My metal roofs are 2 miles direct line of sight to the towers of a 1 MHz
50kw AM station. Needless to say, that RF was everywhere and to measure
any millivolt signals on my scopes I installed a good ground system and
serious AC line filtering.
Since the BCB signal was so powerful, I installed a BCB brick wall
hi-pass filter at my Icom 756 and didn't hear any spurious AM signals or
mixing products. All antennas had good baluns.
One fact I recall from FCC emissions testing of computers is that the
metal to metal (no DC path) overlap such as in a metal roof provides a
lot of capacitance between plates. IMO it's one factor that tends to
reduce the potential for PIM and makes metal roofs appear as contiguous
plates at HF.
I also found that the MFJ-269 was useless for antenna measurements since
it uses a wide-band detector. Without any filtering, the AIM-4170 did
ok with some artifacts that could be easily dismissed as such.
Grant
KZ1W
On 8/7/2011 2:59 AM, Pete Smith wrote:
> 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
> reversebeacon.blogspot.com,
> 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,.
>>
>> 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@pacbell.net>
>>> Sent: Saturday, August 06, 2011 9:02 AM
>>> To: "Jim Hoge"<knowkode@verizon.net>; "towertalk"
>>> <towertalk@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
>>>>> _______________________________________________
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> TowerTalk mailing list
>>>>> TowerTalk@contesting.com
>>>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>>>>
>>>> _______________________________________________
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> TowerTalk mailing list
>>>> TowerTalk@contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk@contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|