Ground Plane Dumb Question

[Vanishing Point]

I'm just double checking before I make a bigger purchase.

I'm mounting my roof rack and as part of the project it is going to double as a ground plane for my two way radios. The frequencies I plan to operate are 150 mhz (2 meters) and GRMS (around 400 mhz or 3/4 of a meter). So I'm going to install a sheet of aluminum on the roof rack. I could go with a solid sheet (cheaper, easier to work with like cutting holes in, better shade for my roof keeping summer temperatures down) or a perforated sheet ( lighter, IMO would look better, easier to keep looking clean). The perforations would be small (depending 3/16 to 1/2 inch). The perforations at those frequencies aren't going to affect the function as a ground plane are they?

The sheet will be 44.5 inches by approximately 54 inches and should be plenty big enough as a ground plane for the 150 mhz radio. Yes, I could use radials but shading my roof (that black roof heats up the truck in summer and even a perforatedsheet would help with that) and being able to place smaller boxes on the roof are advantages to having a more continuous structure.

Thanks

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[Radio Guy]

No difference in performance with or without perforations as you describe for a 2m or GMRS ground plane. The size you mention should work great, way more than adequate for both bands. Its best if you mount the GMRS antenna with at least 6" to the edge and the 2m at least 19" to the edge of the ground plane while keeping the antennas separate as much as possible.

What make/model antennas are you using?

 

[Vanishing Point]

No difference in performance with or without perforations as you describe for a 2m or GMRS ground plane. The size you mention should work great, way more than adequate for both bands. Its best if you mount the GMRS antenna with at least 6" to the edge and the 2m at least 19" to the edge of the ground plane while keeping the antennas separate as much as possible.

What make/model antennas are you using?

Thanks. That will help ease my mind. A precut sheet of aluminum isn't cheap and though I understand the theory (I got my HAM license last summer) I'm not a radio guy and antennas weren't easy to understand. I didn't want to miss some sort of weird effect the holes might have.

Haven't decided on the make or model of the antennas but the requirements for the 150 mhz one will be 1/4 wave and the ability to take it off with a wrench quickly so I can get into parking garages without it scraping. That's going to be one of my main requirements. The GMRS one will be short enough that I won't have to worry too much. My truck isn't lifted.

I hadn't thought too much about how far apart to mount them but the VHF antenna will have fewer choices. It will need to be near the midline of the truck. The GMRS can go near just about any corner of the roof rack. That should keep them far enough apart.

Again thanks for your help.

 

[Radio Guy]

Thanks. That will help ease my mind. A precut sheet of aluminum isn't cheap and though I understand the theory (I got my HAM license last summer) I'm not a radio guy and antennas weren't easy to understand. I didn't want to miss some sort of weird effect the holes might have.

Haven't decided on the make or model of the antennas but the requirements for the 150 mhz one will be 1/4 wave and the ability to take it off with a wrench quickly so I can get into parking garages without it scraping. That's going to be one of my main requirements. The GMRS one will be short enough that I won't have to worry too much. My truck isn't lifted.

I hadn't thought too much about how far apart to mount them but the VHF antenna will have fewer choices. It will need to be near the midline of the truck. The GMRS can go near just about any corner of the roof rack. That should keep them far enough apart.

Again thanks for your help.

The reason I asked about antenna types is a small 6" 1/4 wave whip for GMRS will go unnoticed close to the 2m antenna. If the GMRS antenna is a larger stacked 5/8 over 5/8 colinear then it will affect the radiation pattern of the 2m antenna if its within about 6ft or less. If a GMRS antenna is more than about 2ft away from the 2m antenna it won't be affected much.

 

[Vanishing Point]

The reason I asked about antenna types is a small 6" 1/4 wave whip for GMRS will go unnoticed close to the 2m antenna. If the GMRS antenna is a larger stacked 5/8 over 5/8 colinear then it will affect the radiation pattern of the 2m antenna if its within about 6ft or less. If a GMRS antenna is more than about 2ft away from the 2m antenna it won't be affected much.

See, this is why i like asking questions. I figured one antenna might affect tge other.
Was going to use the 1/4 wavelength for both taking advantage of my ground plane. So the ground plane is going to be 44" x 54". I figure I'd set the VHF antenna towards the front app. 20" back and 22" in from each side. The GMRS antenna was going to go towards the back corner about 10" in from the side and from the back. This would mean the antennas would be about 27" apart. Is that going to be a problem? I could move the VHF antenna up one 1" I think and the gmrs back 3" so i could get them maximally out to about 31 inches apart. But from what you are saying having both be 1/4 wavelength the 27" separation should be fine?

 

[kb5zcr]

Either one of your plans will work fine. Truth be told, you won't notice any difference either way.
I like your idea of on the roof with ground plane, bigger than the hood but higher in the air.

 

[Radio Guy]

See, this is why i like asking questions. I figured one antenna might affect tge other.
Was going to use the 1/4 wavelength for both taking advantage of my ground plane. So the ground plane is going to be 44" x 54". I figure I'd set the VHF antenna towards the front app. 20" back and 22" in from each side. The GMRS antenna was going to go towards the back corner about 10" in from the side and from the back. This would mean the antennas would be about 27" apart. Is that going to be a problem? I could move the VHF antenna up one 1" I think and the gmrs back 3" so i could get them maximally out to about 31 inches apart. But from what you are saying having both be 1/4 wavelength the 27" separation should be fine?

That should be fine for 1/4 wave whips. If you plan on upgrading to a larger UHF antenna later you might consider the 19" and 6" spacings from the edge giving a little more space between antennas.

