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Old 04-21-2018, 8:34 PM
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Default Antenna spacing and hole saw

I’m looking to install a dual band ham antenna and a triband scanner antenna on the trunk of my car. They will be 5 inches from the front edge, 9 inches from the side edges (that deflects up slightly) and 23 inches apart. The ham will be a icom ID-5100 and scanners are whistler WS1095s. I know I will need a ground strap from the lid to the body remove some paint from the holes to make good contact. Also mounting on the roof is not a good place due to other antennas to be mounted there later. Does anybody see any problems with this?

Also, since I don’t need to worry about any thing under the metal, can I just use a standard hole saw of the appropriate size?


EDIT: This is a newer Ford Fiesta.

Last edited by JeffDS3; 04-21-2018 at 10:33 PM.. Reason: Inserted car make and model
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Old 04-21-2018, 9:15 PM
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Best hole saw ive used for NMO mounts is a Cablematic NMO hole saw

HSK - Ripley Tools
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Old 04-21-2018, 9:32 PM
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Quote:
Originally Posted by n3obl View Post
Best hole saw ive used for NMO mounts is a Cablematic NMO hole saw

HSK - Ripley Tools
That looked perfect, though since I only plan on doing this twice, Iíd rather not spend the money if I donít have too. I already have a hole saw that should work.
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Old 04-21-2018, 10:23 PM
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Quote:
Originally Posted by JeffDS3 View Post
Iím looking to install a dual band ham antenna and a triband scanner antenna on the trunk of my car. They will be 5 inches from the front edge, 9 inches from the side edges (that deflects up slightly) and 23 inches apart. The ham will be a icom ID-5100 and scanners are whistler WS1095s. I know I will need a ground strap from the lid to the body remove some paint from the holes to make good contact. Also mounting on the roof is not a good place due to other antennas to be mounted there later. Does anybody see any problems with this?

Also, since I donít need to worry about any thing under the metal, can I just use a standard hole saw of the appropriate size?
There's no need to remove paint around the outside of the hole for NMO's. They ground underneath. The nut that screws on from the outside has an o-ring that may or may not allow the ring to contact metal. It's not necessary.

23" apart is good. More is better, but you need to keep some ground plane around your antenna. 9 inches on the side would be a concern, but it'll work. Just expect that you might have some issues getting the SWR low, and the radiation patter is going to be funky.
Usually 1/4 wavelength at your lowest transmitting frequency is suitable. Do expect some front end overloading/desense of your scanner when you are transmitting.

Also, if the rear window is tinted or has defroster wires, putting a transmitting antenna that close to the glass might be an issue. Top of the vehicle is always the better choice.

For the standard NMO mounts, you need a 3/4 inch hole. Doesn't matter what makes the hole, as long as you don't screw up the paint or sheet metal. The "official" NMO hole saws are certainly worth the investment if you plan on doing more than a few installs. If not, there are suitable hole saws and chassis punches that will do the job.
Do not use a hole saw with big teeth, they will often cause unnecessary damage and heating of the metal. Make sure you get one with fine teeth.
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Old 04-21-2018, 10:33 PM
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Go buy a cheap 9 dollar 3/4" unibit (step bit) from the local auto parts or box store. Works great for trunk lids.

I'd avoid hole saws, unless you can find one for thin metal. Regular bi-metal ones might cause damage and the hole will be bigger than 3/4". That a big no no for a NMO mount. You'll want a fairly accurate hole.
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Old 04-21-2018, 10:41 PM
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Do I need to remove some paint under the lid then or just not at all?

I can slide them a little closer to get away from the side edges but it will still be 5 inches from the front edge and about the same from the back edge. I don’t expect much of an overload problem since I’m not one to talk too much on the radio but if I do end up talking a lot I could always install a coaxial relay to disconnect the scanner antenna (or maybe find a way to automate it). I’m avoiding the top because I will be putting a temporary CB antenna up there sometimes, plus if I ever decide to play with mobile HF radios (the multi band style), their antennas are designed for trunk mounting.

The rear window has aftermarket tinting, so I don’t think that’s a problem, but there are some defroster wires.

