GMRS antenna on smaller vehicles

Chorky

Observer
I am wondering on smaller vehicles like a Jeep, where good locations might be for GMRS antennas. I currently have a 102 whip on the right rear corner via a ball mount (swr is way off so it needs some better grounding). When working, it worked quite well and I was pleased by the range, being CB and all. I am planning to install GMRS now that most people are going that route, but want to keep CB abilities due to many in my area still utilizing it. I am looking for opinions on where would be best to mount. Being a Jeep, roof is not an option. One thought would be on the right front quarter panel right next to the radio antenna. Although I'm not sure what damage to the radio unit that may cause. Another possible location would be right above one of the brake lights, but that seems awful close to the whip. to complicate matters is wanting to find a suitable location for a cell extender as well. I suppose to make it easy I could place the GMRS above the drivers side brake light, and the cell extender above the passengers side (which would be mere inches from the whip).

Looking for helpful hits and suggestions before I drill holes in the body, and cause damage to any component.
 

bitbckt

Member
I use a no-drill NMO fender mount on my JK, from Topsy I think. It’s mounted on the passenger side ahead of the AM/FM antenna.

99% of the time, I have a Laird B4502N installed. It works great for me.
 

Bill Ruttan

New member
Being a Jeep, roof is not an option.
If the (plastic?) roof should become an option, there are (longer?) GMRS antennas that do not require a metal ground plane. (A roof mount should at least give any antenna the greatest possible unobstructed “view”.)
 

Superpanga

Active member
i have the Midland 275, with a 6db whip mounted passenger front forward of the radio antenna on a tab that uses an existing bolt.
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DaveInDenver

Expedition Leader
Where @Superpanga has his antenna should work alright but will have a distinctly distorted pattern favoring the hood side. Given the limitations of a soft top and such it's about the best you can do, although mounting it on the roof rack bars might be an option on his truck.

The important thing with GMRS is that the antenna is as high and clear of the vehicle body as possible. Remember that it's UHF with a wavelength of around 65cm, so you don't need a lot of physical length.

A 1/4λ whip is only about 6" long and any ground plane under it would only need to that size, too. So a 6" disk or square mounted under a whip is all you need to get a decent electrical match. But even if you use a no-ground plane type it's still better if you can get mounted up high with as little stuff around it.
 

Chorky

Observer
Where @Superpanga has his antenna should work alright but will have a distinctly distorted pattern favoring the hood side. Given the limitations of a soft top and such it's about the best you can do, although mounting it on the roof rack bars might be an option on his truck.

The important thing with GMRS is that the antenna is as high and clear of the vehicle body as possible. Remember that it's UHF with a wavelength of around 65cm, so you don't need a lot of physical length.

A 1/4λ whip is only about 6" long and any ground plane under it would only need to that size, too. So a 6" disk or square mounted under a whip is all you need to get a decent electrical match. But even if you use a no-ground plane type it's still better if you can get mounted up high with as little stuff around it.

so wouldn't a no ground plane antenna work better then? what about the left rear above the tail light and having a post of some sort extend the antenna above the fiberglass top? Also, is there much performance difference between a 1/4, 1/2, and full wave antenna considering the constraints from a Jeep?

Thanks for chiming in Dave, you have a ton of knowledge to share :)
 

Superpanga

Active member
Where @Superpanga has his antenna should work alright but will have a distinctly distorted pattern favoring the hood side. Given the limitations of a soft top and such it's about the best you can do, although mounting it on the roof rack bars might be an option on his truck.

The important thing with GMRS is that the antenna is as high and clear of the vehicle body as possible. Remember that it's UHF with a wavelength of around 65cm, so you don't need a lot of physical length.

A 1/4λ whip is only about 6" long and any ground plane under it would only need to that size, too. So a 6" disk or square mounted under a whip is all you need to get a decent electrical match. But even if you use a no-ground plane type it's still better if you can get mounted up high with as little stuff around it.
Thanks Dave for the feedback. I am very much still learning (studying the HAM book now and mired in the antenna section as we speak!)

When I installed the antenna, I had a RTT mounted on the rack. I also have a mounting area on the tailgate/tirecarrier that does place the antenna a but higher, but again, when the tent was in position the antenna would have gotten in the way or damaged. I have been considering relocating it.

