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Author Topic: Increasing efficency of a mag mount.  (Read 1598 times)
W6NZX
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« on: March 27, 2009, 09:37:31 PM »

Increasing efficency of a mag mount.

I hardly ever get on 2M/70CM anymore, so I sold off my 2M rig and kept my VX-2R.

Great little radio and I can get into MOST of the repeaters here in the Utah valley, however I'm no where near full quieting and people say I drop in and out.  I have one of those cheapie imported mag-mounts and i'm wondering if there is anything I can do to make it "work" better.

I ask, because I had an AMAZING experience with the VX by adding a metal ring and wire equal in length to the portable antenna and putting it between where the antenna screws on and the insulator to the frame.  I.e. I made the other half of the circuit larger--the vx-2r has a small body.

So, can I do something similar when using the mag-mount?  a LONGER wire?  ANYTHING to mak the mag-mount bigger?

DE W6NZX SK 0436Z
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K0BG
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« Reply #1 on: March 28, 2009, 04:26:45 AM »

Where are you going to connect the other end? To make it effective, it would have to be connected to the body someplace. I suppose you could make a ground plane out of it by using radials as it were, but the wind load would increase, and so would the likelihood of it dislodging. Why don't you just drill the hole, and do it right?

Alan, KØBG
www.k0bg.com
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W6NZX
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« Reply #2 on: March 28, 2009, 07:03:51 PM »

I was actually thinking of having a heavier wire running from the "frame side" of the vx to one of the bolts holding the seat against the frame, that would give me a big hunk 'o metal.

I think my real problem was the last car I had this setup in was a Chevy cavalier lots more metal to reflect that signal--now I'm in a GEO metro and my HT is almost useless, and the price of mobile amps, makes just buying a mobile make more since.

But I'm a cheap-skate :-)


-Rob
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KI4SDY
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« Reply #3 on: June 08, 2009, 06:35:04 AM »

Your not telling us how far away the repeater is so that we can make a judgement on the performance of your setup.

Right off the bat, the most inexpensive and effortless thing you can do is to buy a better magnetic antenna. I would suggest the MFJ-1729 power gain dual band antenna. It is highly rated on eHam.net and locally here. You can place it on the roof for maximum range or on the trunk for garage clearance. In addition you can move your rig from car to car or set it up as a portable base at home.

If you don't use VHF/UHF that much, I wouldn't drill holes in the car to install an antenna. It isn't necessary.
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AA4PB
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« Reply #4 on: June 08, 2009, 09:56:54 AM »

The larger dia the mount, the more capacitive coupling there will be to the metal roof.
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K5END
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« Reply #5 on: June 08, 2009, 10:49:20 AM »

You are mounting an antenna on the exterior of the vehicle, and not trying to use the HT antenna on the interior, right? That's my first question. Anything below a couple of GHz really needs the antenna exterior to the vehicle.

As far as the other stuff, I agree with Alan on this topic.

Unless it is a borrowed or rented car, or something like a pristine 1935 Super V8 Packard sport coupe, of which there are only a half dozen in existence, just drill a dang hole (and you wouldn't be putting a mag mount on that Packard anyway.)

When possible, drill and do it right. Beware of existing wiring, structural members, air bags, sunroof mechanism, weatherproof, etc; you get rain in UT, from time to time, right?

Metal to metal joint to a bonded car body beats capacitive coupling at any frequency. Mag mounts are just a convenient hack.

In the long run, mag mounts will mess up your paint much more than a pluggable 13/16" hole and they present a safety hazard.
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KI4SDY
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« Reply #6 on: June 16, 2009, 07:47:32 AM »

K5END said; "In the long run, mag mounts will mess up your paint much more than a pluggable 13/16" hole and they present a safety hazard."

Not if your read and follow the instructions that tell you to remove the antenna when you wash the car and replace it when dry. Of course, that would be for people that actually wash their car, assuming it would make their hole ridden heap look better.

I can't think of anything that will mess up your paint more than drilling a hole in the car. Sticking an ugly  rubber plug in it doesn't fix the paint either!

As far as the safety hazard, that is an old wives tale (like global warming) that has no statistical basis. Ignore the hole drillers that spout this silly scare tatic.

If you permanently mount the antenna you also lose portability to move the rig from car to car or as a portable base. The antenna I suggested has been used for both. Just place it on top of a metal file cabinet or ther large metal object. Remember to check the SWR!
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WW5AA
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« Reply #7 on: June 16, 2009, 01:16:07 PM »

"As far as the safety hazard, that is an old wives tale (like global warming) that has no statistical basis. Ignore the hole drillers that spout this silly scare tatic."

You might let the motorcycle rider who got hit last year by a mag-mount CB antenna that bounced off the street and cut his chin pretty good know about this silly scare tactic. I don't know about the statistics, but with 40 years in Law Enforcement, I have seen several. By the way, the lawyer on this case doesn't care about statistics either.

73 de Lindy
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K0BG
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« Reply #8 on: June 17, 2009, 05:48:54 AM »

Larry, if you care to, you can look up the crash which killed a 75 year old women in Cincinnati. If my memory is correct, is was April 2004.

