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Author Topic: Magnet Mounts....Just how safe are they????  (Read 15005 times)
KC7YRA
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Posts: 256




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« Reply #15 on: November 14, 2011, 06:26:11 PM »

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 had mixed results.  I once traveled many thousands of miles 1 summer with my folks.  As a way to shut me up, the let me stick a 20 meter hamstick to the roof with a 3 magnet mount.  It worked fine.

But several years later, I had it on my truck along with several other magmounts (5).  Doing 80mph down the interstate into a significant headwind (as we ALWAYS have here in Wyoming), A tractor/trailer passed me and blew every single magmount off the roof.  I mean EVERY.  I also had it get to oscillating while offroading and tip over a number of times.

So I am 50/50.  I have had extremely mixed results.  I now drill all my antennas.  But at the same time, I'd use a magnet mount for temporary stuff.

Brad 
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W8JX
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« Reply #16 on: November 14, 2011, 09:09:02 PM »

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.

It would be interesting to actually measure capacitance.  I have had good results with double mag on roof.


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.

I think this a stretch as most mobile HF antennas are basically radiating dummy loads. (negative gain antennas)
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KD0PBO
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« Reply #17 on: November 20, 2011, 12:40:29 PM »

I would drill holes for my antennas but seeing that i mostly operate while offroading i'd rather have the magmounts get knock off and leave some scratches than have a NMO mount rip a hole in the roof of my truck...just my $.02 if its even worth that

Miles
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W8JX
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« Reply #18 on: November 20, 2011, 03:35:53 PM »

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 had mixed results.  I once traveled many thousands of miles 1 summer with my folks.  As a way to shut me up, the let me stick a 20 meter hamstick to the roof with a 3 magnet mount.  It worked fine.

But several years later, I had it on my truck along with several other magmounts (5).  Doing 80mph down the interstate into a significant headwind (as we ALWAYS have here in Wyoming), A tractor/trailer passed me and blew every single magmount off the roof.  I mean EVERY.  I also had it get to oscillating while offroading and tip over a number of times.

So I am 50/50.  I have had extremely mixed results.  I now drill all my antennas.  But at the same time, I'd use a magnet mount for temporary stuff.

Brad 

Problem with drilling is that sheet metal is thinner than ever and a hole mount does not provide holding power it once did. Any wind load heavy enough to unseat a 3 mag mount with 5 inch magnets will put a LOT of strain on hole mount and bend metal and likely pull out over time unless hole is gusseted. Most car makers use as thin of metal as possible to limit cots and maximize profit.
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KC7YRA
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« Reply #19 on: November 20, 2011, 04:55:11 PM »

Well, I managed to put a 5 inch HI-q screwdriver, totaling 14 feet tall, on my truck with a single mounting point.  And it stays on at 90mph.

And this is on a Tundra with the thinnest sheet metal In the industry.

Brad
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W8JX
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« Reply #20 on: November 20, 2011, 05:07:05 PM »

Well, I managed to put a 5 inch HI-q screwdriver, totaling 14 feet tall, on my truck with a single mounting point.  And it stays on at 90mph.

And this is on a Tundra with the thinnest sheet metal In the industry.

Brad

I do not think Toyota has thinnest metal in a truck. That honor likely goes to Detroit.
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K0BG
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« Reply #21 on: November 21, 2011, 07:48:16 AM »

The holding power of a mag amount is also based on the thickness of the material it is stuck to. And by all accounts, it is getting thinner as automobile manufacturers strive to meet the new fuel economy standards. Adding some insult, Audi, and MB both use lamented "sheet metal", that is actually aluminum or steel sandwiched together with rubber. A mag mount won't stick even on the steel variety. Oh! And how about the 2012 Ford Mustang? Composite hood, trunk lid, fenders, and lower body. That leaves the top, and it's 18 ga.


PS: For you Ford lovers, the 2012 Cobra Super Snake has 750 HP, and a top end of over 200 mph. It goes to 0 to 100 faster than a Vette can get to 60!
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AC4RD
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« Reply #22 on: November 22, 2011, 04:18:14 AM »

The holding power of a mag amount is also based on the thickness of the material it is stuck to. And by all accounts, it is getting thinner as automobile manufacturers strive to meet the new fuel economy standards. Adding some insult, Audi, and MB both use lamented "sheet metal", that is actually aluminum or steel sandwiched together with rubber.

