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NMO Mount Spacing and installation

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W3AWF

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I'm getting ready to install 3 NMO Mounts on the roof of my Silverado. I have a Whelen Mini Edge light bar on the roof as well. I was curious if anybody happens to know how far the mounts have to be from the light bar in order to not cause destruction to the internals. My main concern is with my VHF Radio causing havoc, a Motorola MCS2000 running at 100 Watts.

Also, I have heard a lot of theories about installing NMO mounts in regards to sealing them weather tight. What is the best way to seal them? The rubber gasket that comes with them? I've heard a lot of people using black RTV to seal them as well. Can anyone shed some light on this one?


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buddrousa

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What are the 2 other antenna mounts? If one is a scanner I would not put one of my high dollar scanners on an outside scanner antenna with a 100 watt transmitter in the truck. That is just me not sure why you are running 100 watts unless your repeater is a long way off or you are using simplex. We work our county and the next county with 25 to 35 watts. We can switch to any department that we respond to for mutual aid with a TK790 mobile running 35 watts.
 

clbsquared

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Using a hole saw specifically designed for NMO mounts, with the o-ring and rubber gasket supplied with the mount and not getting carried away during tightening of the ring will insure a trouble and leak free installation. Always a good idea to make sure the surface is clean and free of debris before final assembly, as well. And I also agree with buddrousa. I wouldn't put a scanner anywhere near an antenna with 100 watts on the other end. With a properly installed and tuned antenna, you should have no trouble hitting a repeater from 25-30 miles(depending on terrain) away with a 35-45 watt mobile.
 

mmckenna

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I agree, install the NMO mount per the manufacturers instructions.
Use the included O-ring with the silicone grease that comes with it.
Using the proper size hole saw and installing per directions is all you need.

I never understood why people needed to slather RTV all over the mounts. My guess is to make up for a sloppy install.

I've got a 10 year old work truck with NMO's I installed when it was brand new. Never a leak. We've got much older trucks at work that have had the NMO's installed to, probably going on 20 years now. No issues.


As for spacing from the light bar, I've seen them pretty close, I don't think you'll have an issue.

Still, I'd give it some spacing, at least 1/4 wave, to prevent making your pattern slightly directional (probably going to happen anyway)

I'd also recommend turning down the power on the MCS. No reason to run them flat out at 110 watts. We turn our high power mobiles down to 80 watts or so, makes them a lot happier in the long run.

Spacing of your other antennas is going to be important. You'll need a lot of separation between the VHF antenna and the scanner. You're likely going to experience some overloading no matter what you do, but you need some space to prevent damage.
 

03msc

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I'd put the scanner antenna down on the front left fender, if possible. Since it's rx only it shouldn't cause any great loss and I don't think you'd notice a huge difference in reception compared to the roof. Then you'd only need to put 2 on the roof, which allows for more spacing.

You can find a vehicle-specific fender mount that uses an existing bolt for more Silverado years. I have one on my '15 and my scanner antenna is mounted on it.
 

kayn1n32008

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Also, I have heard a lot of theories about installing NMO mounts in regards to sealing them weather tight. What is the best way to seal them? The rubber gasket that comes with them? I've heard a lot of people using black RTV to seal them as well. Can anyone shed some light on this one?

As mmckenna stated, done right, NMO mounts on the roof do not leak. There are 2 or three gaskets on a Larsen NMO mount. One for the mount, one O-ring on the coil, and a third that sits between the roof and the coil.

I would also agree with turning the MCS2000 power down. Even @ 75-80w you will see no difference in how far you can talk @ 100w.




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mmckenna

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Here's an install on the roof of a regular cab F150.
LED light bar, VHF and 800MHz.

VHF is a 100 watt Motorola XTL-2500.
800 is a 15 watt Kenwood NX-900.
Not sure what flavor the light bar is.

 

jeatock

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Spacing

The answers to your question are fairly simple and come from two sources: electrical engineering formulas that my wife (Masters in Mathematics) has difficulty resolving, and spaghetti engineering (throw it at the refrigerator - if it sticks its's right).

Generalities: Antennas should be at least 1/4 wavelength apart, and not land on multiples of 1/4 wavelength (1/2, 3/4, etc.). That distance is different for the approximate center of each frequency band.

50 watts into the base of any 1/4 wave antenna sends 50 watts towards the horizon. 50 watts into the base of a 5/8 wave (3db gain) antenna sends 100 (theoretical) watts towards the horizon, and towards other antennas on the same roof. Gain antennas work by "stealing" signal from the birds and earthworms and focusing more energy towards the horizon. If the receiving antenna is a couple thousand feet higher and only a few miles away the signal may go underneath. If the land is flat it works.

Personal experience: I have three VHF 50-watt transceivers, one dual input R2500 receiver and five VHF antennas plus one UHF setup and one Low-Band (45 MHz) setup on the roof of my "company" Expedition. Radios are all 45~50 watts and one VHF transceiver is on a 3db 5/8 antenna (the rest on 1/4 wave). All antennas are spaced about 1/3 VHF wavelength (that math took some figuring). Everything works just fine (going on four years), and I have not been able to detect any directional patterns.

Personal opinion: space at 1/3 wavelength in a triangular pattern and stay as far away from the light bar as practical.

As mentioned, fender/hood/bed mounting your scanner antenna gives you both horizontal and vertical isolation and can't hurt.

No matter what you do, when a radio is in transmit the other receivers will be de-sensed and deaf, but you shouldn't care.

And yes, out here in God's Country all public safety vehicles have 5/8 3db VHF antennas. Remember that the 3db gain works for both transmit AND receive. A 50-watt transmitter has an (actual, measurable) ERP of 90+ watts, and the receiver hears almost twice as well.
 

cmdrwill

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The other missing piece: Groundplane for each antenna. Minimum of 1/4 wave in any direction. Spacing 1/4 wave at the VHF frequency to either of the other two antennas.
 
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