[TowerTalk] Metal roofs and EMI/RFI

[Jim Thomson]

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