Building an NMO antenna base, what a serious pain in the ....

[mmckenna]

The problems I encountered were quite possibly because I was given the incorrect components to fit together. And since the interaction was a form of mail order, I was not able to run down the street, to the local HRO and ask what's up with improperly fitting equipment. I merely presumed I was just having difficulty with fitment. And, as usual for me, I made it work. One way or another, I was not going to let that NMO mount win.

Some of those guys at HRO are not exactly the sharpest tools in the shed.

What's important is the "I made it work." part. That's what's important. You figured out a way to get everything to cooperate. And, you did it yourself, not just buying a pre-packed antenna/base/cable set up. There's a lot to be said for that approach.

So, I asked. And I got great answers. So far, with 14GA wire, a total length of about 7', seems to be working Just fine.

Yep, you done good. 14 gauge is fine for a single amateur radio with a short loop length.

The fuses that came with the original wiring, were the glass tube type.

Most, manufacturers have moved away from those for mobile use.

I installed waterproof blade type on both the negative and positive, 15A each. Ain't blown yet.
Scott

Much better setup. The ATO fuses are well proven in mobile applications.

 

[jim202]

The only thing to remember when installing an NMO mount is to put some silicone grease on the threads and the black rubber gasket. This will prevent the mount from locking up on you from corrosion a couple of years down the road if you have to pull the mount out. Learned this lesson from my mentor many moons ago. Been doing antenna installs for many years and never forgot what my mentor showed me about the NMO mounts.

One suggestion is to find some 3M high vacuum grease to use instead of the silicon grease. The high vacuum grease will not migrate with heat from the sun like the silicon grease that comes with the NMO mount. You don't need to put much of it on the threads and rubber gasket. A little goes a long way.

It also is a good idea to use the roof hole saw for the vehicle mounts and not the hole saw you can get from Lowe's or Home depot. The radio hole saw has a ridge on the outside of it to prevent you from accidentally plunging too deep into the roof metal and tearing up the head liner.

Jim

 

[prcguy]

The regular hole saws from Lowes or Home Depot work great and I can cut a perfect hole in a single layer metal roof with a headliner glued to the underside and not damage the headline except for the pilot hole. If your right handed you hold the drill in your right hand then put your left elbow on the roof and use your left hand to regulate the force and depth of the drill. With little practice you can keep the cutting teeth from going any deeper than the thickness of the metal roof.

Since you usually have to pull some rubber door seal out to run coax, you can slide a sheet of thin plastic or sheet metal between the headliner and roof in the drill area for extra protection from the hole saw if its your first roof hole.

The only thing to remember when installing an NMO mount is to put some silicone grease on the threads and the black rubber gasket. This will prevent the mount from locking up on you from corrosion a couple of years down the road if you have to pull the mount out. Learned this lesson from my mentor many moons ago. Been doing antenna installs for many years and never forgot what my mentor showed me about the NMO mounts.

One suggestion is to find some 3M high vacuum grease to use instead of the silicon grease. The high vacuum grease will not migrate with heat from the sun like the silicon grease that comes with the NMO mount. You don't need to put much of it on the threads and rubber gasket. A little goes a long way.

It also is a good idea to use the roof hole saw for the vehicle mounts and not the hole saw you can get from Lowe's or Home depot. The radio hole saw has a ridge on the outside of it to prevent you from accidentally plunging too deep into the roof metal and tearing up the head liner.

Jim

 

[cmdrwill]

I think Larsen LM, which was a later design, was a little better.

The main thing here is that not all NMO mounts are the same. One needs to get a reputable brand, not some Ching Chang knockoff.

"The ATO fuses are well proven in mobile applications" Actually ATC fuses for all under hood applications.

 

[iMONITOR]

What is it you think makes it a poor design?

I've used them very little and have not experience any problems personally but that's probably because I avoid using them when ever possible. However I've hear countless stories and have seen examples when vehicle roof top installations bend the roof metal, broke when the antenna struck something, leaked water, rusted out, came lose etc. The coax usally has to make a very sharp 90 degree bend immediately at the NMO base. They seem to be prone to cross threading, the center pin connection isn't 100% reliable and not consistent even though it's supposed to be an industry standard. Some antennas have a sping loaded solid brass center pin, while others use a cheap thing flat spring tab. The few ground plane conversion kits I've seen do not have an adaquite diameter to make a good seal with the O-ring in the typical NMO antenna.

 

[BoxAlarm187]

I've used them very little and have not experience any problems personally but that's probably because I avoid using them when ever possible.

Many of these issues seem to be installer-related, not the NMO itself. Tens of thousands of emergency vehicles in the US are using NMO’s 24/7 with little to no issues.



Sent from my iPhone using Tapatalk

 

[iMONITOR]

Many of these issues seem to be installer-related, not the NMO itself. Tens of thousands of emergency vehicles in the US are using NMO’s 24/7 with little to no issues.

Sent from my iPhone using Tapatalk

Totally agree, but maybe a design that doesn't require such precision from the average installer would create less problems.

 

[nd5y]

I think Larsen LM, which was a later design, was a little better.

I bought some of those back in the 80s because they were cheaper than NMOs. They all broke. The plastic insulator that holds the threaded stud weakened or cracked and the stud unscrewed. I threw them away and have used NMO ever since.

