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Author Topic: Gap required for NMO trunk lid mount  (Read 7226 times)
KD5GR
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Posts: 101




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« on: April 22, 2013, 05:54:39 PM »

I've searched and can't find any specs for the gap required for a trunk lid mount (NMO).  It's going on the hatch of a C-5 Corvette that has a 0.165" gap.  I'll have to use something like this so I can adjust the angle.  http://www.diamondantenna.net/k400.html

Does anyone know offhand the minimum required gap typical for these types of mounts?

Thanks,
Chas.
« Last Edit: April 22, 2013, 06:18:14 PM by KD5GR » Logged
K1CJS
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Posts: 6034




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« Reply #1 on: April 23, 2013, 04:38:00 AM »

Usually you'll want more than what the mount requires to avoid body/paint damage.  Most mounts are made with the equivalent of approx. 1/8 inch steel plate formed to make the mount--and the RG58 cable can sometimes require almost 1/4 inch of space to avoid crushing!

This is just another reason that many hams say 'drill the hole' instead.  I do understand that you're talking about a Corvette, take my offered info FWIW.  73!   
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K0JEG
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Posts: 665




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« Reply #2 on: April 23, 2013, 05:27:32 AM »

I've searched and can't find any specs for the gap required for a trunk lid mount (NMO).  It's going on the hatch of a C-5 Corvette that has a 0.165" gap.  I'll have to use something like this so I can adjust the angle.  http://www.diamondantenna.net/k400.html

Does anyone know offhand the minimum required gap typical for these types of mounts?

Thanks,
Chas.

Might be cutting it close, depending on how straight the gap is. On my Audi the mount sits in a gap on the rear hatch that's a little less than a 1/4 inch. Prior to this car I had one mounted on the trunk lid of a Pontiac grand am and didn't have any problem with it rubbing/interfering with the trunk operation. However, there is a tab that sits on top of the body panel that WILL leave a mark on that side. Not terrible, but be aware of it.

It's a nice mount, if you use a 1/2 wave antenna. The thin coax does have a lot more loss over just about anything else, but it also makes it much easier to run in small holes and gaps. It does hold too. I check the screws every so often and they've never loosened up at all, never had any issues with moisture in the coax, it just works (at least the NMO mount, can't speak for the UHF version).
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K7RBW
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Posts: 392




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« Reply #3 on: April 23, 2013, 05:46:03 AM »

For the tab, I put a piece of gaffer's tape on the body underneath the tab. I changed the tape every month or two to keep it from getting stale a gooey. After several years, I took it all off and you can't tell it was ever there.

Gaffer's tape is cloth tape with a non-permanent adhesive. It looks like black duct-tape, but is of a much higher quality (and higher price). It holds securely without leaving adhesive marks behind.
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KD5GR
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Posts: 101




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« Reply #4 on: April 23, 2013, 10:55:18 AM »

Thanks for the information guys, I really appreciate it.  Putting an antenna on a plastic car is an iffy proposition at best, but I saw an article about someone doing it using 2" wide copper foil under the mount and running for some length on either side.  I thought I'd give that a try rather than using a glass mount.  With the defrost wires in the hatch glass, a glass mounted antenna would probably perform even more poorly.

Thanks again,
Chas.
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K0JEG
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Posts: 665




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« Reply #5 on: April 23, 2013, 04:16:25 PM »

Thanks for the information guys, I really appreciate it.  Putting an antenna on a plastic car is an iffy proposition at best, but I saw an article about someone doing it using 2" wide copper foil under the mount and running for some length on either side.  I thought I'd give that a try rather than using a glass mount.  With the defrost wires in the hatch glass, a glass mounted antenna would probably perform even more poorly.

Thanks again,
Chas.

That's why I use a 1/2 wave antenna (specifically the Diamond NR-770NMO). You don't need a ground plane. I don't think the K400 will give you a good ground connection on a 1/4 wave antenna. I've used the NR-770 on mag mounts, the K400SNMO mount and even on a bicycle rack with no SWR problems. I've read some bad reviews, but mine have never been a problem. Note that I use the NMO mount, not the UHF mount. That might make a difference.
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KD5GR
Member

Posts: 101




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« Reply #6 on: April 24, 2013, 10:31:04 AM »

Thanks for the information guys, I really appreciate it.  Putting an antenna on a plastic car is an iffy proposition at best, but I saw an article about someone doing it using 2" wide copper foil under the mount and running for some length on either side.  I thought I'd give that a try rather than using a glass mount.  With the defrost wires in the hatch glass, a glass mounted antenna would probably perform even more poorly.

