Bracing / reinforcing NMO mount

[SailorDan]

TL;DR: If I reinforce NMO mount by adding metal backing between roof and NMO mount, does the metal backing need to be bonded to the roof? Do I need to use a conductive epoxy for this to work or just snug it tight with bare mental?


I’ll be installing a few NMO mounts in my roof. I’m concerned about durability with what appears to thin sheet metal in my roof ( Jeep Cherokee XJ).

So I’m thinking about backing with an additional piece of thin sheet metal. The bottom of NMO has teeth that need to “cut into” the roof. Will the this sheet backing need to be electrical connected to roof?

I may be overthinking this - mmckenna and others have stated several times they have had good success with NMO mount durability - I just want to do this once and not worry about something ripping out later.

 

[RFI-EMI-GUY]

There is a certain thickness maximum most NMO mounts will work to. Too thick and the center pin may not mate. If you are using a "doubler" plate, it should probably have electrical contact with the bottom of the roof around the hole and should be of same material as roof to avoid electrolysis. For steel, I would use painted sheet metal, sanded to make contact and use automotive dielectric grease where the metal is exposed so that rust does not creep in. Epoxy around the periphery if you wish.

 

[SailorDan]

RFI - Guy thank you for the input.

Drilled my holes today and decided to roll the dice and not reinforce the NMO mounts.

I'm thinking that a solid ground and clean installation will be more beneficial than preparing for the "what if's". After all avoiding dinging an antenna just requires me pulling my head out of my heinie, but good radio performance will impact me every day I drive.

 

[Project25_MASTR]

Antenna selection also plays a big part here. Low moment, light weight antennas common in the LMR industry have a massive advantage here to the stiff, heavy, high moment antennas often marketed to amateur operators.

 

[krokus]

On my old XJ, I used a large washer to distribute the load across the underside of the NMO hole. This was after some damage from a stiff antenna smacking into objects, and some deformation of the hole. I used some anti-seize to provide electrical connection between the washer and the body.

As a practical note: the washer was selected based on thickness and OD. I then had the ID bored to fit the NMO.

 

[AK9R]

Oversized washers are often referred to in the hardware store as "fender washers". I did some quick browsing on the McMaster-Carr (large nationwide retailer of all kinds of fasteners and parts for industrial and machine tool applications) website. Found some zinc-plated steel (typically what you'll find in the hardware store) washers that are nominal 3/4" diameter (0.812" ID) and 3.000" OD in thicknesses ranging from 1/8 to 3/8" thick for $2.74 to $7.22 each. Also listed are aluminum washers that are 0.812" ID and 1.875" OD for about $1.10 each. There are several other materials to choose from.

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

 

[serial14]

I have several NMO mounts in my XJ roof with out re-enforcement. They've held up fine over the past ~10 years, enduring all sorts of off road adventures.

Antenna wise, I've had a mix of 18" tall ones up to ~40" tall whips on them, again no issues.

 

[K6GBW]

I had a truck with a thin roof. I needed to put a base loaded low band antenna on it and resorted to one of these:

NMO Antenna Roof Mount #504

3 inches in diameter and 3/8 inch thickaluminum top plate with a solid brass NMOneoprene rubber gasket2 inch round nut with ground screwstainless steel screws for ring terminal connectionrequires only 1 inch clearance on the inside to installNeeds only one 5/8 inch hole to install on sheet metal

breedlovemounts.com

 

[SailorDan]

I have several NMO mounts in my XJ roof with out re-enforcement. They've held up fine over the past ~10 years, enduring all sorts of off road adventures.

Antenna wise, I've had a mix of 18" tall ones up to ~40" tall whips on them, again no issues.

Thank you for your input. I will also be sticking with shorter 1/4 wave antenna with spring bases.

 

[RFI-EMI-GUY]

I had a truck with a thin roof. I needed to put a base loaded low band antenna on it and resorted to one of these:

NMO Antenna Roof Mount #504

3 inches in diameter and 3/8 inch thickaluminum top plate with a solid brass NMOneoprene rubber gasket2 inch round nut with ground screwstainless steel screws for ring terminal connectionrequires only 1 inch clearance on the inside to installNeeds only one 5/8 inch hole to install on sheet metal

breedlovemounts.com

Does that include the backing plate shown on the underside picture?

 

[K6GBW]

Yes, you basically get a very nicely machined 3" aluminum disk with the NMO mounted in it. The only down side is you have to prepare the coax on the underside by dressing the shield and center conductor separately. I hadn't done that since the late 70's when I did it for CB antennas. But it's not hard to do and once it's installed it actually works quite well under a roof liner. I had an Antenna Specialist antenna cut for 52 MHz on there and ran it for several years with no issues. When I sold the truck replaced the NMO mount and capped it off. I kept the Breadlove mount in case I ever need it again. Yes, they are THAT good!

 

[slowmover]

Breedlove Mounts have a large selection. Here’s the backing plate for a large puck mount.

 

[jeepsandradios]

I run 6 NMO mounts in my F150 (aluminum cab) and have yet to find a reason to put any other plates in. When properly installed you should have no issues. I run NMO27 and a NMO40 in addition to 4 other NMO antennas as reference.