 

[Vanishing Point]

So I finally got the major parts of my ground plane project finished. As noted prior this will serve as a ground plane for 1/4 wave antennas for a GMRS radio and another 2 meter/VHF radio. The roofrack will also function as a roof rack if needed and the ground plane will act as shade for the roof keeping the temperature down inside during summer.

The area where the two radio antennas and the Sirius antenna go through the roof. So far no leaks. The Sirius antenna needed to be moved because the metal blocks it from working.

The front of the truck with the roofrack

The antennas and the ground plane. I decided to build a little extra platform for the GMRS antenna because I didn't want it in the "valley" of the roofrack and figured elevating it a few inches couldn't hurt its distance transmission.

It was pretty arduous sourcing parts. All the hardware that is not antenna is aluminum. Aluminum bolts, aluminum spacers, aluminum roofrack, aluminum sheets. I went with the aluminum roofrack because I'm older, working out of a garage, and therefore could use the lighter weight. All the other aluminum parts were to prevent galvanic corrosion processes.

 

[Vanishing Point]

Also I built hatches at each antenna mount and over the pass throughs in the roof so that if I needed to work on something in those areas I could remove the hatch and access the area. The pass throughs have not leaked even in the open and in heavy rain but if they do I'd like to be able to try some sealant or replacing a grommet without removing the whole roof panel.

 

[Radio Guy]

Very interesting and very overkill in many ways. Do I see coax running through various length of rigid conduit?

 

[Vanishing Point]

Very interesting and very overkill in many ways. Do I see coax running through various length of rigid conduit?

Aluminum tubing. Not purposed conduit. Keeps the coax from sagging and rubbing. If you run the antenna cable under the crossbars and hang it from hangers it gets pretty close to the roof. Then if you tighten that up to keep it off the roof you get force at each support where the wire has to turn, tension at the antenna base and tension at the pass throughs. This way it stays put and has no tension on it. I didn't want any tension at the pass throughs.

The only disadvantages to this are it might be more difficult to examine the cable if I need to, it weakens the crossbars to some degree (not that much as it's a hole and not a notch) at every point it passes through and it was sort of more work. Drilling holes in the aluminum cross bars and putting in a grommet and sliding in the aluminum tubing probably was not that much more work then drilling the same amount of holes and mounting hangers. Just bigger holes. And fewer nuts and bolts. Less fussing and adjusting the cable.

 

[Radio Guy]

Aluminum tubing. Not purposed conduit. Keeps the coax from sagging and rubbing. If you run the antenna cable under the crossbars and hang it from hangers it gets pretty close to the roof. Then if you tighten that up to keep it off the roof you get force at each support where the wire has to turn, tension at the antenna base and tension at the pass throughs. This way it stays put and has no tension on it. I didn't want any tension at the pass throughs.

The only disadvantages to this are it might be more difficult to examine the cable if I need to, it weakens the crossbars to some degree (not that much as it's a hole and not a notch) at every point it passes through and it was sort of more work. Drilling holes in the aluminum cross bars and putting in a grommet and sliding in the aluminum tubing probably was not that much more work then drilling the same amount of holes and mounting hangers. Just bigger holes. And fewer nuts and bolts. Less fussing and adjusting the cable.

The various chunks of aluminum tubing will also act as an RF choke on the coax. Not that you need it but its interesting, can't say I've ever seen that in a mobile install.

 

[Vanishing Point]

The various chunks of aluminum tubing will also act as an RF choke on the coax. Not that you need it but its interesting, can't say I've ever seen that in a mobile install.

I'm not an electronics genius and figure tens if not 100's of thousands of HAM and CB folks have run coaxial cables in their cars/trucks without any problems by just running the cable so this wasn't about building a better electronic mousetrap. I just wanted to build it and then over the years as I get older not have to fiddle with it too much. I didn't want to check years from now and find a low hang cable had been rubbing on the roof eventually wearing the cable and the fiberglass roof possibly. Also I figured side tension on the pass throughs might eventually oblong the holes through the grommets and cause leaks.

Most engineers I know will tell you that when you get non engineers (me) to engineer stuff it usually gets overbuilt. My rebuttal is that in a one off some extra cost and effort is not usually an issue. Inefficiencies and over use of materials in a small project aren't really a big deal. At least not to me.

 

[Capngeo]

I am contemplating putting an NMO mount and a No Ground Plane Antenna thru the roof just above the overhead speakers on the Alpine system..... How thick is the fiberglass? I need to order the NMO mount and don't want to get one too short.

Awesome build BTW!

 

[Vanishing Point]

I am contemplating putting an NMO mount and a No Ground Plane Antenna thru the roof just above the overhead speakers on the Alpine system..... How thick is the fiberglass? I need to order the NMO mount and don't want to get one too short.

Awesome build BTW!

I wish you had asked me while I drilling throught it. I would have measured. But I drilled through the Freedom Top and not the back part so I can only speak to that. Be aware that there are parts of the roof that are single layer and other parts that appear like they might be double layer with a space in between. I'd avoid those places as without a spacer any camping force will try to bring those spaced planes together. Not good as far as my non engineer brain can tell.

Given my memory I'd say the roof in areas that aren't spaced is about 3/8 inch thick. That would make sense as any kit car or small boat made out of fiberglass I've ever seen seems to rrun between 1/4 and 1/2 thick. A regular NMO mount won't work. They do make thicker NMO mounts for presumably fiberglass. Those would work and they require a smaller diameter hole I believe than a regular NMO mount.

Thank you for the kind words!

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