Fiberglass Roof Ground Plane

[chassed]

Hi All!!

So I wanted to move the antenna on our fire depts Tanker to the roof of the trucks cab but the roof is fiberglass. So I need to make a ground plane for the antenna. I did some searches and found that I can put sheet metal on the inside to do this. But I wanted to see what you guys think of using Aluminum HVAC tape to create the ground plane? Or would copper tape work better?

Also I am assuming I will need Thick Surface NMO mount. Anyone have an idea how thick the fiberglass normally is? The truck is a Peterbilt Cabover.

Any help would be great!
Thanks

 

[Project25_MASTR]

Thick surface is generally needed for fiberglass roofs. If a NGP antenna can be found, it’s generally the easier solution.


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[prcguy]

I've done lots of antenna installs on fiberglass vehicles and campers using wide aluminum HVAC tape for CB, VHF and UHF. At the minimum try to make an X 1/2 wavelength across so you have four 1/4 wavelength radials if space permits and mount the antenna in the middle of the X. More radials is better, especially if you don't have space for full 1/4 wavelength radials.

I would also go a step further and put a common mode RF choke in the feed line. For VHF/UHF get a large #43 mix ferrite bead, either snap on or solid with at least a 1/2" hole through it, then wrap 3 passes of the coax through the hole for VHF or UHF. You can place the choke right at the mount or if space permits place it 1/4 wavelength down the coax from the mount.

 

[mmckenna]

I've done the aluminum and copper tape thing before.

Trouble I had was this was on a UTV roof. The roof was heavy/thick plastic. Constant vibration and heat/cool cycles caused the tape to eventually tear and start to come apart. You either need thick enough tape to not do that, or look at 1/2 wave antennas (which I eventually did).

You can purchase copper flashing rolls from hardware stores pretty inexpensively. Attaching that to the roof will work, but you'll need something strong enough to hold it in place. Drilling through/into the roof in enough places to do this is going to be problematic.

Depending on what frequencies you run, it can be pretty easy. Some of the larger antenna manufacturers produce pre-made/pre-drilled metal disks, but they are around 6", so really only good for UHF and higher frequencies. You could certainly make a VHF version.

 

[Project25_MASTR]

I've done the aluminum and copper tape thing before.

Trouble I had was this was on a UTV roof. The roof was heavy/thick plastic. Constant vibration and heat/cool cycles caused the tape to eventually tear and start to come apart. You either need thick enough tape to not do that, or look at 1/2 wave antennas (which I eventually did).

You can purchase copper flashing rolls from hardware stores pretty inexpensively. Attaching that to the roof will work, but you'll need something strong enough to hold it in place. Drilling through/into the roof in enough places to do this is going to be problematic.

Depending on what frequencies you run, it can be pretty easy. Some of the larger antenna manufacturers produce pre-made/pre-drilled metal disks, but they are around 6", so really only good for UHF and higher frequencies. You could certainly make a VHF version.

Go find someone who does commercial AC duct work (with the round stuff). They generally have 18”-20” blanks laying around. Last time I acquired some, they were 18 ga with center punches in place. Got 20 of them for $5. Made good targets as well as ground planes…



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[mmckenna]

Go find someone who does commercial AC duct work (with the round stuff). They generally have 18”-20” blanks laying around. Last time I acquired some, they were 18 ga with center punches in place. Got 20 of them for $5. Made good targets as well as ground planes…

That's a good idea, both for ground planes and targets…

At one point I did get a sheet of 20ga steel and used that, but I got tired of the vibration and chose to remove the roof from the UTV all together and moved the antenna to a mount I fabricated on the back of the roll cage.

 

[chassed]

Depending on what frequencies you run, it can be pretty easy. Some of the larger antenna manufacturers produce pre-made/pre-drilled metal disks, but they are around 6", so really only good for UHF and higher frequencies. You could certainly make a VHF version.

We are VHF. I am thinking of buying this disk. https://www.theantennafarm.com/catalog/pctel-k332-5384.html?zenid=bfadb07952651e29bd4fa1c0a49f35d9

 

[mmckenna]

We are VHF. I am thinking of buying this disk. https://www.theantennafarm.com/catalog/pctel-k332-5384.html?zenid=bfadb07952651e29bd4fa1c0a49f35d9

That's the one I was talking about.
It's only 6" in diameter which is suitable for UHF frequencies, but much too small for VHF. You need 18 inches for VHF public safety frequencies.

 

[prcguy]

You can make the tape permanent by rolling on a layer of surfboard resin. Clean and degrease the side you will be applying tape to thoroughly, then apply tape and install antenna mount. After the grounding to the tape is proven to be reliable, use a small roller to apply a layer of resin and your done.

