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Author Topic: Metal roof and my Dipole  (Read 1165 times)
K8AEC
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Posts: 46




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« on: October 19, 2012, 05:14:18 AM »

Greetings,

     I'm thinking of putting on a metal roof.   My dipole is near the house; it runs beside the roof, maybe 15 feet from the front of the house.   The open wire feed (with the insulation intact) runs down my chimney and down into the shack.
     So basically the only contact is the feed over the last 10 feet of the roof.   Will this have an adverse effect?

Thanking the Elmers in advance,

Scott E. Fields, K8AEC
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AA4PB
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Posts: 13032




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« Reply #1 on: October 19, 2012, 05:58:23 AM »

You need to provide some insulators to keep the feed line suspended at least 6" to a foot above the metal roof. You don't want the feed line laying on metal. You also need to make sure that the feed line can't move around in the wind or you may find the SWR changing as the distance between the feed line and the metal roof changes.
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K3VAT
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Posts: 763




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« Reply #2 on: October 19, 2012, 06:41:24 AM »

You need to provide some insulators to keep the feed line suspended at least 6" to a foot above the metal roof. ...

I believe that this separation is both frequency and power dependent.  Six inches is awful close if you're talking about 1 KW @ 3.5mHz.

Greetings,

     I'm thinking of putting on a metal roof.   ...  Will this have an adverse effect?

Scott E. Fields, K8AEC

Depends what you mean by 'adverse'.  Certainly, there will be coupling of electromagnetic energy from the dipole to the metal roof as it is relatively close, especially in terms of wavelength, and so therefore on the lower bands will be affected more.

This is difficult to determine - things are pretty complex in the near field.  Perhaps on one band the metal roof may actually enhance the signal in the desired direction; on another it may substantially alter the dipole's pattern.  May be some of the modeling gurus can chime in and provide some insight.

Finally, I didn't see what were your operating interests/goals nor the parameters of the dipole - these would help us better answer your question.

GL, 73, Rich, K3VAT
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KQ6Q
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Posts: 993




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« Reply #3 on: October 19, 2012, 11:58:33 AM »

If you're putting up a metal roof, think of it as a dandy ground plane just waiting to be used. If you can put up a vertical on a pole near the apex of one end, with either a direct connectiothe n roof for the ground, or even radials laying along the roof, you'll be pleasantly surprised. If it will be a really steep metal roof, you'll have more of a challenge, but think of this as an opportunity, rather than a problem.

Fred, KQ6Q
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K7KBN
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Posts: 2838




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« Reply #4 on: October 19, 2012, 04:56:53 PM »

If the metal roof is in a number of easy-to-work-with separate sheets, you'll have to bond them all together to get one continuous surface for your ground plane/counterpoise (just placing them in contact won't work very long, if at all).  Clean metal to clean metal.

The bonding will also help greatly in reducing the noise that can occur when random bits of metal are in loose contact in an RF environment.
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73
Pat K7KBN
CWO4 USNR Ret.
AA4PB
Member

Posts: 13032




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« Reply #5 on: October 19, 2012, 05:27:34 PM »

The required separation between metal and the feed line (provided the feed line is properly balanced) is dependent on the spacing between the two conductors in the line rather than power and frequency. The separation needs to be at least twice the spacing and far enough to prevent arcing to the metal if running high power. It would take quite a bit of power to arc a distance of 6" to a foot.

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