 

[Paysonscanner]

Paysonscanner,
You're quite welcome and thank you for the nice comments. Now, when it comes time for you to install that NMO antenna base in your roof of your car, might I suggest something, if you haven't already seen or experienced this. Go to youtube and watch a few NMO installs. They use a specifically designed drill that, puts a pilot hole, a 3/4" hole and, there's also a stop built in so you can't accidentally go through the headliner. See what you think.
Scott

Thank you for that suggestion. My husband had put a few mobile antennas in when he was in high school and used a very large diameter drill his dad had. He had done a few more since, but still didn't have the best tool for the job. Then it was time to drill into the new Subaru 28 years ago, so he ordered an antenna drill. It looks kinda like a hole saw, but much better put together. He had me drill the second hole in the Subie. The phrase "measure a dozen times and cut once" was what I used that day, even if he was standing there watching me the whole time.

 

[iMONITOR]

The phrase "measure a dozen times and cut once" was what I used that day, even if he was standing there watching me the whole time.

 

[mmckenna]

I've used them very little and have not experience any problems personally but that's probably because I avoid using them when ever possible. However I've hear countless stories and have seen examples when vehicle roof top installations bend the roof metal, broke when the antenna struck something, leaked water, rusted out, came lose etc.

Installer issues.

I've got an F-350 4x4 with an NMO dead center in the aluminum body roof. On there I have a Larsen NMO 1/4 wave VHF wide band. Last year I was in Las Vegas for a conference and forgot about the antenna. Drove into a parking garage and hit it hard. While those antennas have a spring at the base, they are pretty stiff. The whip material is much thicker than the standard NMO's. Due to a bunch of cars behind me, I couldn't stop to remove the antenna. Just had to deal with the noise.
When I was able to stop, the spring had taken a permanent bend to the rear. The NMO base was still fully intact, including the aluminum roof it was mounted on.
When I got home, I ordered just the spring part, and replaced it. No damage to the truck or the rest of the antenna.

Damaging the roof top is possible, but it's usually done by someone doing a poor job of installing it.

Leaking/rusting is another installer issue. Failure to make the right sized hole can cause that. Failure to properly install and lube the included o-ring at the base. Failure to properly maintain it. All things that require a bit of work, but certainly not hard to do.

The coax usally has to make a very sharp 90 degree bend immediately at the NMO base.

This tells me that you haven't installed one before in a roof mount. The NMO design has the coaxial cable leaving the base at a 90º angle.

They seem to be prone to cross threading,

Sounds like the typical Tram/Browning/Cheap Chinese Crap. They use soft materials on the bases and the antenna coils. They'll cross thread if you don't start them correctly. A good name brand (Larsen, Laird, Antennex, MaxRad, etc) will not do that. Important part is to not try and save a few bucks by using the cheap crap.

the center pin connection isn't 100% reliable and not consistent even though it's supposed to be an industry standard.

I've seen that on a few installs. Sometimes it's due to the installer not using a mount designed for the roof thickness. Sometimes it's a poorly designed antenna coil.
You are correct that there doesn't seem to be a standard to address this, but it's also something that would be checked easily by using an SWR meter or antenna analyzer after the installation.

Some antennas have a sping loaded solid brass center pin, while others use a cheap thing flat spring tab.

"Pogo-Pin". Done right, those can work. Done wrong, they crap out. Motorola 800MHz antennas from the 1990's used a pogo pin that was sheer and utter crap. Those failed very frequently. A really poor design.
Newer ones seem to be much better.
The tab issue usually is from the wrong thickness mount installed and someone trying to bend the tab down to make up for it. Done incorrectly, they will break. If the install is done correctly, it's not an issue.

The few ground plane conversion kits I've seen do not have an adaquite diameter to make a good seal with the O-ring in the typical NMO antenna.

Yes, and again someone that pays attention to what they are doing will catch this. Using a base conversion kit is good for a temporary installation. Using them as a permanent solution, well, that's an installer doing his/her best to get out of doing a proper job.

 

[FIREUP]

I've used them very little and have not experience any problems personally but that's probably because I avoid using them when ever possible. However I've hear countless stories and have seen examples when vehicle roof top installations bend the roof metal, broke when the antenna struck something, leaked water, rusted out, came lose etc. The coax usally has to make a very sharp 90 degree bend immediately at the NMO base. They seem to be prone to cross threading, the center pin connection isn't 100% reliable and not consistent even though it's supposed to be an industry standard. Some antennas have a sping loaded solid brass center pin, while others use a cheap thing flat spring tab. The few ground plane conversion kits I've seen do not have an adaquite diameter to make a good seal with the O-ring in the typical NMO antenna.

Well Sir,
When I put the NMO together, mine didn't have to make a sharp 90 degree bend. It went straight into a tube that was 90 degree to the mount itself. There is absolutely no stress on any section of the coax on this style mount. And, if you look at most of todays open ended mount only (no coax attached, you'll see that the coax does not have to bend.
Scott

On edit: I used a pretty fresh 3/4" hole saw made by Kobalt (obviously a Lowes product here) and, I thought it would make a precise 3/4" hole. Nope, it wandered quite a bit. In fact, in the end, it's a 7/8" hole. Not good but, the antenna base does have enough teeth on the bottom lock nut and the top is well supported by what's left over. My mount (steel) is 3/16 steel. When I told the HRO helper I needed a "thick" mount, he gave me one for upwards of 1/2" to 5/8" thick steel. It's not a big deal. It just looks slightly odd having that much thread sticking out the bottom of the mount, no biggie.
Scott