Yeah, another Jeep Wrangler Antenna thread with options and questions

[11th]

OK, so it is a JK, I'm in a city and my JK is my daily driver as well, usage of amateur and GMRS will be limited to a few times a month east coast off road, mainly Shenandoah and norther Appalachians where repeaters are relatively rare compared to Rockies. I expect to be mainly simplex, but do want to practice repeaters for a trip out west this summer. It is parked outside in a city so I will probably have the antenna portion. I had a ham license back a million years ago in HS. I'm studying to get a tech ticket at the moment. In what is going to be an anathema to some, I just could not help it when camelcamel showed $18.50 for UV 5R and I grabbed a couple, but I do expect to get a mobile unit once I establish it will be best for me.

So I am up agains the normal Jeep Wrangler dual band antenna issues, along with my individual needs being in a city and needing a vehicle mounted dual band actually on the vehicle just a couple of times a month. The vehicle, despite being off road a fair amount the last four years is cherry, I also want to minimize the damage that does in fact occur from antennas. While a lip mount on front driver fender or close to it on hood is something I am considering, I have seen damage from those from simply getting caught in brush.

**First I have a question on terminology and function (and if if this part exists at all) of an antenna part. On JK rear tailgate upper hinge mounts, on on fender mounts, there is 3-4" inches of near vertical metal right next to antenna. To get above that would require an extension of the mount. a) do such things exist? b) should they be non conductive to avoid becoming part of the antenna and detuning it? I see springs on CB type installs, but from what I read this requires antenna trimming or tuning somehow for vhf/uhv dual band) If such a component exists, is there a name for them? I looked under various names such as "riser" or "spacer" and saw nothing.

Second question would be concerning grounding. Tailgate and hood are grounded by hinges (mediocre) and by latches (better) but maybe some of these installs would be significantly better with groundstrap. I do know that electric items on the tailgate are wired both positive and ground, So FCA does not consider tailgate a dead certain ground.

1) mag mount on hood cowl (pic 1a)
Pros: Somewhat asymmetric ground plane but only standard install with a ground plane I also see cowl mounting that isn't a mag mount (1b), that is way easier on newer JL, and I have a JK
Cons: Some damage possibility to paint, but there are ways to avoid most of that risk.

2) fender /hood side edge mount:
Pros: half a ground plane if a riser can be placed.
Cons: if no riser mechanism is available, then a) below ground plane of hood, vertical 3-4" inches of hood right next to lower part of antenna, and way way asymmetric, as hood is is effective as ground plane (pics 2a and b). Risk of body repair requirement damage is possible especially when off road (see pic #2b)

3) tail light mount (not available on JK years due to very weak taillight attachment, prior years allowed very stout mount)
4) Bumper attachment: I am not considering this
5) A-pillar light mount based mount. 1" from vertical steel surface. Not considering it.

6) Security deck mag or non mag mount: (se pic 6a for deck)
pros: something of a ground plane (after ground strap it). it is a flat steel surface that is 13" by 39", all inside but above tub. No body damage potential. No corrosion issues.
cons: limited height 27" when roof is up, unlimited when roof is off. Proximity to rear passengers? virtually all off roading is just two in front seat.

7) Rear tailgate hinge mount: see pic 7a
Pros: little damage potential. high enough to clear tub with a 2" rise of some kind. Also far enough from tire and wheel that blocked area is probably only 30 degrees out of 360 from spare if no riser is used.
Cons: no ground plane. 30 degrees of circle has substantial metal in the way unless some kind of 12" riser is used. Mount is protected off road being at rear, but does not look robust for tall antenna.

8) spare based mount with mount at slightly above spare. Pic 8a
Pros: High. no blockage from talgate metal. No blockage from spares wheel. No blockage from tire, especially if steel belted as many off road tires are
Cons: Not for anyone who is going to use it full time and has garage or uses parking garages. No close grounds plane. Extreme height result (that is a pro and con)


#s 6,7,8 are most protected from trail damage. That is a concern since on-trail would be majority of usage. I am at this point tempted most by #6 to start. If I decide to go with something lese having a mag mount wont be a problem as I can use it ad hoc for our other vehicles , and if I drill a hole for mount there I wont really be upset or worry about corrosion since it is an interior spot.

so in the spirit of nothing being all right or all wrong, do let me know of any major missing pros or cons you can see. I have seen all these set ups on the trail over the past few years. When it comes to people reporting reception though it is pretty much all anecdote with zero controls for severe variables.

