[TowerTalk] Metal roofs and EMI/RFI

[Joe Subich, W4TV]

> I could not run a dipole without a balun, and HF beam antennas had
> to have abeta 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 themixing out of the rig.

A proper high-pass filter on the receive would have resolved the issue
without any need for heroic measures.  Placing a low frequency short
(beta match or trifilar voltage balun) across the antenna terminals is
nothing more than a simple, crude, two pole highpass filter.  That the
simple highpass filter resolved the mixing shows that the issue was a
receiver problem due tot the high signal level, not an issue of mixing
in the environment.

Metal shingles have large areas of interlock the full perimeter of the
shingle.  A typical 16 x 8" shingle is quite small and with any induced
voltage spread over the entire perimeter of the shingle. there is very
little - if any - RF voltage across the joints.  With little or no RF
voltage, there is little chance of rectification or passive IMD.  The
aluminum metal shingle are far different than old style "galvanized"
roof where there is no interlock between sheets and contact between
adjacent panels depends on how tightly the panels are nailed to the
rafters.  Contact between panels on "standing seam" roofing can also
be an issue depending on the way the particular brand of roofing panels
are designed to be attached.

I've had both traditional galvanized and modern metal shingle roofs
at various homes over the years.  Even though I am only about three
miles as theh crow flies from two AM stations, (50 KW daytime), I see
no noise issues from my metal shingle roof,.

73,

    ... Joe, W4TV


On 8/6/2011 12:47 PM, Jim W7RY wrote:
> 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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