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Old 04-07-2017, 07:07 PM   #1
Gashog
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Default Aluminum roof ground necessary?

I've Googled 'till my head hurts.
I've asked on other forums.

I restored a 1984 Sunflyer about 8 years ago.
It's about ready for another dry dock but no one wants to take road trips with me anymore.
I figure it'll make a good ham shack/office but I need one question answered first;

Does the aluminum roof need to be grounded to chassis?

Oh... Edit: KD2MBG
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Old 04-08-2017, 10:05 AM   #2
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Is a "Ground" necessary?

Well most folks do not understand the differece between Ground (Earth ground) Ground (Chassis Ground) and Ground Plane (Something else entierly)

FOr VHF/UHF they make antennas that do not need a ground or ground plane,, But with an aluminum roof.. You can use a standard quater wave or 5/8 wave if you like because the roof will make a perfect GROUND PLANE.. Just make sure you are well connected to it.

For lower bands it depends on the size of the RV.

Also the roof may also be a "Cassis ground" (use ohm-meter to check)
and in worst case some nice heavy braid can connect it to the chasis (use flat braid) and from there to ground...

If using flat braid.. connect it well.. I'd paint the conneciton with one of the "liquid solder:" products (Silver paint, Copper paint) after making a good connection, then Run it down the back and cover with plastic channel, that can be held on with adheasives (Looks better than the wire running down there) and connect to chassis, weld that connection or paint as above after connecting.

Then ground the chassis (I use a 3' long screwdriver with ground strap attached to it ust below the handle as an "Aux Ground" when needed.. Does not seem to make a difference but .. Well I feel better)
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Old 04-08-2017, 11:38 PM   #3
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Ok so that's one vote for maybe.
Another issue is the thickness of the roof.
I hadn't considered that until I had the mounting stud in my hands.
I'll ponder on it tomorrow.
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Old 04-09-2017, 01:14 PM   #4
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Thickness is not all that big an issue if the mount can penetrate and there is something (Wood for example) under the aluminum. But that depends on the mount.

A standard 3/8" "Snap in" I'd not use.
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Old 04-09-2017, 06:19 PM   #5
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I went all over my camper aluminum roof/siding with a Meger did find some corroded fittings but by in large it was well "grounded" at all points including the camp ground, ground (that sounds funny) at the frame any way is it a realistic ground when it is plugged in? That's a lot of wire just under ground.
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Old 04-10-2017, 07:24 AM   #6
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I went all over my camper aluminum roof/siding with a Meger did find some corroded fittings but by in large it was well "grounded" at all points including the camp ground, ground (that sounds funny) at the frame any way is it a realistic ground when it is plugged in? That's a lot of wire just under ground.
Yup as I was looking up at the roof AC yesterday, I figured it probably is grounded, one way or another.
As for the thickness of the roof. I'm installing an 8'-6" fiberglass whip.
Where I plan on setting up gets zero protection from the wind so the mount has to be very sturdy.
A good 70mph gust would rip a hole in the roof if it isn't reinforced with a plate to spread the load. I guess I could guy it too.

I was thinking about a shock absorber washer since they're offset/recessed/concave. I could countersink it into the ceiling so the stud can reach.
I need to drill a hole in it regardless.
I have to see what I have available in my parts piles.
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Old 04-10-2017, 08:11 AM   #7
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If your parked any thing goes the 14X14 vent openings are strong enough to hold an AC unit so it should not be a problem to box in some thing to support your antenna lot more wind loading on an AC unit plus it's 100# weight.
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Old 04-10-2017, 08:15 AM   #8
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Is the aluminum over wood? Screw into the wood, That is my best advice

Else mount it at a frame rail and add a steel plate so that the stronger steel holds it, NOTE that there will be corrosion where aluminum meets steel but there are ways to prevent that.
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Old 04-10-2017, 04:21 PM   #9
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If I had a camper with an aluminum roof parked in the yard, and I was going to use it for a ham shack...

I'd have me some thin steel plates made. I'd put them on the roof and stick mag mounts on them. I would assume, until I proved myself wrong, that capacitive coupling would electrically attach my antennas to that lovely ground plane just like on a car.

I'd do this probably for 10m and up.

I might put a thin rubber barrier between my plates and my roof, made from old inner tubes or what ever. And I'd give it a try and see what it does. It might work.