I definitely echo @Chorky in saying thanks for sharing the knowledge.
 

DaveInDenver

Expedition Leader
so wouldn't a no ground plane antenna work better then? what about the left rear above the tail light and having a post of some sort extend the antenna above the fiberglass top? Also, is there much performance difference between a 1/4, 1/2, and full wave antenna considering the constraints from a Jeep?
No ground plane is a misleading concept. What it means is the antenna does not need the ground plane (e.g. the metal of your truck under and around the mount) to achieve a suitable match to your coax feedline. NGP does not give you the ability to just put the antenna anywhere and automatically expect good performance. A bad location is still a bad location even if you achieve a low SWR.

There's two very basic steps here. First to get the energy from the radio to the antenna. If NGP is necessary to achieve that then you do what you need to. Having a ground plane like a big metal roof is nice in that it gives you flexibility but you can work around that. All mobile radio is a compromise at some level.

When thinking about this don't forget that there's more than just a whip over a plane. You can use many things as a counterpoise to achieve a good match. Running a wire in the opposite direction as the whip can do it, essentially making a dipole turned vertical. But metal in bumpers or racks can be turned into counterpoise if you're creative.

The second step after you've got energy to the antenna is you need the antenna to radiate it as efficiently as possible before it completes it's path back to the radio, which is its ultimate goal. Whatever you've referenced the radio to is that path, almost always the body of the truck.

If you put the antenna right next to the body on a tail light it'll be happy to complete the circuit with the least work possible. But you've not excited many of the electrons around it to actually send a signal anywhere. That's where putting up high and clear of stuff so you don't short cut or absorb RF comes into play. This is why on the lip of the hood usually will work better than a tail light.

It's also why you'll see people say SWR isn't everything. You can achieve an almost perfect SWR without the RF doing any useful RF work. A dummy load gives you perfect SWR but all it does is make heat. Dumping 100 watts into a dummy load might not even give you a radio path to your neighbor. While 1 watt into a great antenna that measures 2:1 SWR can work several states away.

Imagine the RF energy coming off the antenna as though it was light or sound or water. If there's "stuff" around that would prevent those things from reaching you then it's likely it will for RF, too. That's not always true but it's safe to say it'll be extremely rare that anything around the antenna is going to improve performance. If you're lucky the materials around it are invisible to the wavelength you're using, but I wouldn't count on that being true.

ETA: Just realized I never addressed your second question. Short answer is it doesn't make that much of a difference.

First thing is to not bother with a full wavelength antenna. There's no advantages that make it worthwhile. Stick with standard configurations, 1/4λ, 1/2λ and 5/8λ or typical combinations of those, such as collinears.

On a Jeep you probably want to use 1/2λ antennas mainly for their NGP characteristics. That's about the first step of just finding a way to mount an antenna that allows you to transfer as much energy from the radio to coax to antenna. With a 1/4λ you have to have a good ground plane and middle of roofs are almost mandatory.

But beyond that there's propagation characteristics. If you were to use 1/4λ and give it a non-ideal ground it'll have a distorted radiation pattern in all likelihood. So by using an antenna designed for imperfect situations you can usually get decent matching and a reasonably good pattern.

It's most important to mount the antenna in a way that at least gives it clearance away from body panels and RTTs so you'll be exciting more of the aether than using your radio as a mini microwave oven to warm up your sleeping bags just a slight bit. If you have to use a tail light or rear tub corner or swing out then opt for taller antennas mounted as high up as possible. It's about exposing as much radiating surface to bright, clear daylight.

On the edge of the hood then it'll be less of an impact I reckon. In that case going with a taller antenna or one that increases gain will make some difference, but less so than the difference between a stubby antenna blocked by your truck. If you use a 1/4λ on your tail light it's going to be a very distinct difference front to rear. You might not hear a station 1/4 mile ahead of you but will reach some a few miles to the back. It'll probably be very obvious.

Any location other than the middle of the roof will do that but think about it in geometric sense. If you have an antenna on the hood lip draw a line from the middle of it towards your truck. Where it intersects will be where it's blocked.

So while the cab will block it the shadow it creates is smaller than if you put it right against the windshield. On the hood lip you might not hear someone 1/4 mile behind you but only if they're the same elevation or lower than you. If they're perhaps 10 feet higher in elevation and looking down on you even just slightly they could be 5 miles back and still talk.

