Antenna advice

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Antenna advice

Postby MrO123 » Mon Sep 11, 2017 1:15 pm

Hi looking for a bit of advice im planning on putting a 33ft mast on my roof with an antenna on the top of it. I want to increase marine VHF and civil air band range. Please can anyone andvise me of a good antenna that will cover both frequency bands.
I am tempted to buy a Diamond 777 Will this struggle on the marine band though?
Thanks any help appreciated
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Re: Antenna advice

Postby G4RMT » Mon Sep 11, 2017 1:34 pm

The 777 isn't too bad on Marine Band, but it's on the start of the fall off curve. Diamond seem er, 'economical' with their specs, but I suspect the antenna is simply a quarter wave for 120 MHz, that again works on three times the frequency. Marine coverage is going to simply be the 'bit of wire in the sky'. To be honest, I think that the extra height will gain you more than the antenna loses. Unless you really need extra ground coverage on airband, myself, I'd probably put a dual band for the ham bands up, which have useful gain at marine still - and a bit less on airband, where it's rarely critical as the planes are up in the air!

Your lucky you can get a 30ft pole up - that's an awful lot of weight up in the sky, and I assume your linking a couple of 50mm tubes together, with all the guys and stuff? That's too much of a worry for me, too breezy here on the coast to risk that.
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Re: Antenna advice

Postby MrO123 » Mon Sep 11, 2017 1:51 pm

Thanks so a HAM band antenna would be more beneficial than air band on the marine band I agree air band gain isnt difficult to acheive its more the marine VHF line of sight I need to target the antenna at so I will look into some antennas for those freq. The 33ft mast is telescopic light weight secured to chimney with ratchet straps sounds dangerous I wont be listening in during a thunder storm thats for sure.
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Re: Antenna advice

Postby G4RMT » Mon Sep 11, 2017 8:50 pm

Ratchet straps, 33ft and chimney together don't sound nice in the same sentence!

Joking aside, the turning moment on a few courses of brick with 30ft of pole above can be considerable with even a modest headland - A 30 mph breeze up top, a strong pole and decent straps can put amazing forces onto the chimney. Probably fine with a 6-10 stack 3 storey house massive chimney, but with less than a 3-4m circumference around the brickwork, there's probably not enough weight in it. Be careful. Remember that 10mph wind impacting on your antenna on the end of a 10m pole is the same as 100mph at the bottom!
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Re: Antenna advice

Postby m0lsx » Mon Sep 11, 2017 9:47 pm

G4RMT wrote:Joking aside, the turning moment on a few courses of brick with 30ft of pole above can be considerable with even a modest headland - A 30 mph breeze up top, a strong pole and decent straps can put amazing forces onto the chimney. Probably fine with a 6-10 stack 3 storey house massive chimney, but with less than a 3-4m circumference around the brickwork, there's probably not enough weight in it. Be careful. Remember that 10mph wind impacting on your antenna on the end of a 10m pole is the same as 100mph at the bottom!


The part of the chimney that pokes out of the top of a building. And that is a chimney of any size. Sits on a Damp Proof Course at the level of the lower part of a sloping roof, as bricks are very porous & without it, it would thus allow damp to get into the house. And a DPC means the bricks are in effect floating. A Chimney it's self is designed to move slightly in a strong wind, as a rigid structure is in effect brittle, so less safe than something that flexes very very slightly.

Image

On taller chimneys,back in the good old days when we could slap up some flimsy scaffolding of our own. The distance between swaying chimney top & the swaying scaffold could vary by a fair distance.
So a pole with 30 feet of that pole swaying in the wind with an antenna on top of that is concerning. Unless it's one of the very solid large chimneys with a 6 or more pots in it & thus some weight to it.
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Re: Antenna advice

Postby MrO123 » Tue Sep 12, 2017 12:15 am

Thanks for the information think I will loose some hight and attatch the mast on the washing line pole which is very strong safety is more important. Will still give me enough hight just need to find an antenna there are so many.
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Re: Antenna advice

Postby m0lsx » Tue Sep 12, 2017 10:31 am

The washing line pole will not have a lot of concrete around it or be very deep, so make sure you guy any mast you connect to it.
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