Help w/ Antenna placement (Fiberglass roof, etc.)

[Herbie]

OK radio experts. I need help determining where to mount a couple of antennae on my van. The difficulty comes from the GTRV fiberglass pop-top I've mounted that covers the entire roof structure.

Beyond just covering the roof in (movable) fiberglass, there's also a big hole cut out from the center of the roof (highlighted in pink below), so while I'm considering some sort of mount that sticks out from under the pop-top, I'm worried about ground plane.

Pink area approximates the hole in the roof. Green blocks indicate possibly-convenient mounting locations, but none is ideal. The rearmost location would be tricky because of the CHMSL that sticks up above the roof and the top snugs down to it. The front-right corner is where the AM/FM antenna is currently - willing to consider losing/relocating this. The front-left corner would be OK if I used a removable antenna, but would create visibility issues for daily-driving. The driver's-side center spot is where there are already a couple of surplus holes in the roof that would be convenient, but obviously close to the hole in the groundplane. The other location I didn't mark would be left-rear corner. Right-side roof mounting is mostly out because of the awning (grey box).

For any of the roof-mount locations I'd need either a folding or quick-disconnect base, due to clearance issues. Bumper-mounts seem like a bad idea because the van body will block most/all of any antenna.

For the near-term I'll want to mount at least a CB antenna, but long term I'll add (or substitute) a 2m/70cm, as soon as I can afford a radio.

I was thinking of using my existing Firestik CB antenna on a hood/channel mount in the stock AM/FM location and deleting the AM/FM in favor of the AR-1A that would let me (marginally) use the CB antenna for AM/FM reception. I live in an area with strong commercial radio signal so I'd be willing to tolerate reduced performance from the AM/FM path if the CB antenna would function OK this way...

Thoughts? Other ideas? Thanks!!

 

[rusty_tlc]

I would lean to a hatch mount with a 1/2 wave antenna.

 

[xtatik]

^wut he sed.
Don't get too caught up in the jibba-jabba with FM antennas. A well grounded mount(to the lift/swing hatch) is all you'd need. At the most, you could check for continuity twixt the hatch and body. If it's good, it's usually good enough for a small FM antenna. If you want added assurance for the counterpoise, you could run a braid strap spanning the hinges in order to better bond the hatch to the body.
A Diamond K400 type mount and your on the air.

 

[Herbie]

I'll have to check the arc of the rear hatch relative to the pop-top, it seems likely a hatch-mounted antenna would interfere with the pop-top/tent when it was raised, since even at the aft end the top comes up 12" or more.

If that location seems workable, though, then I'll look a little harder at what it would take to mount directly to the roof in that area. A "Z" shaped bracket that put the antenna base above the CHMSL might not be that hard to pull off. An offset mount that put the antenna to the left or right of the CHMSL wouldn't be symmetrical but would almost certainly be doable.

After a bit more research I'm definitely staying away from the splitter arrangement for the AM/FM antenna. Worst case, if I need to use the OE location for a CB, I'll get a separate universal window-mount AM/FM antenna and put it up behind the rear-view mirror.

EDIT: Found a picture of the top up and the hatch open, and pretty sure any antenna mounted to the hatch would foul on the top.

 

[nemoaz]

The antenna won't work as well without a metal ground plane. That's a fact. http://www.firestik.com/Tech_Docs/GP-or-NGP.htm That's why all the serious hammers have cars that look like porcupines with all those antennae down in the middle and emergency professionals do the same. On the top of our fiberglass ambulance roofs, we always mounted a piece of metal under the antennae. It was as simple at pop riveting them in place. You'll need a ground wire too.

A ground plane is only important if you actually want people to hear you at any distance. If you just plan on talking to the vehicle just in front of you or just behind you, it won't matter. Of course, for that you could use an handheld radio.

Window mounts blow. At best they don't reflect as much power and they are unreliable too boot. We have them at work for some of our low profile applications and they are a constant pain in the ********.

Out of the options you mention, I'd probably choose the fender mount. There is a stile that just screws on the lip and give you the place to mount. It's not a bad location. It will definitely reflect and receive better to the front but there isn't as much to the back as you'd think. The glass and most of the interior is no obstacle to the radiowaves anyway. The hatch mount or even bumper mount (especially if you raised it some) would work ok.

 

[4RunAmok]

Go find an 18"x18" piece of copper, affix it to the inside of the fiberglass, drill through and mount a Diamond NMO surface mount.

Removable, and now has a nice ground plane.

 

[1911]

I have a similar problem (fiberglass roof) 0n one of my Toyotas. As has been said, a 1/2-wave 2m antenna is more forgiving of lack of ground plane. On the CB side, so is the Firestik FireFly and the Wilson Flex. Put them wherever is most convenient.

 

[Herbie]

Go find an 18"x18" piece of copper, affix it to the inside of the fiberglass, drill through and mount a Diamond NMO surface mount.

Removable, and now has a nice ground plane.

I had thought about that, and I WILL do something like that if/when I ever redo the headliner in the pop-top, but it currently has the headliner glued in place and it's in decent shape, so I won't be redoing the underside.

OTOH, I did entertain the idea of attaching a plate to the TOPSIDE. If could be reasonable assured of making the penetration weather-tight, affixing a sheet of some sort to the outside/top wouldn't be out of the question....