Here are the pics:

1a (above): magnet. User reports after four years no scratches or harm to cowl

1b above, "Just jeff" install, no mag hood


2a above. Fender, 3-4" of lower part of antenna below ground plane and against near vertical of hood


2b above. Same issues? Also ground strap like tail gate?


2c above potential damage from lip and side mounts



6a above, flat steel security deck surface at tub height. 13"x39" with 27" of height potential. will riders get fried at 5 watts from HT? grounstrapping this would be easy if needed.



7a above. JK hinge mount. To get to tub height blocking 180 forward at base of antenna would need 2"-3" rise. another 5" to get above spare wheel blocking 30 degrees, and total of 12" to get above spare if steel belted. ground strap to tailgate?


8a above: rocky road mount, gets fully above tailgate vertical, and full above wheel and spare/ ground strap to tailgate?

 

[jeepsandradios]

So on my JK I have run the rugged radio mount for years on my public safety VHF/UHF stuff. Its a decent mount and works for my application. On the drivers side I run a Larsen NMO150 for SAR channels and on the passengers side I run a UHF 1/4 wave for GMRS and TLMR for our county. My CB is a rugged ridge CB mount that replaced the AM/FM antenna. On the JK i'll be honest and say i have had issues. I run the same with my JT and its fine. The only other antenna I run is during the summer I have a bracket that goes on the roll bar for the 2M/440. I mainly run APRS there and I just run a short dual band antenna. Last year at JJ2020 I did swap it out to a 5/8 wave for the ride to give it some more help in the NC mountains.

There are not great places to mount atennas as you said but these work pretty well for me. If you go with the NMO brackets from rugged radio buy the Larsen NMO as its sealed on the bottom better than most and will hold up better.

CB Mount/Coupler -

https://www.amazon.com/gp/product/B078XY7D49/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

NMO Bracket -

https://www.amazon.com/gp/product/B07W95NFB2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

[11th]

So on my JK I have run the rugged radio mount for years on my public safety VHF/UHF stuff. Its a decent mount and works for my application. On the drivers side I run a Larsen NMO150 for SAR channels and on the passengers side I run a UHF 1/4 wave for GMRS and TLMR for our county. My CB is a rugged ridge CB mount that replaced the AM/FM antenna. On the JK i'll be honest and say i have had issues. I run the same with my JT and its fine. The only other antenna I run is during the summer I have a bracket that goes on the roll bar for the 2M/440. I mainly run APRS there and I just run a short dual band antenna. Last year at JJ2020 I did swap it out to a 5/8 wave for the ride to give it some more help in the NC mountains.

There are not great places to mount atennas as you said but these work pretty well for me. If you go with the NMO brackets from rugged radio buy the Larsen NMO as its sealed on the bottom better than most and will hold up better.

CB Mount/Coupler -

https://www.amazon.com/gp/product/B078XY7D49/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

NMO Bracket -

https://www.amazon.com/gp/product/B07W95NFB2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

@ kb2ztx. Thanks for the response and links. By its nature a discussion of this involves downsides. So I am not at all net negative on your install, and all installs any of us considers or chooses on a Wrangler involves a fair amount of negatives. So the negatives I see in that are: if replacing factory antenna in that position, that is pretty much the position already most susceptible to trail foliage (I am living in East coast so lot more foliage on trails), and when pulling to the right in case of oncoming traffic on trail the right gets a lot more exposure to foliage than the left. Also that looks to be 4" or so of vertical body sheet right next to it on the fender, and another few inches from the near vertical portion of hood. On the other mount there is a lot of a-pillar or hood vertical metal right next to substantial portion of lower antenna as well.

I guess I am tending toward the three that invovle rear mounting as they avoid both vertical metal as well as are a lot safer from folliage whihc can damage antenna and vehicle. In my case I also do have that tuffy horizonal flat sheet of steel. What do you think of mount there for someone who already has the tuffy security deck? With any of the three rear applications I am looking at I could swap out for a longer antenna when conditions merit.

 

[favin]

Arizonarockyroad.com

www.arizonarockyroad.com

 

[jeepsandradios]

Agree with your statements. I'm on the east coast also (SW VA) and am on alot of trails with the JK. The CB Antenna is further in on the mount than the fenders so If it gets yanked off I have other issues. My mirror is also further out. I had the rear mount CB antenna on the spare tire rack but when I never could get great SWR and it was always in the way of the hard top glass in the winter. Wife hated it. So after I installed it on the JT I ordered one for the JK. In all reality my CB stuff is the guy in front or behind me. At the Jeep Jamboree I still heard guys on other trails so it works ok. I run the 3' fire stick on it.