Thanks again,
Chas.

That's why I use a 1/2 wave antenna (specifically the Diamond NR-770NMO). You don't need a ground plane. I don't think the K400 will give you a good ground connection on a 1/4 wave antenna. I've used the NR-770 on mag mounts, the K400SNMO mount and even on a bicycle rack with no SWR problems. I've read some bad reviews, but mine have never been a problem. Note that I use the NMO mount, not the UHF mount. That might make a difference.
I see what you mean about the eHam reviews on the Diamond NR-770NMO -- they're all over the map.  You got me to thinking about a 1/2 wave antenna and the Comet SBB5-NMO, a 1/2 wave on 2 meters, gets great reviews.  I think I'll try that.  On 70cm, it's 2 5/8 waves, so I'm not sure how that's going to like the Corvette.   Grin

Thanks for the idea.
Chas.
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N6AJR
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Posts: 9908




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« Reply #7 on: April 24, 2013, 12:27:38 PM »

And you could always use a piece of steel or aluminum from under the drivers side rear corner. weld or bolt it to some of the steel chassis and mount the antenna on the short piece sticking out the side. works well and on the drivers side, it is away from little fingers on the sidewalk and trees are higher in the middle of the road.
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K5LXP
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« Reply #8 on: April 24, 2013, 02:08:13 PM »

Years ago I met a ham with a corvette (don't remember what model year) that had a clever antenna installation.  He mounted a pin jack (think old TenTec headphone jack) to the flange of the fender so the opening was centered in the hood/fender gap.  His antenna was a simple 2M whip with a bit of shrink wrap for the bottom inch or so.  So all it amounted to was a whip stuck in a pin jack.  The heatshrink kept the whip from contacting either side of the gap.  When he wanted to, he could easily remove the whip and there was no exterior sign of any antenna installation, and no risk of paint damage from a more conventional mount.


Mark K5LXP
Albuquerque, NM
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M6GOM
Member

Posts: 915




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« Reply #9 on: April 26, 2013, 03:45:47 AM »

I'll have to use something like this so I can adjust the angle.  http://www.diamondantenna.net/k400.html

Does anyone know offhand the minimum required gap typical for these types of mounts?

Thanks,
Chas.

Hi Chas. I've just measured my K400 mount and it protrudes 3mm. I reckon you wouldn't want any less than a 4mm gap. 0.165" is just a hair breadth wider than 4mm so although it'll be close it should be OK.
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KD5GR
Member

Posts: 101




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« Reply #10 on: April 26, 2013, 07:03:02 PM »

I'll have to use something like this so I can adjust the angle.  http://www.diamondantenna.net/k400.html

Does anyone know offhand the minimum required gap typical for these types of mounts?

Thanks,
Chas.

Hi Chas. I've just measured my K400 mount and it protrudes 3mm. I reckon you wouldn't want any less than a 4mm gap. 0.165" is just a hair breadth wider than 4mm so although it'll be close it should be OK.

Thanks for the information.  I feel much better.
Chas.
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K1CJS
Member

Posts: 6034




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« Reply #11 on: April 27, 2013, 03:16:25 AM »

Hi Chas. I've just measured my K400 mount and it protrudes 3mm. I reckon you wouldn't want any less than a 4mm gap. 0.165" is just a hair breadth wider than 4mm so although it'll be close it should be OK.

Thanks for the information.  I feel much better.
Chas.

Don't forget that the coax cable shouldn't be squashed down to fit between the two surfaces.  Sooner or later, continued squashing will result in shorting the coax and finally severing it.  It isn't just the mount you have to be concerned with.
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M6GOM
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Posts: 915




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« Reply #12 on: April 27, 2013, 07:27:12 AM »

It is that really thin stuff barely 2mm in diameter and nigh on impossible to crush with a rubber door or trunk/hatch seal.
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