It's good to get a specific and quantitative view, from someone with
direct experience. Mine is all with galvanized roofing in the big
panels. It is amazing just how strong AM broadcast signals can be. I
live five miles from a 5000-watt daytime, 6-watt night-time (yes 6
watts) station on 1550 KHz, and the typical field strength on my shunt
fed 160M vertical is +1 dBM. I have to wait for them to switch to
night-time power before I can use my antenna analyzer for any
adjustments on that antenna.
73, Pete N4ZR
The World Contest Station Database, updated daily at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at
spots at telnet.reversebeacon.net, port 7000
On 8/6/2011 3:48 PM, Joe Subich, W4TV wrote:
>> 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,.
> ... 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.
>> Jim W7RY
>> From: "Grant Saviers"<firstname.lastname@example.org>
>> Sent: Saturday, August 06, 2011 9:02 AM
>> To: "Jim Hoge"<email@example.com>; "towertalk"
>> 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.
>>> On 8/5/2011 10:57 AM, Jim Hoge wrote:
>>>> Are there any RF noise issues to consider when contemplating a
>>>> 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
>>>> TowerTalk mailing list
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