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Author Topic: Dipole rooftop mounting  (Read 842 times)
WP4NXA
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Posts: 48




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« on: September 16, 2008, 07:10:30 PM »

What's the best way to mount wire dipoles on a rooftop. What mounting hardware should I use at a reasonable price. If there is a non penetrating way- even better cause it's a school rooftop. Also, please link to a company's site if you can.

Also, is just drilling a mast enough to support an HF vertical?

Thanks.

73
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K0ZN
Member

Posts: 1563




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« Reply #1 on: September 16, 2008, 08:59:03 PM »


Hi.

You have multiple antenna questions that really should be addressed via some study of the ARRL Antenna Book; more than can be adequately addressed on here.

Respectfully, purchase of a commercial antenna will not guarantee success. Pick up an older copy of the Antenna Book on ebay. They can be had cheap and the info in it is still totally valid and will help you immensely.

The knowledge you pick up from a little book study will be VERY helpful in reducing frustrations and giving you a bigger signal. Antennas are not complex, but some specific parameters need to be met for them to work and the basic theory section of the Antenna Book will tell you about these. i.e. there is too much for the whole subject to be addressed here in a few blurbs.

The time you put in some book study will be rewarded many times over.

Depending upon what material the roof is made of, it may or may not have a Big effect on the antenna. If it is a metal roof, as many commercial buildings have, you have a much different situation and separation requirements than a non-conductive roof.

Good luck on your project.  73,  K0ZN

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WA3SKN
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Posts: 5559




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« Reply #2 on: September 17, 2008, 04:52:00 AM »

Your question poses questions!
You are trying to set up something for a school... what bands are you trying for and what are you trying to do?  40 meter CW is different than 20 meter DX, which is different than 2 meter FM.
Explain what you are trying to do and we might be able to help!

-Mike.
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AA4PB
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Posts: 13032




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« Reply #3 on: September 17, 2008, 05:16:26 AM »

If its a flat school roof you'd better be checking with the facility manager. Putting any kind of holes in a flat roof can cause serious water problems. Usually you can find some kind of vertical surface that you can fasten brackets to. At any rate, you need to be checking with someone famailiar with the building construction.

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WA7NCL
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Posts: 625




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« Reply #4 on: September 17, 2008, 09:01:54 AM »

I would try something like putting some plastic pipe sections of say 8 to 10ft into some large buckets filled with cement.  Then you could put them on the roof without making holes.  From the support pipes I would hang a 40 or 20 meter dipole fed with coax.  You could also a get more complicated and use two dipole elements fed with same coax.  Each dipole element would be for 20m and 40m.

The antenna wouldn't be very high and would not win any perfomance awards, but it is simple and you don't have to make any holes in the roof.  Seems like thats would be a good start on a school station.
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WB6BYU
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Posts: 13580




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« Reply #5 on: September 17, 2008, 10:07:06 AM »

Have the metal shop make some steel mesh plates with a
rim around the side that will hold 4 cinder blocks each,
with a bolt in the middle.  Get a TV antenna tripod (or
better yet one of the tripods sold for ham roof-mount
antennas) and bolt the legs to the frames.  Put the
cinder blocks on the frames to hold them in place.  Stick
about 15 to 20 feet of TV antenna mast in the tripod.

(For a smaller version, use plastic dish pans filled with
concrete with a bolt sticking out from the center.)

This may need guying, but you can use the dipole antenna
wires for that (two bands = 4 wires running at right angles
to each other.)
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W4JPP
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Posts: 20


WWW

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« Reply #6 on: September 17, 2008, 11:56:52 AM »

my first dipole was simple...I took a cinderblock, an 8' section of pvc pipe, and filled with cement...that was my center support...and the legs slope slightly down and tied off to a similar setup with just 2" sections of pvc...it was on a flat section of roof

I did this for 20 meters, which was great, because making contacts was easy...

some years later I got a commercially made beam, left the dipole up...and although the beam did have better reception/gain....I was still surprised how effective a simple, cheap antenna could be.
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AA4PB
Member

Posts: 13032




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« Reply #7 on: September 17, 2008, 01:27:46 PM »

Common guys. We're talking about a commercial building here - a school at that. I assume that it's some sort of a school club station. If he causes roof damage or the thing comes down in the wind and maybe hurts somebody, or it gets hit by lightning and does damage because its not properly grounded then as a very minimum the club is over. Buckets filled with concrete may be okay for your home but I sure wouldn't do it on a school roof without having the direction and okay of the building manager.
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WB6BYU
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Posts: 13580




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« Reply #8 on: September 17, 2008, 05:33:03 PM »

I'm certainly not proposing that he do anything without
the support and approval of the building manager.  But
he was looking for ways to mount an antenna without
drilling holes in the roof, and that is one way to do so.
I've seen photos of triband yagis mounted that way on a
rooftop - with suitable guy wires, of course.
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K7BAB
Member

Posts: 87




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« Reply #9 on: September 19, 2008, 04:56:12 PM »

First, most modern commercial roofs are membrane roofs. That is, a heat-sealed plasticized membrane with a scrim encapsulated within.  These roofs are not be walked on, there are usually planked to permit walking on for inspection and maintenance. Air conditioning units are supported by dunnage. Therefore, putting any “load,” (as suggested) on this type of roofing will probably damage it and void its warranty.

Is there a parapet on the building?  If so, I suggest mounting a mast at each end and using a TV antenna mount. Lag bolts and lead shields would be a good choice to anchor the TV antenna mount. Also, egg compression insulators would be “fail safe” and copperweld wire would be an improvement over other wire types.
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