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NMO versus Mag Mounts

NMO Mag Mount

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#1 Sasquatch

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Posted 09 May 2018 - 11:11 PM

I hear a lot discussion about the superiority of NMO over Mag Mounts. It usually ranges from very little difference to NMO being a vastly better performer. Those using mag mounts say that if the magnet is big enough and the capacitive coupling solid that there is very little difference. Those with the NMO's say that you can't get a good ground with a mag mount.

 

My question is does anyone have any hard data showing how much improvement can be obtained by drilling a hole and solid mounting the antenna? Someone must have studied this at some point in time or performed some experiments.

 

Thanks!

 

Sasquatch


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#2 PastorGary

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Posted 10 May 2018 - 06:03 AM

Using a properly mounted NMO places the antenna over a ground plane surface at a proper resonant distance and has a DC ground.  If a mag mount is used, the bottom of the antenna itself is above the ground plane surface by at least an inch and the resonant length of the antenna may be off.   It also has no DC ground connected to the vehicle body at that point because of paint layers, etc.

From personal experience - If an NMO body mounted antenna is cut to a resonant length for lowest possible SWR, and that same cut antenna is then placed on a mag mount, the SWR will be a bit higher.  If the antenna on a mag mount is then re-cut to compensate for the additional height of the mag mount above the vehicle body, the antenna will then be non resonant if placed back on a body mount NMO.

I have also found that most (but not all) NMO mag mounts have factory supplied coaxial cable and PL259 that are somewhat inferior to what might be used with an NMO body mount from a frequency sensitive and loss per foot aspect.  


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#3 kb2ztx

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Posted 10 May 2018 - 11:59 AM

So in my 30 years in public safety radio I have seen varied responses on all of this. Normally the higher the frequency the better a magnetic mount will perform. Normally this is because in UHF and 800 you are talking to a repeater or system, whereas VHF or Lowband is normally simplex or a base station. I have had very good results on mag mounts in the UHF and 800 bands, but all tests have been on trunking systems or good repeaters. In the Search and Rescue business 99% of our traffic is VHF simplex. We have some locations we know if they have a magnetic mount forget it. I have done tests at those locations with new mag mount antenna next to the base of a NMO and the NMO will talk out and hear when the mag mount does neither. In addition we have some pretty remote areas we operate in and even the NMO mount can have some effect on the efficiency of the antenna. I found the better antenna NMO mount is the Larsen enclosed mount vs the standard Motorola. I now install these in all my SAR installs for the VHF radio. UHF I stick with the standard Motorola NMO but the UHF radio is on a P25 trunking system.

 

YMMV....



#4 quarterwave

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Posted 11 May 2018 - 07:53 AM

I can echo that I have both personal and professional experience with both, and always prefer the NMO over a Mag. Why? I always saw a Mag as a temporary setup. Over time they can have issues, mostly from water. I have even seen the ones with clear plastic seals on the bottom have water in them. 

 

I observed that reflected power was always better with NMO, and the lower the frequency, the worse it got on the Mag. When I was working at the M shop years ago, our local Sheriff's dept would keep a couple dash mount D51 Maxtrac lowband mobiles, cig plug power cords and Mags for winter, so they could "borrow" a couple of 4x4 Ford Broncos in case of big snow events. Then they would spend 2 hours at out shop while we tried to explain why they were not working right with 60 watts out and 25 reflected at 39 Mhz, and find a sweet spot on the roof where the reflected was better. 

 

The older techs told me 26+ years ago that a 1/4 wave on a NMO was the best, most reliable antenna. I found that out several times over the years and that is all I use today. 



#5 PastorGary

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Posted 11 May 2018 - 09:17 AM

Let's add another possibility to the discussion... Stainless steel "L" brackets for NMO mounts -  I have used these for 40 + years and never had an issue.

 

Larsennmobrakstainless.jpg


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#6 JohnE

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Posted 11 May 2018 - 10:55 AM

I will add that the "closed" cables KB2ZTX mentioned are rated to 6Gc. from personal experience the "closed" cable w/195 or 200 cables work much better. I have seen better TX/RX in my personal vehicle just from the cable change, all else being the same.

MP antenna make some pretty good cables also.

 

https://www.tessco.c...o.do?sku=332348


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#7 Sasquatch

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Posted 12 May 2018 - 12:49 PM

Thanks to everybody for all the great information and the technical explanations of "why" the NMO makes perfect sense. When I do mine I will be picking your brains on how to approach the whole project. I know there's a lot of information here on the site already but you guys are filling in the blanks. I was especially interested to hear from the professionals out there who have the first hand field experience.

 

Thanks!!


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#8 Sasquatch

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Posted 12 May 2018 - 12:56 PM

One other question...do most NMO antennas have a water proof plug option for going through the car was or pulling in the garage. I'm running a 5/8 wave, 2 meter mag mount on my Outback now and it's pretty tall.


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#9 JohnE

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Posted 12 May 2018 - 01:11 PM

you can get "caps" or covers. remove the antenna and put the cap on.

if you want one or two I have a bunch in plastic or metal.


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#10 PastorGary

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Posted 12 May 2018 - 01:40 PM

John has a good offer there - In case anyone ever needs one and John is out of stock, there are several varieties available from EBay, Tessco and others...


https://www.ebay.com...?_nkw=nmo cover

 

https://www.tessco.c...fo.do?sku=85487


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#11 kb2ztx

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Posted 14 May 2018 - 05:42 AM

The L Bracket works well and in our county when we were on lowband installed hundreds on chiefs vehicles. They worked ok but in CNY Winters required a new NMO mount jut about every year. I found the enclosed mount from Larsen did perform better in this environment. Again it puts the antenna not in the best location but we couldn't convince a volunteer chief to drill a hole in his $50K truck. I actually use the flat stainless bracket on my wifes JK for her VHF antenna. Serves the purpose but if i could find a way to mount a 1/4 wave on a jeep thats what it would be. Her UHF and 800 are all pepper shakers inside the rear window on a plate with 3 NMO mounts.






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