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Author Topic: HF9V On School Flat Roof  (Read 703 times)
AE5QB
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Posts: 265




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« on: January 23, 2013, 05:40:33 PM »

I am a bit confused about how I should mount a Butternut HF9V on the flat roof of our school.  It is a tar and gravel rooftop, flat as a board, 30' up, with a clear 360 degree horizon.  I am thinking I should get pretty good results once I get it up there.

My question is, if I mount the antenna flush to the roof surface as if the roof were the ground, would the radial system act more like a grounded radial system or an elevated radial system?  Would the gravel/building structure detune the radials to the point the radials could not be tuned at all and they would function more like a ground mounted system, in which case 30+ radials would be needed?  Or could it actually be tuned with the radials lying on the gravel?

From an install perspective, it would obviously be easier to mount it at roof level and run some radials.  On the other hand, mounting it on a 10 foot mast and sloping the radials might give better results although tie-off points could be an issue.  I have a dictator type facilities manager in my district so running 4 - STR type radials plus a couple for 80m will look a lot less intrusive than a complete 30+ radial field, but I don't know.  I am thinking that tuning a multi-band vertical perched on a 10 foot mast has got to be a chore, even if a tip-over mount is employed.

I can't seem to find much on this subject other than anecdotal evidence such as, "I have mine on a roof tripod with radials lying on the shingles and it seems to work pretty well.
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N6EY
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« Reply #1 on: January 23, 2013, 05:59:06 PM »

If the roof is asphalt on wood, then you will need to use tuned radials.

Check out DX Engineering's flyer on that series on antennas - they do a GREAT job of documenting that fine vertical.

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73,
Jason N6EY
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AE5QB
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Posts: 265




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« Reply #2 on: January 23, 2013, 06:03:01 PM »

If the roof is asphalt on wood, then you will need to use tuned radials.

Check out DX Engineering's flyer on that series on antennas - they do a GREAT job of documenting that fine vertical.



Hmmmm?  Asphalt on wood?  I never thought about that.  I guess I assumed it was asphalt and gravel on corrugated steel or some type of metal sheeting.  I didn't realize they put wood on building rooftops like that.  I'll check out the DX Engineering white papers.
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N6EY
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« Reply #3 on: January 23, 2013, 06:08:11 PM »

I was mistaken - DXE has a paper on the Hustler xBTV verticals.  Still, there might be some good information.

If it's steel, then the game changes - you might need to play with the matching to make it play.

Either way, if you put some effort into the counterpoise, you should have good results. 

GL!

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Jason N6EY
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WB6BYU
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« Reply #4 on: January 23, 2013, 07:38:40 PM »

If you can bond to the roof metal (sheet metal screws covered in roofing compound
perhaps) then it should make a great ground plane.  But facilities managers are not
overly fond of people putting holes in the roof.

A set of radials laying on the roof should couple to the metal by capacitive
coupling.  Even better than wire in that case would be a flat metal strip -
I've seen aluminum foil used for this, but it doesn't last very long before it
rips.  My recommendation would be to use some aluminum or other metal
sheets attached to your antenna base, perhaps 4 radials each 25' long
and 8" or 12" wide.  Effectiveness will depend to some extent on the distance
between the underlying metal and the top of the gravel where you can
put your flashing strips - you might need to sweep the gravel off to
install it.  (Aluminum is going to weather, so it might need some sort of
painted coating, as well as weights to hold it down so it doesn't blow
around in the wind.)

A few sheets of metal roofing could be used, too.  I'd recommend the
flat stuff rather than corrugated because it will increase the coupling
capacitance between the surfaces.
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N4OGW
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« Reply #5 on: January 24, 2013, 10:36:18 AM »

W5YD has a HF6V in this kind of installation on the top of one of the engineering buildings. In our case the vertical is mounted on a tripod which is 3 or 4 feet tall. We did that because the tripod was already available. Then we used two tuned elevated radials which slope down from the tripod to the edges of the flat roof for each band we were interested in. In this case we only put up radials for 40 and 80, because there is also a tribander which is much better for 20-15. We also added a choke balun at the feedpoint. It works well, although there is a lot of electrical noise coming from stuff in the building.

You could also mount the vertical closer to the roof and put down a bunch of radials directly on the roof. In that case you won't be able to tune them (metal in the roof), so you just want a larger number of radials which can be a random length.

Tor
N4OGW
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AE5QB
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Posts: 265




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« Reply #6 on: January 24, 2013, 02:48:47 PM »

W5YD has a HF6V in this kind of installation on the top of one of the engineering buildings. In our case the vertical is mounted on a tripod which is 3 or 4 feet tall. We did that because the tripod was already available. Then we used two tuned elevated radials which slope down from the tripod to the edges of the flat roof for each band we were interested in. In this case we only put up radials for 40 and 80, because there is also a tribander which is much better for 20-15. We also added a choke balun at the feedpoint. It works well, although there is a lot of electrical noise coming from stuff in the building.

You could also mount the vertical closer to the roof and put down a bunch of radials directly on the roof. In that case you won't be able to tune them (metal in the roof), so you just want a larger number of radials which can be a random length.

Tor
N4OGW

Thanks for the hands-on feedback.  I appreciate it.  I have a 10 foot portable mast up there now that I have a VHF/UHF eggbeater combination temporarily mounted on.  This is our backup antenna system for our upcoming ARISS contact.  I was thinking, since it is up there, why not mount the HF9V on it and use the STR twin lead tuned radial system as suggested by Butternut.  Then I got to thinking about taking it up and down ump-teen times to tune it and am now thinking it will be much easier to put it at roof level with a radial system.  I have two spools of DX Engineering 14 gauge radial wire, so that is not a problem.  I have enought for 40 - 50 foot radials or 66 - 33 foot radials which is more than enough.  Now finding a way to achor the outboard ends of 30 or 40 radials that will keep the facilities folks happy may be another issue.  I can't imagine them being happy about putting 30 or 40 bricks up there.  Maybe I can just lay them out and drop a few daubs of roofing tar on each radial along their lengths.  I can say I am helping seal the roof. Smiley

Thanks for the advice gents, I am not quite sure which way I will go yet.  Maybe I should just buy a tribander or a hex beam and run a dipole for 40.  This whole radial thing sure complicates the mechanics of the installation. 

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