I’ll have to probably buy a new hole saw then. Mine are for wood.
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Old 04-21-2018, 11:04 PM
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Quote:
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Do I need to remove some paint under the lid then or just not at all?
I've never needed to.

Remember, NMO's are designed to be installed "blind", as in going through the roof of the vehicle where you cannot necessarily get to the other side. The cable side of the NMO mount has teeth on it that are designed to bite into the sheet metal when tightened down.

It certainly won't hurt the antenna mount to do that.

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Originally Posted by JeffDS3 View Post
I can slide them a little closer to get away from the side edges but it will still be 5 inches from the front edge and about the same from the back edge. I donít expect much of an overload problem since Iím not one to talk too much on the radio but if I do end up talking a lot I could always install a coaxial relay to disconnect the scanner antenna (or maybe find a way to automate it).
Yeah, you are going to need to just deal with the fact that they won't be in an ideal location. As long as you accept that there will be some impact to performance (probably slight). You don't want them much closer.


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Iím avoiding the top because I will be putting a temporary CB antenna up there sometimes, plus if I ever decide to play with mobile HF radios (the multi band style), their antennas are designed for trunk mounting.
The CB antenna will have less of a problem mounted on the trunk. It's long enough that the little bit of tinted/defroster window bits won't be an issue. I'd suggest considering putting the dual band antenna on the roof, dead center. The CB antenna can still go up there, or on the trunk.

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The rear window has aftermarket tinting, so I donít think thatís a problem, but there are some defroster wires.
It depends on what material they used. If there is any sort of metallic content, it'll impact things.

Also, having an antenna running 50 watts VHF and/or 35watts UHF that close to any rear seat passengers is not ideal. Putting it on the roof would be safer.

4 watts from a CB won't be an issue.

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Originally Posted by JeffDS3 View Post
Iíll have to probably buy a new hole saw then. Mine are for wood.
If you expect to be doing this more than one or two times, investing in the right tool for the job can be a good idea.

However, in a pinch, these will work just fine:
https://www.amazon.com/DEWALT-D18001...ole+saw+dewalt
Plus, you can find them at Home Depot and other hardware stores. I had to do an install and I didn't have the right one, so I was able to run by Home Depot, pick one up and complete the job. It was on an aluminum body Ford, too.
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Old 04-22-2018, 2:03 AM
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Ok. I’ll probably sand some off the inside just to be safe. I don’t feel like pulling apart everything again if I have bad contact.

I’m ok with some performance impact, plus it puts everything closer together since everything will be mounted in the trunk. The CB antenna will be mag mounted since I don’t plan on using it much, mainly for road trips, so most of the time it will be disconnected and put away.

I believe it’s non-metallic since it’s the same my local law enforcement uses (the shop does their tinting as well). Passengers aren’t a problem. The only people who ever ride in this car is me and maybe someone in shotgun. I’ve only had one person in the back ever come to think of it.

I would be willing to buy a expensive hole saw, but I only plan on drilling 2 holes so I don’t want to spend money on something that will just sit in the tool box. I put that exact one on my list just before you mentioned it. I will however buy a quality crimper because I will be needing plenty of custom cable lengths.
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Old 04-22-2018, 2:03 AM
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I would try for more separation if possible, particularly if you intend to TX on VHF. Get a watt meter and a dummy load and connect them to the scanner antenna lead, and see what kind of RF power reading you get when you key up on VHF. Test a sampling of frequencies at full power. With 36" of separation and 25W TX, I've hit my 436 with 500mW without frying anything, but that's probably close to the limit of what scanners can handle without something losing its magic smoke.
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Old 04-22-2018, 10:17 AM
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Quote:
Originally Posted by JeffDS3 View Post
I believe itís non-metallic since itís the same my local law enforcement uses (the shop does their tinting as well).
Don't trust the police department or the tinting shop to make a good decision about that. I could tell you storiesÖ.

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I will however buy a quality crimper because I will be needing plenty of custom cable lengths.
YES! Always a good choice. And buy a few extra connectors so you can practice.
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Old 04-25-2018, 5:25 PM
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Here's a long thread with photos about the hole saw I used with great success.

https://forums.radioreference.com/ra...direction.html
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