Yesterday on the highway, I saw one of those big crew-cab pickups with the dual rear wheels, enormous thing ... and the big protruding rear fender was cracked and broken, with pieces missing.  That fender, to my surprise, was *plastic* of some kind.   :-(
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W8JX
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« Reply #23 on: November 23, 2011, 09:35:23 AM »

Yesterday on the highway, I saw one of those big crew-cab pickups with the dual rear wheels, enormous thing ... and the big protruding rear fender was cracked and broken, with pieces missing.  That fender, to my surprise, was *plastic* of some kind.   :-(

No surprise here. It obviously must cost less than metal and anything to save a buck on build costs. Personally I have always though these dually pick ups a bit silly.  Unlike really dual wheel trucks that narrow track of inner tires to limit rear axle width, with dual pick ups they just basically slap another wheel on it with a plastic fender and let it all hang out. Cheaper to do that way.
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AA4HA
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« Reply #24 on: November 24, 2011, 01:18:03 PM »

I have three antennas mounted on an MB. I did not drill for any of the antennas nor did I use magnetic mounts. They are lip mount antennas at two corners of the rear trunk lid (by the tail lights) and the third in the center (right in front of that auxiliary brake light. I am changing out two of the antennas for a quad band (10,6,2 and 70 cm) in the prime position and a shortie 1/4 wave whip for APRS at one of the trunk corners.

The car is a nightmare to put antennas on as it has them hidden all over the bodywork, front and back deckwork (dashboard) and under the bumper covers (cell antennas and TELEAID antennas hidden under there). I do not want to drill holes and my big sacrifice was to find an open weatherproof grommet to get from the battery lid to where the FT-8900 is screwed down to the floor under the front seat. They thoughtfully put an air conditioning vent beneath the driver and passenger seats so the rear seat passengers do not get sweaty toes.

I bought a few spray cans of onyx black paint so the antenna bases are all painted the same color as the car. I have not encountered any alloyed metal on the car that would be paramagnetic. Either it has been solid steel or that plastic/rubber stuff on bumpers and some trim pieces.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AB4D
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« Reply #25 on: November 25, 2011, 06:47:58 AM »

I've learned over the years, it is just better to drill the hole than to use magnet mounts.  If you use a magnet mount, I can almost guarantee it will scratch up the paint on the surface.  I've yet to see where it hasn't.  I've traded in vehicles with residual antenna holes that I plugged with rubber plugs or installed a cell phone antenna, and I've never heard a word mentioned about them.  I once traded in vehicle with scratches from a mag mount, they dropped the value $500 to allegedly repaint the roof.

For me, the safety factor is why I will not use a magnet mount.  Why take a chance of one coming off?  Likely, probably not, impossible, don't bet your life on it.  To me, it's just not worth the risk.  What if it comes off and launches into the windshield of a car behind you?  Or you accidentally hit another vehicle and it flies off hitting a pedestrian  What if it causes a fatal wreck on a highway? Think about these possible situations where you would be held liable.  I just don't believe it's worth the risk.  Drill the hole or take it to an installer for a much safer antenna install.  

Performance wise, I have not performed any research or tests that suggests which mounting method works better.  However, common sense suggests that a directly coupled antenna vs. one that is magnetically coupled, probably should work better, all else being equal (mounting location, etc).

I can also attest to the quality of the metal on the Toyota Tundra, very thin.  I am not sure if it's the thinnest, but it has to be close. The truck bed quarter panels are so thin, Toyota factory installed pieces of dynamat type material on the inside of the panel to help control noise and vibration. I did the same thing to the roof (second skin damplifier) and also had to attach a pair of three foot long steel rods to the underside of the roof to help stabilize the sheet metal.

