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Author Topic: minimum between hex beam and flat roof  (Read 1933 times)
KU7I
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Posts: 127




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« on: March 16, 2013, 04:38:00 PM »

Looking for ideas on what the minimum distance should be between a hex beam and the flat roof of my house? The support pole will be attached to the house. The flat roof is about 22 feet above the ground. I am thinking about five or six feet above the roof would be fine.
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W1JKA
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Posts: 1821




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« Reply #1 on: March 17, 2013, 03:07:55 AM »

   With the standard mantra [higher is always better] in mind and  depending on local conditions at your qth 22 ft. should give you satifactory results.My hex is on a mast at 23 ft. and no problem with daily 2 way qrp/cw contacts with other hex users in Europe or W.coast USA whose average height is between 20-30 ft. Being on a roof is no problem unless it is metal or in some attics that contain HVAC units which may have some effect,but the experts will have to weigh in on that issue. GL
« Last Edit: March 17, 2013, 03:19:35 AM by W1JKA » Logged
N4CR
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Posts: 1702




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« Reply #2 on: March 17, 2013, 04:20:29 AM »

The mechanical minimum is enough that the antenna doesn't scrape the roof while it is being rotated.

http://www.karinya.net/g3txq/hexbeam/height/
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73 de N4CR, Phil

Never believe an atom. They make up everything.
K2MK
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Posts: 407




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« Reply #3 on: March 17, 2013, 06:17:11 AM »

The first location for my Traffie hex beam was close to the house. About 1/3 of the antenna was over the roof. The SWR would rise over 2:1 as the driven element was oriented over the roof. During that time I had the mast extended to 30 feet, 25 feet and 20 feet. The SWR irregularity was worse the lower the elevation. At 20 feet the antenna just cleared the sloped roof by a few inches. Maybe things would have been different if the roof was flat.

73,
Mike K2MK
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N4UFO
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Posts: 270




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« Reply #4 on: March 17, 2013, 07:17:54 PM »

Based on the observations of a ham I used to know (K3GC; pics of his hex just above roof on his QRZ page), you will put it up on a dry day 5-6 feet above the roof and think, "Well that's not so bad."  And then you will turn on the rig on a rainy day and have a whole different impression. Not only will the rain coating the wires on the antenna change the resonant frequencies (same as with a dipole or any other antenna) but the water coating the roof will cause reflections and wreak havoc on your pattern. Usually 15m is where things are the worst. His amplifier would actually go into fault on 15m if his antenna was pointed a certain direction on a dry day (roof not flat obviously) or the roof was wet after a rain.

If you want to DX anytime the roof is wet... get it higher. If you want a good pattern even on a dry day... get it higher. The antenna is going to capacitively couple to the roof wet or dry... but wet it's going to be worse. That coupling will both detune the antenna and it will pull the radiated energy of the antenna down... Let me put it another way, would you run that antenna five feet above the ground and expect it to work well? It would be a very quiet antenna on receive... but also a very quiet antenna on TX!!! The ground will suck up your power. So will the roof if it is metal or when it is wet.

Use a 5 foot (or if you can find it a 9'4") Rohn tripod then go up 10-15 feet and guy it within a foot or so of the antenna. With respect to the wind and wind load, you want that antenna secured within a foot of the base. Take it from a guy who had one get blown over. (Thank goodness it was in my back yard and not on my roof!) With each foot of extra mast above the highest guy and/or mounting point, the lateral force from the wind load grows exponentially. You want that distance as short as possible.
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KU7I
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Posts: 127




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« Reply #5 on: March 18, 2013, 12:16:04 AM »

N4UFO, Kevin, great post, very useful info. Thanks.

Lane
Ku7i
JH1JCM

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