I've had friends do something similar on a fiberglass camper where they installed an antenna mount in the middle of the roof, then with the camper upside down they completely lined the underside of the roof with aluminum window screen and a few layers of surfboard resin.
prcguy

I've done the aluminum and copper tape thing before.

Trouble I had was this was on a UTV roof. The roof was heavy/thick plastic. Constant vibration and heat/cool cycles caused the tape to eventually tear and start to come apart. You either need thick enough tape to not do that, or look at 1/2 wave antennas (which I eventually did).

You can purchase copper flashing rolls from hardware stores pretty inexpensively. Attaching that to the roof will work, but you'll need something strong enough to hold it in place. Drilling through/into the roof in enough places to do this is going to be problematic.

Depending on what frequencies you run, it can be pretty easy. Some of the larger antenna manufacturers produce pre-made/pre-drilled metal disks, but they are around 6", so really only good for UHF and higher frequencies. You could certainly make a VHF version.

 

[SteveC0625]

Also I am assuming I will need Thick Surface NMO mount. Anyone have an idea how thick the fiberglass normally is? The truck is a Peterbilt Cabover.

It's hard to guess the thickness of a fiberglass truck roof remotely. I would suggest that you first decide whether you are going to use a 3/8" or 3/4" NMO. They're both available in thick surface versions. There's also some different thickness thick surface mounts available. Source your mount first so you know you can get the right one. Then drill the hole and measure the thickness. Put a temporary plug in it and order the mount.

If you go the tape route for a ground plane, you probably won't have worry about the added thickness, but it you use a metal disc, you might. Verify the thickness of the disc and add it to the roof thickness to be sure.

 

[n0nhp]

Granted they were on Concrete mixer trucks that take acid baths and lots of water to clean etc. so we wanted very sturdy mounts. We had the local sheet metal specialty shop make up some stainless steel 24ga plates with mild breaks to each corner for structural strength and pop the 3/4 hole in the center. I used zinc plated self tappers 3 to a side and RTV underneath. Drilled a 3/4" hole in the fiberglass center of the roof for coax and antenna base, standard NMO mount to the SS, glue and screw the plate to the roof and never had RF or mounting problems.
The deep NMO mounts to fiberglass I used on a pair of tow trucks were always a problem no matter what I tried with aluminum and copper tape and ferrites. We eventually convinced them to go to the SS plates that solved the problem.

Bruce

 

[lmrtek]

For VHF, you will need at least 20 inches of ground plane in all directions since ground planes must be at least 10% larger than 1\4
wavelength.

 

[cmdrwill]

For VHF, you will need at least 20 inches of ground plane in all directions since ground planes must be at least 10% larger than 1\4 wavelength.

Exactly..

 

[prcguy]

The 5% or 10% longer for ground radials is an old myth, they should be 1/4 wavelength resonant if possible. That provides an exact 1/2 wavelength round trip from the feedpoint to the end of the ground radial and back to the feedpoint, mimicking the feedpoint impedance.
prcguy

For VHF, you will need at least 20 inches of ground plane in all directions since ground planes must be at least 10% larger than 1\4
wavelength.

 

[madrabbitt]

All the fiberglass truck roofs i've drilled in recent memory were about 1/2 inch thick, maybe less.

The NMO mount we used was LARSEN NMOKHFCXTHK

 

[BushDoctor]

Fire truck antennas

Hi All!!

So I wanted to move the antenna on our fire depts Tanker to the roof of the trucks cab but the roof is fiberglass. So I need to make a ground plane for the antenna. I did some searches and found that I can put sheet metal on the inside to do this. But I wanted to see what you guys think of using Aluminum HVAC tape to create the ground plane? Or would copper tape work better?

Also I am assuming I will need Thick Surface NMO mount. Anyone have an idea how thick the fiberglass normally is? The truck is a Peterbilt Cabover.

Any help would be great!
Thanks

Back in the 50s when i worked for a motorola shop i use to use a motorla TAD 6042 coxial antennas side mounted on a flat vertical surface for cabless engines or trucks with the ladder resting on the cab roof they worked fine and we didnt use a repeater at that time for the FD. maybe a motorola shop can assist you. As i remember the mounts were 2 pieces of flat metal with the centers concaved to wrap around the bottom of the antenna to mount it in 2 places with 2 nuts and bolts each mount i never had any problems the FD was on 154.43 other antennas were used for 450 band same design but shorter. I have noticed our local dept here in strasburg,va has trouble hearing an engine with a nmo mount 453.3 1/4 wave mounted on a metal roof 3 inches from a ladder i happened to see a tech workin g on it checking fwd pwr one day i told him he needs to check reflected pwr with an antenna that close to a ladder he replied all i worry about is power out he was young fellow so i just went on my way. It is still noisy some 6 years later. Practical knowledge helped me a lot seems like not so much nowadays for younger techs