The other mounts are closer than I like to the windshield but the NMO gain VHF works really good on SAR stuff. We use a lot of simplex and it works good. Id prefer center of roof but couldn't do that so this was the best I found. GMRS use is the same so where the 1/4 wave sits doesn't seem to have issues. 99% of the time its on the county trunking system so its also not an issue. I did swap it out to a "pepper" shaker style and it didn't work as well on GMRS so went back to the 1/4 wave.

I did have a diamond clip style mount at one point on the rear gate for APRS during the winter but found it really sucks for 2 reasons, first is its down next to the side as you state and second the cable loss is ridiculous as its that mini cable.

On my JT I have cowl light brackets and put my all band APX antenna there. Works fine for VHF/UHF/700/800 so far. I run a back rack with my APRS Antenna.

 

[CCHLLM]

Antenna science can be compared to snake oil, mirrors and magic. Speaking from decades of building, owning, and driving hardcore off road vehicles in the Appalachians and other venues, and as a professional communications tech and amateur radio operator, I can say that two things are pretty much necessary to do what you want to do: a no-ground antenna design, and an antenna analyzer, but that's only the start of the journey to proper antenna and proper mounting. Anything metal higher than the antenna, or closer than a wavelength at the operating frequency, etc., etc., will skew the radiation pattern which directly affects the SWR parameters. The analyzer will cut many, many steps out of the experimentation process.

To find the sweet antenna, mount, and location combo, you will have to experiment with your particular vehicle and its configuration of structure, accessories, and mounting locations that affect the RF propagation pattern. I have never found a dual band antenna that worked equally well on both bands unless there is a nice flat ground plane that is part of the body. That said, with some experimentation and perseverance you can get the antenna and feedline below 2:1. That ratio is not perfect, but it will keep the radio happy and still possess an operational propagation pattern, though it will not be equal in all horizontal directions. That would require that big flat groundplane of a metal foof.

Without getting into antenna theory and physics here, the goal is to get the capacitive reactance and the inductive reactance of the antenna and feedline combo to be equal. In other words, the inductive and capacitive reactances need to be 1:1. Less than 2:1 is acceptable in order for the radio to be comfortable and the antenna radiation pattern to be somewhat efficient. The analyzer will tell you if the antenna/feedline combo is actually providing a proper match at the frequencies you want to use by indicating the SWR and the actual impedance. That will tell you if the radiating element is to long or too short for the desired frequency application and which way it needs to go. Most of the time with the structural configurations of a Jeep, no amount of antenna element adjusting will get both/all bands of a multiband antenna to be acceptable if there's any compromise in the mounting situation.

 

[11th]

Antenna science can be compared to snake oil, mirrors and magic. Speaking from decades of building, owning, and driving hardcore off road vehicles in the Appalachians and other venues, and as a professional communications tech and amateur radio operator, I can say that two things are pretty much necessary to do what you want to do: a no-ground antenna design, and an antenna analyzer, but that's only the start of the journey to proper antenna and proper mounting. Anything metal higher than the antenna, or closer than a wavelength at the operating frequency, etc., etc., will skew the radiation pattern which directly affects the SWR parameters. The analyzer will cut many, many steps out of the experimentation process.

To find the sweet antenna, mount, and location combo, you will have to experiment with your particular vehicle and its configuration of structure, accessories, and mounting locations that affect the RF propagation pattern. I have never found a dual band antenna that worked equally well on both bands unless there is a nice flat ground plane that is part of the body. That said, with some experimentation and perseverance you can get the antenna and feedline below 2:1. That ratio is not perfect, but it will keep the radio happy and still possess an operational propagation pattern, though it will not be equal in all horizontal directions. That would require that big flat groundplane of a metal roof.

Without getting into antenna theory and physics here, the goal is to get the capacitive reactance and the inductive reactance of the antenna and feedline combo to be equal. In other words, the inductive and capacitive reactances need to be 1:1. Less than 2:1 is acceptable in order for the radio to be comfortable and the antenna radiation pattern to be somewhat efficient. The analyzer will tell you if the antenna/feedline combo is actually providing a proper match at the frequencies you want to use by indicating the SWR and the actual impedance. That will tell you if the radiating element is to long or too short for the desired frequency application and which way it needs to go. Most of the time with the structural configurations of a Jeep, no amount of antenna element adjusting will get both/all bands of a multiband antenna to be acceptable if there's any compromise in the mounting situation.

