 06:10:04 PM »

I just think of why "mag mounts" were invented, which is for temporary operations.

I've also measured performance of many mag mounts over the years, and found that on UHF they're actually pretty good.  On VHF, a bit less good.  On HF, not very good at all; they're just not sufficient capcacitance through the magnets and paint to create any kind of low loss system for grounding.  40-50 pF won't cut it.  Frankly, 1000 pF won't do it.  10,000 pF would probably be okay for a 50 Ohm antenna, but you can't get that with 1-2-3-4 magnets, it's just not going to happen.   Impossible.

A friend of mine in Florida sent me a "Firestik" mag mount he was using.  I measured it.  Its efficiency was horrendous, but then I made some more measurements to find out why: They were relying on the magnet to be a conductor.  It wasn't.  The resistance from the coaxial feedline outer conductor to the magnet was thousands of Ohms, since magnets are not good conductors.  So, in all cases, it was half an antenna.

I modified it to add copper foil from the feedline to below the magnet, and just epoxied that in place.  Efficiency increased from 9% to about 50% with this simple modification which took ten seconds.



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