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Author Topic: One Roof Mount for Multiple Antennas?  (Read 1324 times)
WILLIATY
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Posts: 80




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« on: June 22, 2008, 02:58:56 PM »

Hello,
I currently use CB and have just gotten my Tech license (KD8IVL!) to try to get away from the morons on CB who make it nearly useless (and because I'd like to contribute to ARES/RACES). However, I won't be abandoning CB entirely due to the fact that many people I convoy with use it exclusively. I would like to easily be able to run CB or VHF/UHF depending on my need that day.

I am thinking of putting a mount in the roof, running coax down the B-pillar to under the passenger seat, and then connecting it to whichever radio I need that day. Obviously, if I'm going to drill a hole in the roof, I'd like to be able to just screw out/in antennas to change from CB to VHF/UHF. Is there such a thing as a "universal" base that will allow me to swap antennas like this?
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WILLIATY
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« Reply #1 on: June 22, 2008, 03:05:48 PM »

Since I can't edit my initial post, I'll add that the CB antennas all seem to be 3/8-24 based. So I guess what I'm really asking is:

1) Can you get good UHF/VHF antennas in a 3/8-24 base?
2) Is there any reason I can't us a standard through-the-roof mount that I can get at a local CB shop for both? (I have to mail-order everything ham-specific)
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W3LK
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« Reply #2 on: June 22, 2008, 03:26:19 PM »

<< 1) Can you get good UHF/VHF antennas in a 3/8-24 base? >>

Not that I have ever seen. I wouldn't buy one anyway because no self-respecting US antenna maker would make one and I don't buy Pacific-Rim-made antennas.

<<2) Is there any reason I can't us a standard through-the-roof mount that I can get at a local CB shop for both? (I have to mail-order everything ham-specific)>>

Forget putting both antennas on the same mount UNLESS you install a US-made NMO mount (from Larsen, Motorola or Antennx). Put a Larsen NMO270B on the mount for 2m/70cm and use an 3/8x24 to NMO adapter for the CB antenna.

Personally, I'd install separate antennas; swapping is a pain in the butt, especially if it's raining.

73,

Lon - W3LK
Naugatuck, Connecticut
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W3LK
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« Reply #3 on: June 22, 2008, 03:27:28 PM »

PS - congratulations on the amateur license!

73,

Lon - W3LK
Naugatuck, Connecticut
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WILLIATY
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« Reply #4 on: June 22, 2008, 03:33:15 PM »

<<Forget putting both antennas on the same mount UNLESS you install a US-made NMO mount (from Larsen, Motorola or Antennx). Put a Larsen NMO270B on the mount for 2m/70cm and use an 3/8x24 to NMO adapter for the CB antenna.>>

1) I was unaware that 3/8-24->NMO adapters existed, thanks for pointing this out.

2) So on something like the NMO270B, that black bell-shaped thing at the bottom actually has the mount somewhere underneath it? Basically, you screw off the black base and you're left with the actual mount in the roof? Is the mount still waterproof in that configuration?
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WILLIATY
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« Reply #5 on: June 22, 2008, 04:51:58 PM »

And yet another reply to my own post... I fail it.

Anyway, another consideration is that the antennas will be unmounted when the car is put into the garage. The almost-34" tall mag-mount I have right now doesn't clear the garage door itself even when fully up. The 4' CB antenna or something like the 35-ish inch Larson NMO2/70 won't clear at all. They'll be stored in the car until needed and then mounted.
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W3LK
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« Reply #6 on: June 22, 2008, 06:01:35 PM »

<< 2) So on something like the NMO270B, that black bell-shaped thing at the bottom actually has the mount somewhere underneath it? Basically, you screw off the black base and you're left with the actual mount in the roof? Is the mount still waterproof in that configuration?>>

Yes, the mount is under the antenna base. Properly installed, an US-made NMO mount is watertight with ther antenna removed. I run my through car washes without the antenna all the time. I have installed more of these than I can count over the years and have never had one leak.