For HF work I'd get something like a Butternut vertical and mount it to the tongue. Might need the radial kit. I wouldn't make them permanent, so I could roll them up to mow the yard and roll them back out again.
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Old 04-11-2017, 07:56 AM   #10
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Quote:
Originally Posted by N3LYT View Post
If your parked any thing goes the 14X14 vent openings are strong enough to hold an AC unit so it should not be a problem to box in some thing to support your antenna lot more wind loading on an AC unit plus it's 100# weight.
The roof is aluminum over what would be called SIP panels.
They're like a layered wafer cookie....But with plywood and styrofoam.

There are steel beams and I think the AC is boxed in between two of them.
There's a cutaway diagram on the sales brochure showing the framing and showing how strong it's supposed to be.

As for wind load, I think if you did the math, you'd see that a 9' whip has a bit more wind load then an aerodynamic AC unit sticking up 12" off the roof.

I have some old mirror brackets. I was thinking about finding one of those beams and clamping the bracket right to it.
I'd have to seal a huge hole with rubber pipe flashing sore something.
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Old 04-11-2017, 08:19 AM   #11
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The big problem with a 9' Whip on top of a trailer is this
Say the trailer is 10 feet, the whip adds 9 feet that's 19 feet.. you are going to be hitthing bridges, traffic signals, and may well earn yourself the attention of the police less you fold it down when in motion.
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Old 04-11-2017, 11:15 AM   #12
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Quote:
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As for wind load, I think if you did the math, you'd see that a 9' whip has a bit more wind load than an aerodynamic AC unit sticking up 12" off the roof.
Depends on what you call "a bit more".

Aerodynamically, it's a difficult question. There will be some wind load on the A/C unit, but it will be minimal because the air moving down the top surface is slowed by friction on that surface and by any other protrusions on that surface; and the wind load will be further reduced by any turbulence in the flow. Basically the wind load on the A/C ought to be quite small.

The whip, on the other hand, is in undisturbed laminar flow, sometimes at perhaps 70 to 80 mph. The wind load will be much larger, and is applied about 5 feet from the mount. That's enough moment (torque) at the mount to tear a petty big hole in the roof, or at least wrinkle the skin considerably. So you need a reinforcing plate, on which the whip is mounted, large enough to distribute that load. I'd think a four-foot square of 1/4" 6061-T6 aluminum plate would probably handle it OK.

Indeed, that "a bit more" would really be "a heck of a lot more".
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Old 04-11-2017, 07:20 PM   #13
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Height does not matter it is the surface area that produces wind load. Leverage maybe I have a 21 foot dual band fastened to my tower 40 feet up with hose clamps. No metal in my 14X14's makes it's too hard to fasten vent screws to.
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Old 04-11-2017, 07:31 PM   #14
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Replace the vent with a 14X14 aluminum plate and mount any vertical. This thing is going to be parked any way, you in a tornado area? If so I think the last place in would want to be in is a trailer.
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Old 04-12-2017, 06:41 AM   #15
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Replace the vent with a 14X14 aluminum plate and mount any vertical. This thing is going to be parked any way, you in a tornado area? If so I think the last place in would want to be in is a trailer.
It'll be parked in the black dirt region in Pine Island/Goshen NY.
We've only had two tornadoes in 40 years that I can recall.

When I move it, the antenna will come down obviously.

Is there any down side to guying it?
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Old 04-12-2017, 07:58 AM   #16
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I have fiberglass poles like 24' of them for my rotatable dipole, at 17 meters it's 36' I made a walk up device fastened to the rear bumper of the camper and in that case guying is a must. I have an R7 it's off the ground about 8' Cushcraft says it's OK not guyed they do however make a kit to guy it! Mine is guyed because winds off the mountains have exceed 60 MPH here. I really don't think you would have a problem with 9' of antenna We have 2 18+ feet of single point mount antennas at our club repeater site @ 1600 feet.
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Old 06-19-2018, 07:08 AM   #17
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I might add most metal is not RF continuous in a RV metal roof. Most are not like a vehicle roof. So another ground might be needed.
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Old 06-30-2018, 04:18 PM   #18
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It actually is not bad I have been all over mine with a megohmmeter mine has a rain gutter with probably 20 screws on each side. The roof is folded over the siding then the rain gutter is installed with the screws driven through the roof edge and the siding. The siding is grounded to the utility power ground and the DC ground.
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Old 07-01-2018, 07:25 AM   #19
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Hey guess what!?
We had a tornado!!

I'm framing a plywood observation deck over my aluminum roof as per some innovative youtube guy.
I'll be rethinking my antenna again.
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