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Larsen 2/70 NMO

2RT307

2RT307

Sr. Member
Nov 22, 2011
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Texas
I found a good deal on a Larsen NMO 2/70B. Not the highest gain figures out there, but I have seen solid reviews on them at QRZ and Eham. I wanted something that would hold up to my stunt driving. :laugh:

I'm new to NMO mounts, and was wondering why they specify 3/4 and 3/8 inch holes for them? I plan on modifying an existing Firestik mount by drilling a bigger hole for the mount. Do you guys have any tips on doing this the right way? Is it really as simple as just drilling a 3/4" hole? I can't see splurging for the NMO hole saw that is 60 bucks!

Also, have any of you tried the SO-239 to NMO adapter that Larsen makes? I'm considering using one on my Wilson 5000 mag mount to adapt the 2/70B to that as well.

73,
RT307
 

2RT307 said:
I found a good deal on a Larsen NMO 2/70B. Not the highest gain figures out there, but I have seen solid reviews on them at QRZ and Eham. I wanted something that would hold up to my stunt driving. :laugh:

I'm new to NMO mounts, and was wondering why they specify 3/4 and 3/8 inch holes for them? I plan on modifying an existing Firestik mount by drilling a bigger hole for the mount. Do you guys have any tips on doing this the right way? Is it really as simple as just drilling a 3/4" hole? I can't see splurging for the NMO hole saw that is 60 bucks!

Also, have any of you tried the SO-239 to NMO adapter that Larsen makes? I'm considering using one on my Wilson 5000 mag mount to adapt the 2/70B to that as well.

73,
RT307
Click to expand...

The NMO mount is a lot wider than the others, which is why you need that big hole. You'll see what I mean as soon as it arrives. I suppose it doesn't really matter how you get the right size hole (hole saw, drill, etc.) just as long as you do. Regarding those S0-239/NMO adapters - they work fine for smaller antennas but don't get any ideas about mounting a giant antenna on them....they just aren't strong enough.
 
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The hole for that NMO mount is sort of exacting, it really doesn't hurt to have the bit required to cut that hole (not cheap, but better than having to patch and re-drill... been there, ain't no fun). The circular blade in those thingys can be replaced when they get worn, figure on replacing it if you do many holes.
As for the adaptor you specify, I have no experience with them at all so can't say anything good/bad about them. I have used such type adaptors, they do work, and there are some 'qualifications' about how to use them. They all add a tiny bit of 'length to the antenna, so, you -may- have to re-tune if you use one. And then it may not be worth the trouble of re-tuning, that's up to you.
- 'Doc
 
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Cool beans, guys! I appreciate the feedback. My neighbor is a mechanic, and has some nifty mic tool that he can measure so we get the right bit. I may still try the Wilson mag mount with the adapter on the 2/70, as it is a fairly small and lightweight antenna. This is a "travel option", since I fly and use rental cars quite frequently.

Thanks again and 73!
RT307
 
I use a regular 3/4" hole saw with several layers of emery cloth rapped around about 1/4" from the bottom and secured with a hose clamp, this gives depth control + it cleans the paint off the roof so I get a good ground. A lot cheaper than the special cutter that's recomended.
 
Blue Jay said:
I use a regular 3/4" hole saw with several layers of emery cloth rapped around about 1/4" from the bottom and secured with a hose clamp, this gives depth control + it cleans the paint off the roof so I get a good ground. A lot cheaper than the special cutter that's recomended.
Click to expand...

Good idea Blue Jay. I just remembered my Firestik mount is stainless steel. I may just take it to a local shop and see if they can use a drill press on it. Or maybe I should just buy a new NMO mount. :whistle:

73,
RT307
 
Well, the good news is I got the hole drilled at on of my customer's shops. A 3/4" hole in stainless steel ain't easy! Then, to get home and find out that the hole was just a tiny bit too small. Out came the file, and I trimmed away the material to make it fit. The NMO mount is now hooked up to the Firestick bracket. I put the antenna on, and now I am not hitting the repeater that I CAN hit from inside my truck with my Chinese duck. The antenna is right behind the cab of my truck. CB antenna works fine here, but one of the local hamsters said I might detune the antenna being that close to the cab. The antenna is about 38" tall, and clears the roof of the cab by about 10 inches. I've seen installs on the front cowl area of pickups and they seem to work just fine, so I figured no real difference between that and my install. I don't have any analyzer equipment to check resonance, and don't even have an SWR meter that will cover these frequencies yet. I checked the connections and actually loosened one because I think I might have tightened it so much that it would short the mount. Will the location of the antenna be enough to stop it from working correctly?




73,
RT307
 
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Something to keep in mind about that antenna (or any brand) is that the things are not all that flexible and are tall. That can introduce some complications. It wasn't that brand, but I had one of the 5 - 6 feet antennas at one time. It certainly worked, but the limitations caused me to go back to a shorter one. The differences weren't that great in performance, but it sure was in convenience.
- 'Doc
 
I almost went with a Comet SBB7, which is also a tall high gain unit. I was going to buy it off a local. Turned out it had a cracked loading coil, and I passed on the deal. More research showed that to be a common problem with that antenna, along with allen screws backing out (which can be fixed with loc-tite). I went with the Larsen for the simplicity of design and reliability.

73,
RT307
 
Had a thought.... there is a tab in the base of the antenna. I remembered it was bent at a funny angle. Straightened the tab, reinstalled it, and it is now hitting the repeaters on 2m and 440. Yea me! (y) Now to do some driving around and see how it does.

73,
RT307
 
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Hey Eric, I've actually been playing with both. I have the Wilson mag mount from my 5000, and used a Larsen adapter SO-239 to NMO. Works pretty good on the rental car so far (center of the roof of a Ford Edge). Tried it that way on my F150 super crew too, and it did well. The permanent mount on the truck is working good, too. But, the repeaters are very close in the DFW area and I may try one of the Comet or Diamond antennas with more gain just to see how I do from further out.

73,
RT307
 
You can't go wrong with a Larsen! I have been running them mobile for over 15 years and I only had one to go bad due to a bad coil.

I also have tried a NMO "L" bracket mounted to the side of my tool box. The antenna did not like seeing the truck cab and I never got good results until I mounted the NMO up on the roof. What a difference that made!!

I think you made a good decision by going with a Larsen!
 
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