Proper installation means using either a 3/4" chassis punch (NOT a conduit knock-out punch) or an Antennx hole drill. DO NOT use anything else.

BTW, both Motorola and Larsen make caps that screw on the mount to protect them from damage is you leave the antenna(s) off the mount.

73,

Lon - W3LK
Naugatuck, Connecticut
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KF4URY
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« Reply #7 on: June 23, 2008, 04:43:05 AM »

by W3LK on June 22, 2008

<< 1) Can you get good UHF/VHF antennas in a 3/8-24 base? >>

----Not that I have ever seen. I wouldn't buy one anyway because no self-respecting US antenna maker would make one and I don't buy Pacific-Rim-made antennas.----


Actually, http://www.hamstick.com/products.htm has a few that work.  They might not be the prettiest things around, but I know most of their products are rugged. I agree on not using any of the comet/diamond/whomever stuff.



----Personally, I'd install separate antennas; swapping is a pain in the butt, especially if it's raining.----


Once again, I agree 110%



I'm really glad the thread went in the direction it did.  I had the misfortune of seeing another thread by someone who was suspected of using a CB, and the OFs got out their pitchforks and torches.  That's why I like to browse eham...
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W0GSQ
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« Reply #8 on: June 23, 2008, 09:43:01 AM »

Larsen also makes a base load coil for 27mhz.
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N8EKT
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« Reply #9 on: June 23, 2008, 11:01:20 AM »

Larsen NMO 270/BK all the way for vhf/uhf
And I would suggest you invest in a Larsen NMO27C
27mhz 1/4 wave NMO antenna $25.95 at R&L Electronics
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W3LK
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« Reply #10 on: June 23, 2008, 03:12:24 PM »

Good suggestion. I had forgotten about Larsen's 27 mHz line.

73,

Lon - W3LK
Naugatuck, Connecticut
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WILLIATY
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« Reply #11 on: June 23, 2008, 06:29:24 PM »

OK, so it sounds like a Larson NMO mount is going into my roof.

Does anyone know anything about reinforcing the mount to prevent it from tearing up the roof of the car if the antenna hits a tree or other solid object?
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W3LK
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« Reply #12 on: June 23, 2008, 07:55:39 PM »

I have never seen the need for reinforcement. Unless you hit something solid three or  four inches above the roof, you'll likely never be able to pull the mount out. Damage the antenna maybe, but not the mount. Properly installed, it's VERY difficult to dislodge the mount.

I have a Larsen NMO270B that's on it's third vehicle, with over 370,000 combined miles and it has yet to be damaged from hitting low tree limbs. airport parking garage ceiling beams and so forth. As long as the obstruction is more than 15-18 inches above the roof of my Windstar, I don't worry about it.

Unlike the average 3/8x24 thread CB antenna, the American made NMO mount antennas are very flexible and tend not to break off. The same cannot be said of Pacific Rim-made antennas such as the Opek, Diamond, Comet and Maldol. I avoid these like the plague.

73,

Lon - W3LK
Naugatuck, Connecticut
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WILLIATY
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Posts: 80




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« Reply #13 on: June 23, 2008, 08:04:25 PM »

Well, I was planning on using a NMO->3/8-24 adapter to run a 4' Wilson flexible fiberglass antenna for CB so that could transfer more force. Even if I went with a base-loaded antenna with the standard stainless stinger, I'd be worried due to the fact that you can make the roof make pop can noises by sitting a gallon of milk up there.

Are you sure it'll be alright?
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W0GSQ
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« Reply #14 on: June 24, 2008, 10:07:51 AM »

Having used that type of fiberglass antenna on vehicles at work for thirty years I would NEVER put one on the roof of my private vehicle. They just don't have enough give to them. Time and time again our 1\8 inch thick stainless steel antenna mounts get bent when the fiberglass antenna hits tree branches. The whip on the Larsen base load antenna is very flexible but will take much abuse. I have one mounted right now on my pickup roof to use with a 10M FM radio. Mine has the optional spring installed.

Just my opinion....
Steve
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