Subject: Re: [TowerTalk] Metal roofs and EMI/RFI
I've had two verticals on metal roofs, old galvanized barn and shed
roofs at that. A 80m hat loaded 40' vertical was terrific for DX,
particularly when using a Beveridge or dipole for lower noise receive.
An elevated vertical with a large metal ground plane is an excellent DX
antenna IMHO.
The antenna support structure, connectors, and coax PIM (passive
intermodulation) issues are an issue on cell towers with lots of full
duplex radios, but I think unlikely in common amateur service. Anyway,
you likely have many thousands of metal to metal joints in RG8 shields,
metal fences, tower parts, house wiring, audio systems, etc. etc.
There is an interesting application note from Andrews that compares
braid coax PIM with hardline, as an example.
Grant
KZ1W
On 8/5/2011 10:57 AM, Jim Hoge wrote:
> Are there any RF noise issues to consider when contemplating a residential
> reroof from composition to metal shingles? The last thing I want to do is
> increase my background noise level here in suburbia.
>
> Tnx, Jim W5QM
### large galvanized barn roofs... and similar on flat roofs is one thing, BUT
metal shingles, [ by the hundreds] is something entirely different again. Way
more joints and overlaps with the use of hundreds of metal shingles.
## seems to me, these new metal shingles are coated on the top side at least...
so they don't wake up the dead ..when it rains out. Metal shingles/metal roofs
went out of vogue back in the 30-40's cause folks didn't like the racket they
made every time it rained..or when they started to rust.
## are u sure these new style shingles are steel ? They might well be
aluminum..
I think some of them were coated with plastic composite, or powder coated +
some kind of composite. I would check this out in fine detail 1st.
## back in 1994, I bought a new condo...that had the usual flat roof. I put
the
fluke DVM on ohms... and measured 0.0 ohms between 3rd prong of outdoor
120 vac outlet on outer deck....and the Powder coated new Aluminum railing.
I also measured 0.0 ohms between same grnd on 120 vac outlet.... and new
AL gutter's above the deck, at roofline.
## Thinking all was well... and ran some wire off the top floor of the condo..
almost to
the grnd... using either minimal counterpoise, then the AL gutter's, then the
AL railing. This was a 4 floor wood frame building, with wood decks, etc.
It also had stucco on the outside... with miles of chicken mesh, just below the
stucco.
## I ended up with IMD city for the next 4 years ! The local 50 kw + 2
more 10 kw
AM broadcast stations sitting 3 miles out in the water. on local small islands
would show
up at random..all across the 80m band. noise galore, imd, and all sorts of
other crap.
The signals across the ham bands were WAY down in level... and every ant tried
had flat swr
and would work good at any other location. It was like I was inside some
kinda electronic vortex.
I even tried some small verticals + 3 x isotrons plus some other configs.
Various chokes used,
even tried battery operated ham gear..and totally isolated from the 120 vac.
## Nothing improved...and eventually sold the condo...and bought a house. 4
x years of
constant experimenting, re-configuring, etc... very frustrating. I'd be real
wary of hundreds
of metal shingles, all slightly overlapped, and all nailed together. IF you
end up with problems,
you will never resolve em, and they will just get worse.
Jim, VE7RF
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