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Author Topic: VHF Antenna Efficiency-Mag Mount vs Drilled Hole NMO  (Read 11643 times)
AD1DX
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« on: August 23, 2011, 04:05:40 AM »

I don't wish to get into the magmount controversy.I understand that a mag mount does not function as well as a properly installed nmo.I am also aware that some feel that there are safety issues.What I am trying to understand is how much antenna efficiency am I losing by using a mag mount .I am presently using a tri mag mount for my 5/8 wave 222 mhz whip.Has anyone actually done any measurements to support this ?

Thanks and 73
AD1DX Ceasar
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K5LXP
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« Reply #1 on: August 23, 2011, 05:56:17 AM »

It's going to depend on the actual mag mount of course, but I would venture to say that VHF and higher with any nominally sized magnet (say, 3" or bigger) the efficiency would be pretty close.  I would lay odds it's well within a dB of each other.  Not sure you're going to find any measured data unless you do it yourself.


Mark K5LXP
Albuquerque, NM
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K0BG
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« Reply #2 on: August 23, 2011, 06:08:25 AM »

There is measured data, and it is referred to in Larsen's antenna catalog. The biggest issue isn't loss, so much as it is common mode current. No matter the magnet size, there will always be some common mode. Whether this causes you any problems is perhaps moot, but it can and usually does.

You didn't mention how you were routing the coax into the vehicle, but sooner or later coax run over, or under, the weather stripping is going to cause a problem.
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KI4SDY
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« Reply #3 on: August 23, 2011, 06:46:15 AM »

None of that has made any difference in the small mag mount antenna on the lady ham's Mustang she has been using for the last half decade at our house. She likes it and it performs well. The weather stripping has not suffered or leaked, nor is she experiencing any common mode problems.  Grin
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K5LXP
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« Reply #4 on: August 23, 2011, 07:24:08 AM »

You're right Alan, they show a difference of .2dB.

I would think the common mode issue is highly installation dependent (other disparaging issues with mag mounts notwithstanding).  


Mark K5LXP
Albuquerque, NM
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M6GOM
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« Reply #5 on: August 23, 2011, 05:15:11 PM »

I notice that nobody has mentioned the SWR being affected by the co-ax getting pinched as it enters the vehicle due to the impedance of the co-ax being changed as the ratio of center to spacing of outer is massively reduced.
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WB6BYU
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« Reply #6 on: August 23, 2011, 05:44:59 PM »

That depends on the type of coax you use - foam dielectric is a particularly poor choice in this
circumstance.  Solid polythene (or polyethylene as they say on this side of the Pond) actually
holds up pretty well to a door jam as long as it has some weatherstripping.
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K0BG
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« Reply #7 on: August 24, 2011, 06:19:41 AM »

There are a lot of common practices in amateur radio, but just because they're common, don't mean they are the correct way of doing things. The truth is, permanent mounting has a lot of advantages, and one of those is even the cost involved in removing one (a fact which will elicit a few rash comments).

Another is using the cigarette lighter socket to power a transceiver. It is a very poor, and potentially dangerous practice. Yet, far to many do so adding the comment, I never had any problem!

It just makes me cringe to see some of the installation practices some amateur use. It sort of reminds me of old adage: If he spent as much effort working, as he did staying out of work, he'd be rich!
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KI4SDY
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« Reply #8 on: August 24, 2011, 06:29:14 AM »

The SWR on the lady ham's $14.95 dual band MFJ 1722S Ultra LiteMag mount antenna is 1.2 in the middle of the band where we operate most of the time. Great low profile antenna and quick to install!  Wink
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KF6DBZ
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« Reply #9 on: August 24, 2011, 07:51:01 AM »

I use 2 mag mount antennas on my Honda Fit. Both are Wilson brand antennas. I have the 2 meter mag mount and i havent checked the actual performance of the antenna other than the SWR meter in the radio (Icom 706 MiiG) it works perfect for me as i can remove it when i want or switch to my truck.
The other antenna ia a Wilson 500 for CB. I matched it to the Honda and i use it on 10 meters (28.400) I dont think most people get down to all the checking and stuff with the radio equiptment, just enough to get by. I look at it like if it is safe, does what you want and you are happy with it, then it must be the perfect setup.
 For the power i put a 12 volt marine battery in the car and run the radio from that. This gives me a clean power source and i dont have to worry about killing the Honda's battery (which would take about 30 minutes)
Just my 2 cents. Grin Grin Grin
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K0BG
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« Reply #10 on: August 24, 2011, 12:07:45 PM »

There is a local amateur here with a Honda Fit, IC-7000, and ALS-500. I have never heard his say anything about the stock batter being too small. It does have a 105 amp alternator, which will run almost anything you could install in one.

By the way, using a second battery is no guarantee that the power will be clean. It will, however, assure that the input voltage, those power out, will both be low.
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KF6DBZ
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« Reply #11 on: August 24, 2011, 01:04:30 PM »

If i park and leave the car running the radio would do fine hooked up to the car battery but when i park to use the radio i don't want to leave the engine on. If i was to use the car battery with the engine off the battery would die within a hour. Hondas are known for having small batterys.
By using the marine battery i can use the radio for hours before i need to start thinking about it. When i get home i just charge it up and its ready to go again.

This is not a perfect situation, just one that works for me.
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K0BG
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« Reply #12 on: August 25, 2011, 06:39:34 AM »

You're spreading a false rumor! Honda's SLI batteries aren't any smaller than any other brand. The size is based on the accessory load, and little else.

My Ridgeline has a size 34, as does my wife's 2006 Civic. The only Hondas with a small battery are the Hybrids.

BY the way, the battery you should be using is one with a large RC rating. A marine type is designed to sit uncharged for a long period of time (up to two years), and still have 80% of its charge left.
« Last Edit: August 25, 2011, 06:41:15 AM by K0BG » Logged

KI4SDY
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« Reply #13 on: August 25, 2011, 06:50:50 PM »

What is a "stock batter?"  Huh
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KF6DBZ
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« Reply #14 on: August 25, 2011, 11:39:56 PM »

Alan,
I will measure my battery and post it tomorrow. So you are saying i can hook up the 706 to the Honda Fit battery and talk on the radio for several hours and the engine will start? Maybe it will. I just remember when i bought the car if i left the headlights on by accident for 30 min the car wouldnt start. might turn over but wont start.
I measured it now the dimensions are 8 inches x 5 inches x 8 inches deep. There is a cover on it so it might be smaller than that. It is a Yuasa (SP) battery. I did buy the car used so maybe this isnt the right battery for a Honda Fit?
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