[TowerTalk] Metal roofs and EMI/RFI

[Jim W7RY]

Back to the original subject....

If you live near any AM broadcast transmitters, (5 miles) I would not 
recommend a metal roof of any kind.

When I lived near several, I would get mixing in my metal rain gutters when 
I lived on the south hill in Spokane.

I had to use DC ground antennas to keep the mixing away on most bands. A DC 
ground antenna has very low resistance at low frequencies.

I could not run a dipole without a balun, and HF beam antennas had to have a 
beta match (Hy-gain mono banders, TH6 etc) and or a trifiler would balun, 
which shows a short at DC.  A choke type balun would not keep the mixing out 
of the rig.

The statement about mixing is correct. The only time it's an issue is when 
there are multiple transmitters at the same location.

73
Jim W7RY



--------------------------------------------------
From: "Grant Saviers" <grants2 at pacbell.net>
Sent: Saturday, August 06, 2011 9:02 AM
To: "Jim Hoge" <knowkode at verizon.net>; "towertalk" 
<towertalk at contesting.com>
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
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