Hey, thanks so much. Yeah, I read about a zillion antenna installs all over the net before requesting feedback on the eight or so methods I have seen on a jeep that I outlined here. And I actually know a bit about wave theory, but I know nothing about the practicalities first hand. I am not even sure of the construction method of the antennas. Do they achieve resonance of fraction (1/4 wave, 1/2 wave, 5/8 wave) by having coiled wire at their bases? If so would not the coil be most susceptible propagation and reception issues from vertical surfaces in many hood/fender or near a-pillar mounts we see?

When you say no ground, is that no ground plane antenna? Do you think the horizontal 39" by 13" steel security deck is too small for any decent ground plane effect application for dual band? I can cut a hole in that, and although it shows excellent consistent continuity to ground, it is actually easy to ground strap that with braid. Id lose about 30% at my 180 due to a bit of the spare's aluminum alloy wheel of wheel sticking up, but that is not much different with anything. My jeep is a two door so my tufyy is 13"x39. But in the four door it is like 22'x39' so for four door jeeps that would seem to be best location. Is 13x39 of flat, well grounded steel worse than a no ground plane antenna?

above: NOT MY deck as I have two door. Tuffy deck for 4 door jeeps, steel, at level of tub (jeep's main body) about 22"x39" small blockages at 90 and 270 roll bar stanchions, small blockage from spare's wheel at 180 degrees


above: tuffy security deck for two door, 13"x39.


Also on a practical question to show you my ignorance: on the Arizona rocky road (the high post seen in #8); how is the entire 2' long steel post it is made from stopped from acting as an antenna? thus lengthening it and totally detuning it? How are springs shown in many mounts not part of the antenna? I have to assume they are conductive metal?
And is there way to get another 3" to 6" up on a tailgate hinge mount per example #7 without detuning? ?

 

[11th]

Agree with your statements. I'm on the east coast also (SW VA) and am on alot of trails with the JK. The CB Antenna is further in on the mount than the fenders so If it gets yanked off I have other issues. My mirror is also further out. I had the rear mount CB antenna on the spare tire rack but when I never could get great SWR and it was always in the way of the hard top glass in the winter. Wife hated it. So after I installed it on the JT I ordered one for the JK. In all reality my CB stuff is the guy in front or behind me. At the Jeep Jamboree I still heard guys on other trails so it works ok. I run the 3' fire stick on it.

The other mounts are closer than I like to the windshield but the NMO gain VHF works really good on SAR stuff. We use a lot of simplex and it works good. Id prefer center of roof but couldn't do that so this was the best I found. GMRS use is the same so where the 1/4 wave sits doesn't seem to have issues. 99% of the time its on the county trunking system so its also not an issue. I did swap it out to a "pepper" shaker style and it didn't work as well on GMRS so went back to the 1/4 wave.

I did have a diamond clip style mount at one point on the rear gate for APRS during the winter but found it really sucks for 2 reasons, first is its down next to the side as you state and second the cable loss is ridiculous as its that mini cable.

On my JT I have cowl light brackets and put my all band APX antenna there. Works fine for VHF/UHF/700/800 so far. I run a back rack with my APRS Antenna.

Hey thanks again. Are your cowl off to the side/outboard? On the newer JL and gladiator the bolt placements are strong enough to use inboard cowl bolts, but on the JK apparently not. Inboard gives one more of a ground plane effect (albeit asymmetric) as well as less damage risk

above, inboard light mount cowl bolts used as antenna mount on JL or gladiator

I've searched and searched and could not find this for JK. Seems like it would reduce damage potential. In fac there are rubber bushings between the cowl an the mount on top. There are some other of this type of mount that are not angled and could be spun around to bring antenna about 3" further forward and still not hit the hood when opening -- but again only JL.

 

[11th]

Arizonarockyroad.com

www.arizonarockyroad.com

yes that is the one I pictured and labeled rocky road in the original post. Since I am a new account I didn't want to link and have moderator think I was promoting a brand.
It looks great.. I am not clear if long rod it is made from doesn't effectively become part of antenna and detune or not. Also that is very high (a pro and a con). I am definably considering that

 

[GlobalNorth]

It's a Wrangler. There are no optimal places to install antennae and even fewer places if you have a soft top. With the increasing amount of plastic and non-metals in them, it only gets more problematic the more recent the vintage.

