Mounting VHF/UHF Antenna on Power-Coated Aluminum Vehicle Roof

[Abies]

I had planned to use NMO mounts for three antennas to go on the aluminum aftermarket top that will soon arrive. The main radios will be dual-band VHF/UHF and GMRS. However, I'm also interested in scanning up to 33cm and down to CB, and I may want to transmit on 6m and 10m.

The top is intended to be the ground plane. However, I'd forgotten that the top has an access port. In principle I could use non-invasive mounts and run the coax through the port. I'd prefer this if it's workable and wouldn't cause serious performance losses. The top is powder-coated aluminum so a magnet mount would not work directly. Ideas I've though of:

1. I could put each magnet mount on top of a small steel plate or bar held by to the roof by adhesive.
2. I could also install the manufacturer's aluminum accessory tracks, which attack to the roof via rivet nuts. I could mount the magnetic mounts on these somehow, but then they'd be off the ground plane by an inch or so.

With either idea, depending on hardware details, there may or may not be a direct electrical connection between the antenna and the ground plane.

Do people see any workable solution here, other than just drilling the holes for the NMO mounts?

 

[mmckenna]

Mounting directly to/through the ground plane is the way to go.

Yeah, there's lots of ways you could do this and save drilling a hole, but I don't think any of your proposed ideas offer a good trade off.

1. I could put each magnet mount on top of a small steel plate or bar held by to the roof by adhesive.

That would not provide a direct connection to the ground plane. You'd have capacitive coupling to the steel plate, then capacitive coupling to the aluminum ground plane. It'd probably work well enough, but if your antenna is not tuned really well, it'll light up the coax with RF.
I'd also be concerned about corrosion, rusting, damage to the powder coating, possible electrolysis, etc. The electrolysis will cause all kinds of issues when hit with RF.

2. I could also install the manufacturer's aluminum accessory tracks, which attack to the roof via rivet nuts. I could mount the magnetic mounts on these somehow, but then they'd be off the ground plane by an inch or so.

Mounting the antennas hovering above the ground plane results in a poor ground plane. It think you'd see a distinct impact on performance.

Do people see any workable solution here, other than just drilling the holes for the NMO mounts?

Your roof, your antennas. But I don't see a good trade off on either of those.
You didn't say what kind of vehicle this was, that may play into this. Any sort of marine application, and dissimilar metals/corrosion becomes the challenge.
UTV/Off road, durability becomes the concern.

There really is a benefit to drilling a hole and doing the install correctly. Having a direct connection to the ground plane, the antenna base right on top of the ground plane, and everything sealed up with the correct mount will give you a lot longer service life and greatly reduce issues.

 

[jeepsandradios]

Personally I just drill holes. My last work truck had an aluminum cap. Only thing I did not mine was put a small 1/8" plate under the skin to help with tree damage. Going up some of hte mountain roads to tower sites damged a few of the mounts on a previous truck. Now I have a fiber glass cap but with a rack on it to mount antenna's.

 

[Abies]

Mounting directly to/through the ground plane is the way to go.

Yeah, there's lots of ways you could do this and save drilling a hole, but I don't think any of your proposed ideas offer a good trade off.


That would not provide a direct connection to the ground plane. You'd have capacitive coupling to the steel plate, then capacitive coupling to the aluminum ground plane. It'd probably work well enough, but if your antenna is not tuned really well, it'll light up the coax with RF.
I'd also be concerned about corrosion, rusting, damage to the powder coating, possible electrolysis, etc. The electrolysis will cause all kinds of issues when hit with RF.



Mounting the antennas hovering above the ground plane results in a poor ground plane. It think you'd see a distinct impact on performance.



Your roof, your antennas. But I don't see a good trade off on either of those.
You didn't say what kind of vehicle this was, that may play into this. Any sort of marine application, and dissimilar metals/corrosion becomes the challenge.
UTV/Off road, durability becomes the concern.

There really is a benefit to drilling a hole and doing the install correctly. Having a direct connection to the ground plane, the antenna base right on top of the ground plane, and everything sealed up with the correct mount will give you a lot longer service life and greatly reduce issues.

Thank you so much! I appreciate your thorough answer.

I wanted to get a feel for the tradeoffs, which not having a deep technical background I didn't really have. It felt like I was juggling a lot of disconnected facts with no sense of their relative importance, plus some knowledge gaps.

The vehicle is a Ford Bronco (the new one). It goes off-road so durability is a concern. I just read this morning that another Bronco owner installed an FTM-500 in his rig and ended up with a lot of interference with the vehicle's electronics. Chokes didn't help. I have to wonder if his coax was radiating, which again relates to your answer. I don't know what his antenna setup was.

Looks like the new top will be getting some holes.

 

[AK9R]

A friend has a Chinese dual-band mobile in his Bronco with plastic roof. He hasn't mentioned getting interference from the vehicle electronics. OTOH, his transmitted audio doesn't sound great.

 

[Abies]

A friend has a Chinese dual-band mobile in his Bronco with plastic roof. He hasn't mentioned getting interference from the vehicle electronics. OTOH, his transmitted audio doesn't sound great.

This was interference in the other direction, with the vehicle's electronics malfunctioning.

In any case, I'm seeing virtually no good ground plane antenna installations on Broncos because they don't have metal roofs. Before I decided on the aftermarket metal top, I was going to install a roof rack and retrofit it with metal screen of some sort to create a ground plane. I haven't seen anybody do that either.

 

[mmckenna]

Thank you so much! I appreciate your thorough answer.

Happy I could help.

The vehicle is a Ford Bronco (the new one). It goes off-road so durability is a concern. I just read this morning that another Bronco owner installed an FTM-500 in his rig and ended up with a lot of interference with the vehicle's electronics. Chokes didn't help. I have to wonder if his coax was radiating, which again relates to your answer. I don't know what his antenna setup was.

There's a ton of ways this can be happening.
Where, exactly, they pick up power for the radio matters.
Quality of coaxial cable, connectors, mounts, antenna location, it all matters.

Hams sometimes have a tendency to cut corners on installation, then spend tons of time and money chasing issues with chokes, DC power filters and a bunch of other gimmicks. Often it turns into an "anything but do it correctly" approach.

I've been doing mobile installs for a few decades now, not just my own amateur stuff, but at work on public works/public safety/police vehicles. It is imperative that the installation be done correctly and using proper techniques, or one will spend weeks chasing down nagging issues like this. Always, always better to do it right the first time.

Looks like the new top will be getting some holes.

It really does make a difference. Not only performance, but also durability and looks. I know it's often hard to drill a hole in a perfectly good vehicle, but once you do it right, you'll never go back.

 

[Abies]

It turns out that the Bronco owner in question mounted a COMPACtenna just outside the driver-side A-pillar. No wonder it interfered with the vehicle's electronics. He later changed to a lower-power GMRS setup with the antenna on the bull bar above the bumper, in part in an attempt to reduce his exposure to RF.

 

[mmckenna]

It turns out that the Bronco owner in question mounted a COMPACtenna just outside the driver-side A-pillar. No wonder it interfered with the vehicle's electronics. He later changed to a lower-power GMRS setup with the antenna on the bull bar above the bumper, in part in an attempt to reduce his exposure to RF.

Yeah, about what I'd expect from most. Lack of good ground plane, compromise mount, it often leads to trouble.

But for most hobbyists, the criticality of communications isn't the same as public safety, so those sorts of mounting arrangements fall squarely in to the "good enough" category. Most off road trail type use need a relatively short range. Getting spread out on the trail is the issue, but rarely are people that far apart.

In other words, if it works, it works. Only challenge is the RFI issues.