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Author Topic: Reusing an antenna-- SBB5-NMO, was mounted for seven years...  (Read 4329 times)
KC0LPV
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« on: August 09, 2010, 12:44:23 AM »

I have a Comet SBB-5NMO that has served very well in the seven years I've had it on my Neon.  I'm transferring my radio equipment to a Tacoma, and I would like to reuse this antenna.

On dis-mounting it from the Maxrad mount, I found that there was a little bit of corrosion on the mount, and some slight corrosion contamination on the antenna inner contact.  I cleaned it up with a Q-tip and alcohol, and it looks fairly decent.

I suspect that the water may have infiltrated from the bottom, since there are notches in the brass mounting ring that are used to tighten it down, leaving barely a thread to fully seal to the antenna when tightened.

I would figure on using some deoxit on the contact and threads of the antenna before mounting it to a new mount.  I never noticed any performance problems from this antenna, but I haven't checked the SWR since I tuned it to the Neon.   

Is there anything I should check on the antenna? 

Should I replace the rubber/neoprene seal in the antenna base?

Is there a particular paint I should use (or avoid?) to repair the finish on the antenna where it has been pockmarked from seven years on the road?
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K0BG
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« Reply #1 on: August 09, 2010, 11:37:10 AM »

Part of the issue is, the Comet's NMO is fully compatible with the Maxrad NMO mount. The latter is an exact copy of the New Motorola (NMO) mount. At the least, you should replace the rubber seal.
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KC0LPV
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« Reply #2 on: August 09, 2010, 12:22:42 PM »

Are you saying there is a question about whether the Comet NMO and the Maxrad NMO mount are a good match?

I think I ought to replace the NMO mount anyway--this one seems worse for the wear, and I've never felt really good about the fact that the end of the coax was not weathertight--I used black silicone to make a halfway decent seal.

Is the rubber seal something I need to buy from Comet?
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K0BG
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« Reply #3 on: August 10, 2010, 07:06:51 AM »

The NMO mount from Comet, Diamond, and other Pacific Rim companies is slightly different in design. For example, the threads are of a slightly different pitch, albeit you can get them to mate. What usually happens is however, the NMO outer ring stays in the antenna when you remove it. Most of the time, they're darn difficult to remove. On at least two of the Diamond antennas, the center conductor won't touch the center of a real NMO mount without bending it out further. The reason this is so, is the center of the Diamond NMO mount sticks up about 1/8 inch higher than the real a NMO. One has to assume this was a way around paying royalties to Motorola.

While your Comet has lasted a long time, the facts are, they're not nearly as sturdy as say a Larsen NMO2/70BK. And, there's virtually no difference in their performance, published spec notwithstanding.
« Last Edit: August 10, 2010, 09:09:58 AM by Alan Applegate » Logged

KC0LPV
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« Reply #4 on: August 10, 2010, 04:55:14 PM »

Interesting...I know that this Comet antenna fit well on the Maxrad NMO, or at least seemed to.  The antenna center conductor is gold plated and has a spring button center, and I see the mating mark where it rested on the mount center conductor.

I suppose if this antenna goes bad I'll look at the Larsen you recommend.
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N9KTW
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« Reply #5 on: August 11, 2010, 10:01:04 PM »

Just one quick thought here. I have always used a small amount of " Penetrox" ( a trademark of Burndy corp), antioxidation compound on the MNO threads, and a very small drop on the center pin. I retired a a truck several years ago, and the antennas screwed right off, and the NMO mounts were not corroded.

Antioxidation compound is readily available at most of the larger home centers and electrical supply houses.


Howard, N9KTW
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KI4SDY
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« Reply #6 on: August 11, 2010, 10:56:59 PM »

When I install antennas on a new or newer vehicle, I use all new coax and mounts. If you are going to spend time doing the installation, why take a chance on equipment that has been exposed to the elements for a long period? Buy new coax and mounts to get the best performance out of your gear! Wink
« Last Edit: August 29, 2010, 07:31:25 AM by Guy "Vern" Wells » Logged
KI4SDY
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« Reply #7 on: August 24, 2010, 06:43:33 AM »

Gee, I guess these NMO mounts aren't as waterproof and wonderful as the e.Ham.net experts would have us believe.Wink I use stud mounted antennas myself for strength, positive contact, easy inspection and maintenance. I never had a problem except when a loose antenna wore all the threads off a brass 102" whip stud years ago. I periodically check to make sure they are tight now. If you have worn thread issues replace the item in question.

