Discone proximity to metal roof?

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sanityno

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Hello:
I am new to both scanning and this website so i apologize in advance if this question seems moronic.
I received a Uniden BCT15X for Xmas and have decided to purchase a quality discone to complete my base station. The highest feasible point i can mount the antenna is off my chimney, on the perimeter of the house, about three ft. above the apex of a vast metal roof. Is that mounting location ok or am i asking for trouble? Any help would be greatly appreciated. THX
 

SCPD

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Good location

Hello:
I am new to both scanning and this website so i apologize in advance if this question seems moronic.
I received a Uniden BCT15X for Xmas and have decided to purchase a quality discone to complete my base station. The highest feasible point i can mount the antenna is off my chimney, on the perimeter of the house, about three ft. above the apex of a vast metal roof. Is that mounting location ok or am i asking for trouble? Any help would be greatly appreciated. THX
Others may disagree, but I think mounting the antenna above a metal roof will aid reception, as the roof will act as a good ground plane.
 

Jim41

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Attic antenna with metal roof

What about mount discone in the attic below metal roof, how this will affect reception?
In theory, the metal roof will act as a shield and will significantly attenuate the radio frequency signals. Thus, this would not be a good solution.

From a pragmatic standpoint, take your radio with its own antenna up in the attic and see how well it works. It is an easy and low cost way to assess how well an attic antenna will work in your specific location.

Jim41
 

kb2vxa

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I don't see any problems with your intended way of mounting the antenna but I would NOT put it under an oversize tin foil hat.

Oh and I disagree, the antenna has it's own "ground plane" and another won't help a bit. The metal roof causes reflections but what effect they'll have on reception good or bad is impossible to say.

A large metal sheet is both a shield and reflector depending on which side the antenna is on and where the signal is coming from so putting the antenna between the ground (also a reflector) and the roof is like putting it in a tunnel with reflections coming from every angle and direction. By putting it above the roof most of them are eliminated thus mitigating most of the problems.
 

jackj

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I would suggest that the antenna not be mounted to the chimney close to the vent output. The combustion products of most furnaces are corrosive and will reduce the life of the antenna. If that is the only logical place to mount it, try to mount it at least 10 feet above the top of the chimney.
 

sanityno

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Thanks for the replies all. I originally thought there may be some hard and fast rule about locating an antenna near that much metal. Your help has taught me what a "ground plane" is, that the radials of a discone create their own ground plane and that chimney discharge could corrode my antenna. I will adjust my plans accordingly...

Thanks again,

sanityno
 

Air490

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I have never had a problem with a discone above a metal roof... in fact quite the opposite. This has always been on flat roofs. Mounting it at the apex of the roof should (theoretically) have no effect at all. Go for the height and don't worry about the iron roof!
 

questnz

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another dumb question !!! I am waiting for Scantenna to arrive, can I mount on the same pole together with VHF/UHF TV aerials or move away, how far if move away?
ps Sorry for asking I just want best solution
 

jackj

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You can mount it to the same pole you use for your TV antenna. A general rule of thumb is to side-mount antennas a minimum of 1/4 wave length away from the pole / tower. This will reduce the directional effects of the pole / tower on the side-mounted antenna. At 150 MHz a 1/4 wave length is about 19", 6" at 460 MHz.
 

prcguy

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If you mount the Discone too close to the metal roof it will push the pattern up away from the horizon a bit and you will essentially loose gain where its needed. The Discone is a ground independent antenna and the roof will not be part of a counterpoise or ground plane that will aid reception. As a rough guess I think 6ft or higher above the metal roof should be ok for 150MHz and higher frequencies.
prcguy
 

commscanaus

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If you mount the Discone too close to the metal roof it will push the pattern up away from the horizon a bit and you will essentially loose gain where its needed. The Discone is a ground independent antenna and the roof will not be part of a counterpoise or ground plane that will aid reception. As a rough guess I think 6ft or higher above the metal roof should be ok for 150MHz and higher frequencies.
prcguy
I can agree with prcguy on this topic. My discone is 6ft above a pitched corrugated iron roof and I have not noticed any detrimental effects. The antenna is mainly used for VHF airband, and I can hear most of the air traffic and control towers without issue. Even aircraft on the ground at 30 miles away are quite easily heard.

Another smaller discone is mounted around 5ft above the gutterline and is shielded by the roof in the westerly direction. Certain services west of this location cannot be heard very well on that antenna, but since my interest is to the east and north, it performs very well for what I need.
A dual band vertical antenna is raised above the peak of the roof so there is no effect whatsoever.
Placing the antenna in the roof is not an option. Even the digital cellphone is next to useless in the roof space.

It saves having to wear tinfoil hats when you have a metal roof over your head! ;)
Those marketing satellites can't tell what I'm thinking!

Commscanaus.
 

k8tmk

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The distance from the discone's horizontal elements and its "droopers" is what sets up the antenna's wide-band characteristics. The shorter this distance, the higher the frequency. In other words, the antenna picks up higher frequencies where the droopers connect to the center of the antenna. The antenna picks up lower frequencies at the opposite ends of the droopers. Other frequencies are picked up along the droopers between these two points as the distance between the horizontal elements and the droopers vary. Its all a matter of spacing. And, although the horizontal elements might make you think otherwise at first, the way the antenna works actually makes it vertically-polarized.

Personally, I see no reason why a metal roof would affect a discone's operation.

Randy
 
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