The other known fatality was listed on the NHTSA statistics page. This one occurred after the NHTSA required reporting telemetric use. It occurred in 2006.

As Lindy alluded to, there have been dozens of others which didn't fall under the fed reporting mandate.

This brings up a side light.

Folks drive around with all manner of heavy objects laying around in their vehicles. Brief cases are a good example. You don't read about these objects flying about the vehicle during a crash or evasive maneuver; the fed doesn't mandate such things. However rare, this very thing has happened to me, and I have the scar to prove it! After the incident, I started buckling my brief case in with the passenger seat belt. Over kill you say? I personally don't think so.

On my web site, I admonish folks for using Velcro to attach all manner of devices. Yet, the interior trim of most modern vehicles are held in by the stuff. But, there is a vast difference between a 4 ounce piece of plastic, and a 4 pound radio!

As I recently stated on these pages, the odds may be slim, but like getting hit by lightning (the proverbial chance in a million), it does happen quite regularly.

Speaking only for myself, I prefer to walk on the safe side, even if that means facing the traffic!

Alan, KØBG
www.k0bg.com
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AA4PB
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« Reply #9 on: June 17, 2009, 06:36:13 AM »

Where you mount the radio, even if firmly attached, can have serious results as well. Many years ago my wife did serious damage to he knee in an accident when her leg hit the CB radio mounted under the dash. I also have wondered about the results of an air bag deployment hitting the edge of a center mounted radio.
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KI4SDY
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« Reply #10 on: June 17, 2009, 07:28:55 AM »

WW5AA said; "You might want to let the motorcycle rider who got hit last year by a mag mount CB antenna that bounced off the street and cut his chin pretty good know about this silly scare tactic."

I would be interested to know how the CB antenna came off the car with the coax cable attached! How about giving me the accident report number for that incident so I can get a copy? I would also like the accident report numbers for the "several" other mag mount accidents that you reportedly saw in 40 years. Maybe they have a phenomenon of these things in Arkansas, like the Bermuda triangle. After 25 years of law enforcement, I never saw or heard of anything like that with magnetic antennas, including my own, that stayed on the patrol vehicle at 140 miles per hour while I was chasing criminals.

One thing I learned after many years of law enforcement was the biggest tellers of fish tales wear a badge. Old timers are especially bad about this because they figure the records are gone and nobody can check up on their stories. Unfortunately, I have seen bad police department policy created as a result of these urban legends.
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K0BG
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« Reply #11 on: June 17, 2009, 10:59:18 AM »

Vern, one of the problems finding data on crashes, is the lack of data bases. It wasn't until 2005 that the NHSA mandated telemetric use on crash reports. Even then, there are a lot of crashes where telemetrics were in use, but didn't end up on a police report for whatever reason.

I found the one in Cincinnati, quite by accident, if you'll excuse the pun. The report on the second one was via e-mail.

You ask how an antenna could fly off when it is connected by a coax cable. Well, weigh a mag mount antenna, take your 140 mph report, and calculate the kinetic energy. You just might be surprised to find out just how much there is. Even at 60 mph, an average weight 2 meter transceiver will have hundreds of pounds of kinetic energy. Obviously, not all of the energy would be expended before it hit an occupant, but one thing is for sure. Enough is left to main and kill. Whether it does is moot.

Alan, KØBG
www.k0bg.com
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K5END
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« Reply #12 on: June 17, 2009, 11:17:42 AM »

>
"Larry, if you care to, you can look up the crash which... "

Alan, I agree with you about mag mount hazards.

Are you preaching to the choir? hihi

73
LK
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K5END
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« Reply #13 on: June 17, 2009, 11:24:29 AM »

Personal experience with a mag mount coming off.

About 10 years ago I had a mag mount cell phone antenna, and had snaked the coax through the door gasket to the exterior roof.

For some unknown reason the mount came off and was dragged by the coax.

Because it was held in place by the door, the assembly was dragged under the rear tire. The tension on the coax between the door and the tire promptly snapped that coax right off and made a heck of a racket under the car.

I went back to clear the antenna remains from the road but never did find it. I'm happy no one was hurt and the only loss was the antenna itself.

The coax is plastic and copper. It doesn't take much tension to snap it.
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KI4SDY
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« Reply #14 on: June 17, 2009, 07:24:45 PM »

I haven't seen or experienced any of these flying mag mount phenomena. Maybe I am better at placing antennas on a car than you. If the placement looks like it defies the laws of physics, it probably does. You have to use some common sense!

I have seen (and I know you have) several permanent roof mounted antennas that are missing the radiator wire, leaving only the mount and or loading coil. This usually results in the radio transmitter being burned up, before the operator realizes the wire is gone.

I have never heard of anyone getting hit in the eye with a radiator wire, but I could start a rumor, like the mag mount stories. Alas, I am not willing to make up stories to make a point on eHam.net, so I guess I will pass on the opportunity. However, permanent mount antennas have the potential to be just as theoretically "dangerous" as mag mounts!

I would have a better chance of being a global warming victim than being killed by flying antenna parts! Find better reasons to talk neophyte hams into drilling holes in their cars when it is not necessary or even convenient.
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