As far as repainting the 7 year old antenna; after cleaning and retuning (if necessary), just use $.99 flat black spray paint from one of the box hardware stores. It will look like new and protect the antenna from corrosion.Grin   
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KB1LKR
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« Reply #8 on: August 24, 2010, 03:40:23 PM »

"Gee, I guess these NMO mounts aren't as waterproof and wonderful as the e.Ham.net experts would have us believe.Wink"

Probably due to the pacific rim antenna (Comett in this case) not quite perfectly fitting the NMO mount or not fully tightened/able to be fully tightened.

He didn't report ANY leakage into the vehicle, so the mount appears to have been waterproof, regardless of the antenna applied to it. Stud mounts are good for long heavy HF antennas, but not as practical for VHF or  UHF whips anyway.
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KI4SDY
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« Reply #9 on: August 25, 2010, 08:11:09 AM »

"Probably due to the pacific rim antenna (Comett in this case) not quite perfectly fitting the NMO mount or not fully tightened/able to be fully tightened."

The short explanation; another piece of NMO junk! Grin

"Stud mounts are good for long heavy HF antennas, but not as practical for VHF or UHF whips anyway."

Gee that's funny. I have been using stud mounts for VHF and UHF whips for years. Would you like me to send you a picture so you can see how it works? Wink
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KB1LKR
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« Reply #10 on: August 25, 2010, 02:05:15 PM »

KI4SDY:

Yes, at your convenience, if you could sent a picture to: <my call>@arrl.net that would be great. It might still be possible to teach this old dog a new trick!

Steve -- KB1LKR
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WB6DGN
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« Reply #11 on: August 25, 2010, 08:02:41 PM »

"The short explanation; another piece of NMO junk! Grin"

Yeah!  Somebody needs to teach those Motorola engineers how to do "it" right!  Are you volunteering?  Wonder how many MILLIONS of those there are in use, not counting the cheap offshore knock-offs!
Tom
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KI4SDY
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« Reply #12 on: August 26, 2010, 07:08:43 AM »

I wonder how many millions of stud mounts there are in use? Almost every trucker driving down the road has them attached to their rear view mirrors, and not just for CB either. Wink

As far as the Motorola NMO is concerned, I would rate it a half step up from the Comet or Diamond SO239 mount, which isn't saying much. I like to give credit where credit is due. Grin

The main thing I like about the stud mount is that it was named after me! Shocked
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K0BG
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« Reply #13 on: August 26, 2010, 07:27:50 AM »

Part of the bad press for NMO mounts is due to the myriad of copy cat designs. A real, honest-to-john NMO (New Motorola), properly installed, will not leak, even with the antenna removed. And for good reasons.

The underside of the outer ring includes an o-ring seal. The typical copy cat has a flat rubber washer.

The real NMO has a serrated under plate, with a step which keeps it centered in the hole. Most copy cats don't have either.

The center pin (it sticks up, rather than being flat) of the copy cats leak if the antenna is removed, as do all SO 239 type mounts.

If you really want a cheap mount, use one of the old Antenna Specialists 3/8 inch snap in mounts. What a POS! Not only did they leak like a sieve, one little hit from a tree branch, and they popped out of the hole.

As for the stud mounts, they've been around longer than most of us. In the early days, everybody used ballmounts, equipped with a 3/8x24 thread. Been that way every since, albeit the ballmount has sort of gone away.
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K3GM
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« Reply #14 on: August 26, 2010, 08:11:11 AM »

Jim (the original poster), you'll find Motorola NMO mounts in all sectors of motor vehicles; commercial, industrial, government, you name it and you'll find there there.   As you know, the mount itself has an O-ring to prevent water intrusion into the vehicle itself.  But most quality antenna bases will have 2 additional gaskets.  You find either an O-ring or a rubber skirt around the coil base base as a first line of defense.  On the underside of the antenna base, up by the contact, there is a hose washer style gasket to prevent moisture from reaching the contact area.   A properly installed NMO mount with the right antenna is bulletproof, and will last the life of the vehicle.  I did a quick check of Comet, Diamond, MFJ, and of course Larsen and found that all of their VHF/UHF antennas use either the UHF or NMO mounts.  If I'm wrong, it isn't by much.  Only Hustler's use the 3/8-24 stud on their colinears.  The reason they do is because they're extremely heavy, and have a large surface area.  I've have their 2m colinear in my mobile arsenal, although I haven't used it in years.  Generally speaking, you'll find Hamsticks, and CB whips use the 3/8-24 stud while lightweight VHF antenna will use something else.
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