Follow CCHLLM's advice for the best possible result.

 

[CaptDan]

I have used these on several Jeeps, have used the double mount - a CB on one side with a uhf/vhf on the other side. Never an issue. Currently have one on a 2013 Jeep for vhf/uhf works great !

 

[sdu219]

I had an older wrangler and I used the holes from the rubber windshield bumpers on the hood. When I traded it it i just silicone'd the bumpers back in place and no problems. I had a VHF and UHF larson unity gain in each hole and it worked fine.

 

[mmckenna]

There is a ton of useful info on this thread:

Multiple Antenna Question for 2M/70CM

I am looking at installing an ICOM IC-2730A mobile radio for 2M/70cm in my Jeep for off-road communications. I currently have both a Midland GRMS radio (MXT400) and an older Cobra CB. The MXT400 is connected to a Midland MXTA11 6db gain antenna which is mounted to my rear tire carrier and near...

forums.radioreference.com

Very professional install on a recent model Jeep Wrangler. Several of the antenna challenges were well addressed and specific info on grounding a power feeds are well documented. The Original Poster has documented most of the build on their own website.

 

[11th]

I have used these on several Jeeps, have used the double mount - a CB on one side with a uhf/vhf on the other side. Never an issue. Currently have one on a 2013 Jeep for vhf/uhf works great !

Yes, I have seen them up close and last week talked with the guy in Arizona who makes them about different hole sizes on that small upper plate. I don't have an interest in CB so I don't need twin for now.
I guess with comparing the other rear mount option, #7 on the hinge one could get a longer antenna on the hinge, and be at same final height, or the same length as one mounted on the Arizona and less likely to it anything overhead.

So the hinge mount is still in the running.

On any tailgate mount I think the extra step of an RF ground strap is needed. I do get a DC ground when tailgate it is closed (and it is getting an extra connection at driver side latch). But I don't think it is RF ground. And even FCA runs that third brake light with two leads menaing they don't have full confidence in the DC grounding consistently occurring

 

[mmckenna]

But I don't think it is RF ground. And even FCA runs that third brake light with two leads menaing they don't have full confidence in the DC grounding consistently occurring

There are a number of posts on this site where people have faced the same challenges. The hinge/latch makes an intermittent DC ground at best, as you stated.
Success for antenna installation has usually required braided straps added to provide a good RF ground, however that is usually only possible on the hinge side. That can be an issue on lower frequencies like CB, but not so much for VHF UHF, etc.

As for the mounts you linked to, I see you are "not interested" in the twin mount for now. I'd add that even if your needs change down the road, having two antennas that close together would result in a number of issues. Detuning due to the antennas being so close is one, the other is transmitted RF coupled into the other antenna.

Also...
Do some research on 'sleeve dipoles' or 'elevated feed' antennas as an option. Those can be used on a mount like this and provide pretty good performance with the less than ideal ground plane.

 

[11th]

I had an older wrangler and I used the holes from the rubber windshield bumpers on the hood. When I traded it it i just silicone'd the bumpers back in place and no problems. I had a VHF and UHF larson unity gain in each hole and it worked fine.

That is one possibility I thought about, but out of maybe 50 antennal installs I saw in person, and lots more on the net, I did not see it once.

The sheet metal on my first jeep decades ago was much thicker. The modern hoods keeps getting thinner and I worry about robustness and the damage that could occur, even from simple rocking stress. I can flex that hood in the center with my finger.

 

[11th]

As for the mounts you linked to, I see you are "not interested" in the twin mount for now.

I am interested in a single stalk of the Arizona as I noted in my original post. They are sold single or dual Where I am no one in the two clubs I have ridden with is using CB. Those using the Arizona are virtually all using a dual band on single stalk version.

Just to go with process of elimination. Is the security deck mount a no go? It is all steel, nice and flat and drillable. I think that comes to something like 507 square inches/1/3 a square meter on the two door. Is that too small a ground plane to be of any value? Thanks!

 

[CCHLLM]

The antenna feedline termination is the antenna feedpoint at the base of the antenna assembly where it attaches to the feedline. That is where the 50 ohm match takes place. If using test gear to do any antenna system measurements, make sure the vehicle is outside away from any metallic objects, structures, utility poles, and trees or your measurements will be affected by RF reflected by metallic and the moisture in trees and plants within 50 feet of the antenna. Think not? Consider that vegetation has the ability to block radio waves, especially the higher the frequency is and the greater the veggie density is. That's why cell phones work better in a given location during the winter than in the summer. You may have 3 bars in the winter , but only 2 or maybe 1 in the summer. Heavy rain and snow and high wind velocities also affect how far you can talk. high winds blowing past towers and antennas create static electricity fields which attenuate the radio waves, especially when the humidity is low. Ain't nature fun?

