Will XM and GPS Antenna Work Under Cap?

[NJT5047]

Saw a post not too far back that indicated that XM and GPS antennas would work satisfactorily beneath the front fiberglass cap.   Want to verify that I was not hallucinating or conjuring up something??  I've got enough holes in the roof now.  Just mounted a permanent AM/FM antenna on the top of the cap, and have all the interior in that area removed.  Bought another XM antenna to mount on the bus.  The antenna doesn't have  good way to mount other than bed it in adhesive sealer.   If it'll work OK under the glass cap, I"ll mount it on the steel reinforcing structure and forget it. 
I  found that it will work in the garage (home, wood frame)...usually.  I don't know what the difference is, but sometimes it'll talk continuously after pulling into the garage?  Going thru the roof?  You can move a few inches and it'll quit..pull up and it loads up and talks again.  Yet, you can drive under trees and it'll take a crap.  Won't work when trees cover the area.  What's the difference between trees and fiberglass caps? 
I set a plastic bucket over the antenna and it continued to work....that may have answered my question. 
I believe this may have originated from Craig?  I'm going to check the archives and see if it turns up...but would like to get on with installing it this weekend. 
Thanks, JR

[Nick Badame Refrig/ACC]

JR,

Thoose pucks seem to work in alot of different installations but, I would keep them as clear as I could.

The less they are obstructed the less signal problems you would have under trees, stormy weather, ect...

Just makes sence.

Nick-

[DavidInWilmNC]

Hi JR,

I don't use XM; but I've had Sirius for about a year and a half.  As for mounting the antenna, I had thought to put mine under the cap like you're describing.  I've pretty much reconsidered and am probably going to mount it either on top of the front A/C housing or paint a small steel plate and glue that to the roof.  The magnet on my antenna is very strong and will hold it securely.  I'm not sure if the metallic paint on your cap (yours IS metallic, isn't it?) will interfere or not.  They do make RV/marine Sirius antennas, and I assume they do for XM as well.

As for the interruption in signal, I was told that it has to do with the moisture content in the object between you and the sat.  A tree, like you mentioned, will kill the signal, but a solid, dry object won't interfere as much.  I don't know if XM has terrestrial repeaters, but Sirius does.  Last year, when hunting for a bus, we were in Norfolk and went through an underwater tunnel.  We had perfect reception, which was cool.  Here in Wilmington, there are none.

Let us know how you mount it and how well it works!  See you in Timmonsville (hopefully) with my bus.

David

[NJT5047]

Thanks Nick and Dave,
AC mount is an excellent idea...if the AC wiring was easily accessable...which it ain't.  These are things that would have been worked out with a little more thought before I began this trek.   Nick's right about blocking it...probably every time the roof gets wet the XM would quit with antenna under the fiberglass...?
I may just drill a hole in the cap and bed it down.  As you say, the magnet is strong...and if XM has repeaters, I'm not aware...they may in major cities.   
I'm going to drill a hole and pray for no leaks.  The hole will have to be approx 1/2" to get the plug thru it.  These XM antenna plugs don't come apart.  Some do.   If it fails, I may get one of the marine type 10" antenna...what's another thingy sticking up!!    I've long since lost the smooth-top look.
Dave, glad to here you're coming to Timmonsville...bringing the bus?  I gotta check out your genset install.  I was going to look up your email and see if you were coming up.   Haven't heard much from you lately!
See you soon, JR

[Homegrowndiesel]

I have played with on top of roof and under cap, wet and dry and have not seen much difference.

Biggest problem I have found has been cheap antenna wire.

They want more for an antenna than I paid for the whole system.

Support your local BUSNUT.

Bill

[JackConrad]

JR,
  We have 2 GPS antennas and 1 XM antenna installed against the underside of our fiberglass front cap. When we were building the bus, we had 2" of spray foam applied to the bottom of the cap. When we were rerady to install the antennas, I took a knife and cut out a piece of foam that allowed a snug fit of the antenna against the bottom of the firgerglass cap and reinstalled the foam plug. They have worked flawlessly and no additional holes in the roof.  We have never lost a signal during a rain.  Remind me at Timmonsvile and I will show you the installation.  Jack

[Clarke Echols]

XM and GPS all run on microwave-frequency signals.  Anything that is "microwave safe" for your kitchen
will likely be OK for GPS and XM.