Thanks for the help everyone!

 

[xtatik]

I had thought about that, and I WILL do something like that if/when I ever redo the headliner in the pop-top, but it currently has the headliner glued in place and it's in decent shape, so I won't be redoing the underside.

OTOH, I did entertain the idea of attaching a plate to the TOPSIDE. If could be reasonable assured of making the penetration weather-tight, affixing a sheet of some sort to the outside/top wouldn't be out of the question....

Thanks for the help everyone!

Doesn't need to be a "plate".....Four very small gauge wires sandwiched under fiberglass tape on top of the cover would serve just the same. Whether it's a plate or radials the radiating element of the antenna won't see it any differently. If done on the underside of the cover (interior) you could leave a cople of inches of tag end (furthest from center) exposed. This would allow you to not only tune the radiating (vertical) element, but you could also tune the radials by simply trimming or lengthening them.
Being mounted to the fiberglass and being removed from the bulk of the metal portions of the vehicle, your vertical antenna is going to function as an "elevated vertical". Elevated verticals require the counterpoise (elevated radials) to be tuned as well as the vertical radiator in order to function properly.
Given your situation and wanting to keep it simple......I'd look for a method to mount off the side of you rear corner pillar. With an FM antenna there will be NO noticeable difference at either end of a transmission between mounting there or being mounted ********** in the center of your roof. If you were mounting an HF antenna it would begin to get more complicated. I haven't looked around but I'm sure someone makes a "bulkhead" type mount that will allow an approximate 90 degree antenna adjustment from the mount angle.
I'll have a look around the interwebs.

 

[gary in ohio]

Doesn't need to be a "plate".....Four very small gauge wires sandwiched under fiberglass tape on top of the cover would serve just the same.

Check the marine stores, you can find thin adhesive backed copper pads that can be used as well.

 

[oO_Rogue_Oo]

Given your situation and wanting to keep it simple......I'd look for a method to mount off the side of you rear corner pillar. With an FM antenna there will be NO noticeable difference at either end of a transmission between mounting there or being mounted ********** in the center of your roof. If you were mounting an HF antenna it would begin to get more complicated. I haven't looked around but I'm sure someone makes a "bulkhead" type mount that will allow an approximate 90 degree antenna adjustment from the mount angle.
I'll have a look around the interwebs.

I agree; I would do the same. Check with Hank Breedlove for a mount; if he doesn't already have one that will work he will custom machine one to your needs. The quality is second to none and he is fast and a great guy on top of all that.
http://bellsouthpwp.net/b/r/breedlove1/index.html

 

[xtatik]

I agree; I would do the same. Check with Hank Breedlove for a mount; if he doesn't already have one that will work he will custom machine one to your needs. The quality is second to none and he is fast and a great guy on top of all that.
http://bellsouthpwp.net/b/r/breedlove1/index.html

Thanks for hunting that down. I didn't know if Hank was still in the bidness. I know there used to be links to his site from K0BG's site, but I hadn't visited there in a while. He may be able to machine a "right-sized" ball mount for a UHF (239) type attachment rather than the 3/8 stud. He makes very sturdy stuff.

 

[xtatik]

Check the marine stores, you can find thin adhesive backed copper pads that can be used as well.

Good point Gary,
My local West Marine has copper foil tape (a bit too wide) that could be easily cut into narrower strips if needed. I'm not sure if it has an adhesive back, but that would be easily dealt with.

 

[DaveInDenver]

Good point Gary,
My local West Marine has copper foil tape (a bit too wide) that could be easily cut into narrower strips if needed. I'm not sure if it has an adhesive back, but that would be easily dealt with.

I've used 3M #1181 with success providing a return under a 70cm antenna. This is adhesive backed, 1/2" wide copper, 3 mil thick foil tape. We get the stuff by the case at work, so I usually piggy back one when they order it over in manufacturing and it ends up around $15 for 18 yards (I use it when building things to seal RF leaks). One thing to be aware of with adhesive foils is that the sticky side has distinctly worse conductivity than the other side. If you really work it with an orange stick or your fingernails the adhesive side will make OK contact.

 

[4RunAmok]

In terms of weather-proofing my rooftop NMO mounts... I chose the part with this in mind.

There are essentially two NMO mounts made for going through a surface easily available. One made by Larsen Antenna, which requires a 3/4" hole in the surface, and the other made by Diamond, which requires only a 3/8" hole in the surface.

I bought both.

I chose to use the diamond (http://www.hamradio.com/detail.cfm?pid=H0-009384) because theirs comes with an O-ring on the outer edge of the exterior piece, and it uses the 3/8" hole... going on the logic of smaller hole, less problems, besides, how the heck was I going to get a decent 3/4" hole in my sheet metal without mucking it up somehow! (You would have less problem with this, but still, smaller hole logic applies)

Not only did I pack the underside of the mount with Vaseline, and put Vaseline on the O-ring, but I also put Loc-Tite of the waterproof variety on the threads, and tightened it down and let the Loc-Tite dry.

I also keep silicon gel lube on the rain caps and the antennas themselves, just to keep the water out, and to keep the threads lubed.

I'm on 5 years (almost to the day! wow!) with TWO of these mounts on top, and not a single drip has made it through.