73
« Last Edit: November 25, 2011, 06:56:35 AM by AB4D » Logged
W8JX
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« Reply #26 on: November 26, 2011, 06:22:32 AM »

I can also attest to the quality of the metal on the Toyota Tundra, very thin.  I am not sure if it's the thinnest, but it has to be close. The truck bed quarter panels are so thin, Toyota factory installed pieces of dynamat type material on the inside of the panel to help control noise and vibration. I did the same thing to the roof (second skin damplifier) and also had to attach a pair of three foot long steel rods to the underside of the roof to help stabilize the sheet metal.

I doubt rods in roof are the because thin sheet metal. It is likely for crash protection and part of a total design. They go to great lengths to do well in crashes and roll overs and no longer depend solely on sheet metal. Also they use a LOT of sound deadening material in cars these day and it adds weights so the try to recoup that with thinner metal on some area. I was in a serious wreck in a Toyota car many years ago and walked away for it because car held together well. Even though front end was heavily damage and pushed in the "cockpit" was intact and all 4 doors opened and closed fine and were still aligned. This was because Toyota braced body well for side and front impacts. I have little doubt that had I been on a US car of similar size I would have fared much worse. 
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AB4D
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« Reply #27 on: November 26, 2011, 11:01:58 AM »

I can also attest to the quality of the metal on the Toyota Tundra, very thin.  I am not sure if it's the thinnest, but it has to be close. The truck bed quarter panels are so thin, Toyota factory installed pieces of dynamat type material on the inside of the panel to help control noise and vibration. I did the same thing to the roof (second skin damplifier) and also had to attach a pair of three foot long steel rods to the underside of the roof to help stabilize the sheet metal.

I doubt rods in roof are the because thin sheet metal. It is likely for crash protection and part of a total design. They go to great lengths to do well in crashes and roll overs and no longer depend solely on sheet metal. Also they use a LOT of sound deadening material in cars these day and it adds weights so the try to recoup that with thinner metal on some area. I was in a serious wreck in a Toyota car many years ago and walked away for it because car held together well. Even though front end was heavily damage and pushed in the "cockpit" was intact and all 4 doors opened and closed fine and were still aligned. This was because Toyota braced body well for side and front impacts. I have little doubt that had I been on a US car of similar size I would have fared much worse. 
v

The rods ARE there because I installed them myself, they are not factory installed.  As factory manufactured, the roof sheet metal is very thin by earlier standards, and is unsupported, except for a lateral support made from stamped steel that runs side to side, mid cab.  If you were to look at a few of the Toyota Tundra forums on the net, you will find complaints about the "new" Tundra regarding thin sheet metal and a noisy interior.  A lot of folks recommend sound deadening in the Tundra, as I do, the noise reduction in the interior is substantial.

73, Jim.
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W8JX
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« Reply #28 on: November 26, 2011, 11:28:59 AM »


The rods ARE there because I installed them myself, they are not factory installed.  As factory manufactured, the roof sheet metal is very thin by earlier standards, and is unsupported, except for a lateral support made from stamped steel that runs side to side, mid cab.  If you were to look at a few of the Toyota Tundra forums on the net, you will find complaints about the "new" Tundra regarding thin sheet metal and a noisy interior.  A lot of folks recommend sound deadening in the Tundra, as I do, the noise reduction in the interior is substantial.

73, Jim.

I stand corrected on this matter. I think I would have tied adding another layer of metal beneath roof to stiffen it rather than rods. I could be reverse curved somewhat to preload and stiffen roof when metal is secured and bonded into place. You have helped a lot for a roof mount antenna. I still would buy a Tundra though if they made a 3/4 ton.
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K2OWK
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« Reply #29 on: November 26, 2011, 03:04:05 PM »

I have an Laird magmount. I use a dual band 2 M/ 70 CM antenna. Never had it fall off at any speed. In fact the magnet is so strong it is hard to get it off when you want to. I attached a piece of thin felt to the bottom to prevent scratching (no it does not interfear with the ground plane coupling to the car). I have used mag mounts for 50 years and have never had one fall off. I always buy the best magmounts available. I do not purchase the cheepies. Buy a good quality mount and you will not have any problems. Some of the new rare earth magnets are so strong it is hard to remove them, but they are not cheep.

Just my opinion.

73s

K2OWK
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