The correct length of a given antenna rod is determined by the frequency range and design of the antenna, such as 1/4 wave, 1/2 wave, 5/8 wave, etc. Adding a spring to the radiating element will mean having to shorten the rod by an amount corresponding to the length of the spring assembly. For example, if the spring is 2.5 inches long, you'll need to shorten the rod by approximately 2.5 inches, There is a piece of insulated flexible conductor wire in side the spring that maintains the radiator's true electrical length as the spring flexes.

The spring construction and material itself can cause the SWR number to vary a bit, so be aware that a few spring assemblies may not be a simple compensatory measure-cut-install operation. It shouldn't make much difference, but if it does, it does. Just cut the rod a bit long so you can cut to adjust since there is no such thing as an antenna stretcher. If you want to eliminate the the spring, you'll either need to invent an antenna stretcher or you'll need a new antenna rod.

We are talking 50 ohm coaxial feed line here, so antennas are expected to show 50 ohms impedance at the antenna feedpoint at the base. Antennas may be equipped with a matching coil in the base, or in the radiator element, or both. 1/4 wave antennas are a 50 ohm match when cut to the required length and most don't have a matching coil at the base unless they have a device in the base that increases band width.

1/2 wave and 5/8 wave antennas are not a natural 50 ohm match at the antenna, so a coil or coils are added to give the desired 50 ohm match at the antenna feed point at the nominal frequencies. Where the coil(s) are placed in the antenna will also determine what the radiation pattern will be horizontally and vertically. 3/4 wave antennas are a 50 ohm match but the radiation pattern is not great.

Another important point regarding sustained RF exposure in antenna location is how close a person's body is to the antenna. You don't want the antenna mounted within a few feet of a living being's head or torso, so the decktop is probably OK for a reasonable though not perfect ground plane, but not a good thing for the back seat passengers or doggers. The amount of RF exposure goes up by the square with power output and proximity. A 4 watt CB is not so much of a threat, but 25 to 50 watts within 2 to 3 feet of a real radio's antenna is another prospect altogether.

 

[CCHLLM]

Looking at the stalk on the carrier in picture 8a brings me to a question, and that is what is the length of the stalk? It may be long enough to act as a 1/4 wave VHF counterpoise since it is grounded, and if so, it may actually also function as a UHF counterpoise. That would make an antenna mounted on it narrowly tunable in VHF and UHF, especially if the antenna is a no ground type. I'll have to research the possibility that some of the dual banders are compatible, but don't expect much bandwidth without some ground plane. 5/8 wavers won't tune at all I'd bet, and the no ground plane variety will probably have weird radiation patterns if they do tune. Remember, the higher the gain figure, the narrower the band width. What I don't know is how much the wheel proximity will affect the questionable counterpoise action that may be OK with the spare off, but not with the spare mounted. An analyzer would tell us what's what.

 

[mmckenna]

Just to go with process of elimination. Is the security deck mount a no go? It is all steel, nice and flat and drillable. I think that comes to something like 507 square inches/1/3 a square meter on the two door. Is that too small a ground plane to be of any value? Thanks!

It would work if it's metal.

Ideally, you want a 1/4 wave length at your lowest frequency in all directions under the antenna to give it an ideal radiation pattern and allow for the best match.

Looks like it would be a bit narrow for VHF use, but for short range work I doubt you'll have an issue.
The other issues that would concern me is that it's close to any possible/future rear seat passengers. I know you said it's usually only the front two seats in use, but consider future possibilities. I'd not want a child/family member sitting that close to a transmitting antenna unless it was low power.
There also may be some interaction with the rear part of the roll cage. That may make tuning difficult.

If was me, I'd put a dual band antenna that was based off a 1/2 wave design on a bracket off the hood. Put it about halfway between the front edge of the hood and the windshield, like this guy did: Part 3 - Midland MXT400 MicroMobile GMRS Radio & Icom IC-2730A Mobile Ham Radio Install in a Jeep Wrangler – Part 3 | Southeast 4x4 Trails

Short of drilling a hole in the center of the hood (I know a guy that did that with his VHF antenna), the hood mounting is probably your best option.