When microwaves hit water the water absorbs them.  So does fat and sugar solutions.  You have the
same problem with trees, rain and snow, other vegetation, physical obstacles, etc.  The reason you
can sometimes use them inside a garage is if the garage is wood and it's very dry.  It's the moisture in
the tree leaves that kills most of the signal.  Same goes for Dish or DirectTV receivers.

If you can mount directly to the inside of a fiberglass shell or panel that consists of nothing but
fiberglass mesh or mat and polyester or other totally non-conductive resin, it should pass the
microwaves from the GPS or XM satellites with little attenuation (signal loss), and should work
with no problems.  However, if there is any metal mesh or anything conductive in the panel,
forget it.  It won't work.

Hey!  I think this is post 100 and I'm no longer a Junior Member!  WHOOOPPEEEEEE!!!!
(It's about time. )

Clarke

[Dallas]

!!!!Congratulations Clarke!!!!

!!!!!Happy 100!!!!!

[/i][/b]

[NJT5047]

Thanks for the ideas Clarke.  I'd like to place it under the cap..and I don't have metallic paint on top of the coach...metallic on the sides.  There is nothing above the steel frame, other than foam and cap. 
I'm gonna try it under the cap and see how goes.  If it doesn't work, I'll move it. 
Glad I was able help you with your trek towards BusNut petaflop posting status!  I'm working on Super Hero status....Note Dallas' "Hero" status! 
JR

[Burgermeister]

I haven't read an installation manual, but what, if anything do they say about establishing a ground plane for the antenna if it's mounted under the cap?

Ground planes, if useful to the installation, does alot to improve antenna reception.

Onward and Upward

Marc Bourget

[gumpy]

I just bought a GPS this summer, and got a small magnetic external antenna also. I put the antenna on the framework just under the center of the front fiberglass cap and used it on our trip to WY and CO. Under normal conditions without an antenna attached, the GPS will give accuracies down to about 12-13 feet on a really nice day with no obstructions. With the antenna in the bus, I was getting accuracies down to 8 feet. I won't be moving it from under the cap.

Don't have XM so can't help you there.

craig

[FloridaCliff]

JR

As others have testified about there personal results, just put it under the cap.

GPS is a very hardy signal and the wave length at 1575 MHz is quite small, thus the tiny antenna's.

The only real concern is any roof obstructions, but I doubt you would have enough to be unable to see at

least three or more of the GPS birds.

And anyway, your just getting directions, not landing at JFK. 

Best of Luck

Cliff

[ArtGill]

May of the commerical antenna's have fiberglass radomes.  I often mount VHF and UHF antennas in attics to protect them from wind and salt on the coast.  At the high frequencies of XM and GPS and the fact they are only receiving a ground plane should not me needed.  As others have said you still need a path clear of metal to the sky.

Art  NJT Eagle 20

[NJT5047]

Regarding the need for a ground plane, the instructions state that it needs a ground plane...but, as Art sez, it don't.  It needs ferrous metal (or a large dollop of adhesive) to hold it in place.   I can advise that the antenna works laying on the plastic dash of my vehicles.  As I've stated, it works fine with a plastic bucket over it too....with the antenna wire stretched out and the antenna laying on concrete.  Seems to work anywhere that no metal or trees obstruct the view of the sky. 
There's a couple of 2X2 steel frame members immediately beneath the cap and that's where I'm going stick it.  The magnet in the antenna will clearly hold it in place.  Nothing above but foam and fiberglass (R&M cap).   No metallic paint on the top of the coach either.  Just white paint. 
Only the XM antenna is being installed at this time.  I don't have a perm mount GPS.  Got a little Garm that mounts on motocycle bars....I haven't really found  a use for it yet.   I generally know where I'm going...obviously, I don't go "trekking" often.  It'll interface with a laptop for larger display.   May give it a try this winter and see if I can find Ardadia! 
Thanks for all the ideas!  I'll post the results soon.  I think it'll work fine under the cap.  Be trying it out on the way to Timmonsville...JR