Think about in your mind. Right at the base of the windshield the radiating angle is basically just a 90° pie wedge pointed forward. Now move the antenna a foot forward of the cowling and the blocked angle swings down 45° or 60° so a smaller sliver of pattern is blocked. The ideal is having a 180° half sphere but the only way that's possible is in the middle of a perfect roof.

That's what's know as having a "line of sight". Think of it like you're looking at each other with binoculars or a scope. If the two antennas can "see" each other then it's just a matter of simple signal strength (even 5 watts is likely to work). If they can't "see" each other then almost no amount of power is going to work.
 
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drrobinson

Member
Assuming a roof mount (NMO) on a mid sized pickup truck, what configuration would provide better performance for two antennas. 1 dual band 70cm/2m and 1 GMRS.

1st option
Both antennas in the center of the roof front to back, and spaced equidistant left to right. (So 1/3 from each edge with 1/3 between).

2nd option
Both antennas on center of roof left to right, and spaced equidistant front to back.

Thoughts?
 
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DaveInDenver

Expedition Leader
Assuming a roof mount (NMO) on a mid sized pickup truck, what configuration would provide better performance for two antennas. 1 dual band 70cm/2m and 1 GMRS.

1st option
Both antennas in the center of the roof front to back, and spaced equidistant left to right. (So 1/3 from each edge with 1/3 between).

2nd option
Both antennas on center of roof left to right, and spaced equidistant front to back.

Thoughts?
Locating two VHF and/or UHF antennas on the roof is tricky. It's been discussed, in fact.


I put two antennas centered left-right on my roof using your option 2 if I understand what you're saying. Forward one is about 8 inches behind the windshield and the rear one is about 8 inches in front of the back edge of the roof. That put them about 20 inches apart and as I mentioned in that thread this is much too close from an isolation standpoint.

The reason I did it this way was to maximize side-to-side clearance on the trail. With the whips down the center line I tend to hit less overhanging stuff along the trail edges.

So my recommendation would be to mount them however it seems they'll be the easiest to live and then figure out how to get the most distance you can between them. Which will pretty much be not enough for same band radios (e.g. 70cm ham and GMRS) no matter what you do.
 

drrobinson

Member
I’m usually using GMRS when in a convoy, but do sometimes use VHF when that is the common capability of the group. I’d like to be able to monitor national calling frequency on VHF even when running GMRS for local comms, but It looks like it would be hard to get enough separation on the roof based on [mention]DaveInDenver [/mention]‘s advice.

I’m thinking about only installing one NMO on the roof for the 2m/70cm and maybe put the GMRS on the bed rack. Don’t need max distance for GMRS anyway.
 

DaveInDenver

Expedition Leader
If you can keep separation do that, of course.

But understand @drrobinson that there's ideal and there's realistic. So you may have to prioritize what you do to compromise when running multiple radios and antennas. It's not a definite "this is enough" situation where you can say you are safe or not. It's pretty much impossible to have enough distance on a vehicle to use two same-band radios with zero interaction. At minimum you'll likely get some de-sense.

One thing that may be an option is if you only use one service at a time you could use just a single antenna with a switch or another option is a diplexer/duplexer.

If you're not familiar duplexer (or diplexers, the terms are muddled in common ham use, they are not the same thing) are devices that basically direct RF between two ports from a common port. For example this MFJ-916B gadget allows you to run a dual band antenna into the common and connect a GMRS radio to the 350-540 MHz port and a monoband 2m VHF radio to the 1.3-225 MHz. They could both be operating at the same time and not harm anything. The concept could work the other way, too. You could run a dual band radio into the common and have two monoband antennas for it.

MFJ-916B.jpg

It would get complex if you want to run a dual band ham radio and GMRS on the same antenna. A simple diplexer like this wouldn't be an option. It could be done using a cavity duplexer, at some expense and significant space penalty.

And for the record, I don't follow my own advice on co-location...

IMG_0273_mid.jpg
 
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drrobinson

Member
And for the record, I don't follow my own advice on co-location...
I really appreciate the advice.
I have no problem drilling holes in my roof, but I want to do the best placement I can.

This has been helpful in evolving
My thinking. I think going on centerline with as much spacing as I can get and using lowest power possible seems to be the best option.
 
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