Selecting a VHF antenna for fiberglass bodied bus

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KC9MDQ
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Selecting a VHF antenna for fiberglass bodied bus

Post by KC9MDQ »

Heya gang.

I'm looking for a little input. I have a 1997 Ford E-350 with a fiberglass Eldorado Aerotech bus chassis that I'm converting into a motorhome. I just got my 110w Spectra installed and I'm trying to decide what story do for an antenna. My two work buses have the obligatory 1/4 wave on an L bracket hung on the driver's side fender. I'm wanting to get the antenna on my rig a bit higher, like on the roof. The pickle here is that the bus coachwork is all fiberglass, so there's no ground plane. I already have all of the stuff to do a fender mount like the work rig, but was hoping for elevation. I also have a mirror mount setup that I was going to use for a secondary VHF radio, or maybe a hammy dual band.

What are the thoughts of the collective?


PS, I'm not shy about drilling holes.
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Bill_G
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by Bill_G »

An end fed half wave will operate without a ground plane, and if has one, it exhibits some gain.
jpu535
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by jpu535 »

Larsen, aka Pulse Electronics, used to have a no ground plane antenna. I personally never had any luck with them. The one's I did install were cable length and cable run sensitive. It was a chore to get the correct lead dress and even when I had it tuned as soon as the head liner went back up it went out of tune. Your best bet might be to get a piece of sheet metal, cut it to 20 square and glue it to the underside of the fiberglass roof then install a standard antenna mount through a hole in the center of the sheet metal.
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Bill_G
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by Bill_G »

That is a problem, but a balun fixes it. Three neat wraps of RG58 in a two inch circle as close to the base as possible generally takes care of it.
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KC9MDQ
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by KC9MDQ »

So, maybe a 5/8 wave NMO on sheetmetal secured to the roof?
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Bill_G
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by Bill_G »

Worth a try. Lay the metal up there first. Put a mag mount on it, and see what the vswr looks like. Then you can dial in the size of the metal.
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SteveC0625
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by SteveC0625 »

In the old days, it was pretty much standard practice to get a piece of copper window screen material and fasten it to the underside of the roof to create a ground plane. You want at least 1/4 wave in all directions for decent radiation and pattern. For VHF, about 36" diameter will work fine unless it's lower end like the ham band. Then you'd want nearly 40" in diameter. Make sure the antenna mount makes good contact with the ground plane and you're good to go. We used it successfully on fiberglass bodied vehicles and in boats.

If your bus has a headliner, drop it, install the screen and antenna, and replace.
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KC9MDQ
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by KC9MDQ »

Negative on the headliner. It's a two piece fiberglass clamshell on the inside, and the outside layer is a corrugated fiberglass with an exterior fiberglass shell bonded to it. The layers are about an inch and three quarter thick, so I'm really scratching my head about how to go about this.

These have all been really good ideas that simply aren't going to work with the way the damned thing is built. It looks like the metal fastened to the roof is still the most viable option.

I may just say "f*** it!" and do the L bracket off of the fender.

If anybody comes up with anything else, I'm open to suggestions. Thanks for all the good ideas so far.
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Bill_G
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by Bill_G »

Yep. Know exactly what you're talking about. If you can access the wind dam area above the windshield, and it hasn't been kitted out as a storage compartment, you can easily get to the inside of the roof. Or, you can try the cavity above the driver - usually has electrical panels, but sometimes has storage compartment "sleeve with door" that can come out. Then, there's the area above the door mechanism.
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SteveC0625
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by SteveC0625 »

Is this a cut-away van cab and chassis with a module on the back or a chassis with the entire body added by the coach builder?
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Bill_G
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by Bill_G »

SteveC0625 wrote:Is this a cut-away van cab and chassis with a module on the back or a chassis with the entire body added by the coach builder?
He said it's an Eldorado Aerotech which is a chopped van that keeps the forward section minus the roof and passenger door. Pretty common coach, but comes in a variety of configurations. Roof access is difficult because the top was meant to take some abuse from low overhang.
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SteveC0625
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by SteveC0625 »

A lot of radio shops will just park a VHF spike in the center of the cab roof of Type I and III ambulances and similar vehicles. It creates some directionality but seems to work well except in remote fringe areas.

