Which antenna for my roof.

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Which antenna for my roof.

Postby Southwales » Fri Feb 09, 2018 12:29 pm

Hi, will this be good antenna for my scanner and uv5r, my main interest is around 446 pmr, 2m\70cm and mil air. Although where I live pmr 446 is mainly all I receive from farmer use etc. the antenna is available in 1m length or for a few pounds more 1.55m there much to be gained by gong for he 1.55m length
https://damsu.co.uk/two-way-radio/anten ... n2EbKCnw1I

1.55m version https://damsu.co.uk/two-way-radio/anten ... n2Hr6Cnw1J

What coax do you recommend?, also any tips of fitting to my chimney. Would this bracket be suitable. https://www.ebay.co.uk/itm/Mercury-120- ... SwpLNX~ekQ Thank you for your help.
Last edited by Southwales on Fri Feb 09, 2018 1:46 pm, edited 1 time in total.
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Re: Which antenna for my roof.

Postby Southwales » Fri Feb 09, 2018 1:39 pm

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Re: Which antenna for my roof.

Postby G4RMT » Fri Feb 09, 2018 8:17 pm

Forget that bracket. It's too light and the bolts too close together to give reliable fixing to chimney brickwork.

A normal TV antenna lashing kit - but TWO of them spaced is much more reliable and more friendly to the stack. Use 35mm tube. If you have to use brackets that fix to the brick, use two standoff brackets and again 35mm tube. If the tube is going to be longer than about a metre, maybe a 1.5m above the top bracket, I'd go for thick wall, or even 50mm tube. Those antennas are not lightweight. 50mm tube can go higher of course.

Cable wise a minimum of RG-213, or better. The antennas use SO239 sockets which are not waterproof or even water resistant - so make sure you waterproof the, properly with self-amalgamating tape, or water will get into the cable very quickly.
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Re: Which antenna for my roof.

Postby m0lsx » Fri Feb 09, 2018 9:59 pm

I am a apprentice served Bricklayer & a chimney is NOT connected to the brick work in the house, as it sits on a Damp Proof Course, (DPC.) To stop water ingress. Bricks are VERY porous & transfer water very efficiently. There are various places where damp protection is used & the chimney is one of them.
The other reason a DPC is there is to allow some movement, a rigid stacks would be brittle & having a DPC allows for some movement & thus the chimney is not damaged by strong winds.
The roof slopes so if the chimney does not come out of the ridge, it needs two DPC's, one at the highest point of the contact with the roof & another at the lowest point. This prevents as much moisture as possible from reaching the inside of the house.
So putting a large antenna on a chimney can be unwise.
I have stood with one foot on scaffolding & another foot on 10 foot tall chimney & had the chimney moving several inches under my foot. Not because it was unstable, but because it needed too when the wind is strong.
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