With the fiberglass coach, the directionality should be less for a cab mounred antenna.
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fineshot1
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by fineshot1 »

KC9MDQ wrote:Negative on the headliner. It's a two piece fiberglass clamshell on the inside, and the outside layer is a corrugated fiberglass with an exterior fiberglass shell bonded to it. The layers are about an inch and three quarter thick, so I'm really scratching my head about how to go about this.

These have all been really good ideas that simply aren't going to work with the way the damned thing is built. It looks like the metal fastened to the roof is still the most viable option.

I may just say "f*** it!" and do the L bracket off of the fender.

If anybody comes up with anything else, I'm open to suggestions. Thanks for all the good ideas so far.
Yes - this type of a bus can present a problem. If the mirrors on each side are able to handle
it perhaps a mirror mount will have to do. I know it is not optimum but it may be your only
practical choice. This is sometimes what i must do at work but we have a uhf simulcast system
and this works well on that but you may have to try it on vhf perhaps with a 1/4 wave nmo mount
mated to the mirror mount and route the cable inside anyway you can.
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radiokid
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by radiokid »

I have used Maxrad 1/2 wave no ground on semi tractor and combines with great luck.
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KC9MDQ
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by KC9MDQ »

Well, since I already have a 1/4 wave and a 5/8 wave NMO, I think I'll pick up one of the 1/2 wave antennas and give it a shot off of the L bracket. When warm weather returns, I think I'll try the hole in the end cap and brass grid ground plane method with the 5/8. Thanks for the suggestions, everyone.

It's great to see so many ideas and thoughts floating around here.
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KC9MDQ
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by KC9MDQ »

Image Here's the rig in question
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fineshot1
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by fineshot1 »

Please post the results when your done - I'd like to see you successful
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Andy Corbin
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by Andy Corbin »

My concerns from looking at the picture are the high profile nature of the vehicle. Tree limbs and even some utility lines are going to be banging the hell out of the antenna and may ultimately rip the antenna and the mount right out of the fiberglass. I would consider some sort of "L" bracket u-bolted onto one of the exterior mirrors or a trunk groove mount between the hood and the fender. If the vehicle will be used in an area with no overhead objects, a roof mount, on the end cap or even center roof might be OK. Just my $.02 worth.

AC
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fineshot1
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by fineshot1 »

The area at the top of the wheel chair access doors looks like it may possibly
be metal. If so that area may be ok to drill and mount a NMO mount with a 1/4
wave wip.
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Wiregeek
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by Wiregeek »

Just noticed the bus, we've been forced to go with the groove mount on the fender as described above. It's for 850 mhz trunked, and works OK. I'd hate to think about VHF into that vehicle, but I'd probably go with the fender mount and see how I did.
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by Satelite »

Hello:
I read you decided a option of mount but I just noticed in the picture you had 5 orange roof clearance lights.
Any chance removing the center one and getting access between the two roof and ceiling and drilling nmo mount hole aways back and feeding the coax forward to clearance light area and pushed to one side or the other to gain access to inside cab to radio ?
Might be able to with a bit of work get a pc of rolled up metal ground plane in through clearance light opening and unrolling it inside the roof space although ill admit it wont be easy but where theres a will theres a way.
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jpu535
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by jpu535 »

How about one of those trucker style clamps they use on west coast style mirrors. You can clamp it to the fender mounted mirror on the passenger side then screw a no ground plane antenna to that.
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KC9MDQ
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Re: Selecting a VHF antenna for fiberglass bodied bus

Post by KC9MDQ »

Jpu, that's exactly what I wound up doing. It seems ok for now, but I am definitely going to give the screen and hole method a try when it's warmer. I'll post a pic later.
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