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Author Topic: HF Wire antennas for condo rooftop - suggestions  (Read 1631 times)
HS0ZIB
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Posts: 434




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« on: August 15, 2012, 01:16:20 AM »

I'm making good progress towards the issue of a permanent ham license for operation from my condo in Yangon, Myanmar, (Burma).

My condo building has an accessible flat roof where I am permitted to erect my HF antennas.  But an HF beam would probably not be acceptable to my landlord, so I propose to install a hexbeam for the higher HF bands.  (A 6 meter beam should be OK to install as well).

But what about the lower bands of 40 and 80 meters?  What type of wire antenna or vertical antenna could I use which will provide the best for DX QSOs?  Is a top band antenna even feasible?

I welcome any suggestions, but note that it is also unlikely that I can install radials on the flat roof, (because the condo caretaker will almost certainly trip over them and inadvertantly launch himself off the eight-floor building...)

Thanks in advance for any useful advice

Simon
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N4UM
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« Reply #1 on: August 15, 2012, 03:26:55 AM »

You need to tell us a lot more about your roof.  What are it's dimensions?  How high is it?  What sort of structures are already there?  Is there room for radials if you go with a vertical?  160M is probably not an option. You can probably get away with a Hustler 6-BTV at least for 40 meters...maybe even a narrow slice of 80M if you have room for radials.  If the roof is quite high (75 feet ior more) you might get away with a dipole for 40 and 80...again it all depends on the dimensions and the presence of existing structures alredy on the roof.
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HS0ZIB
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« Reply #2 on: August 15, 2012, 06:21:40 AM »

The building has 8 floors, so the roof is about 70 feet up, with a clear view over the neighbouring and lower condos.  The roof length and width are not great, perhaps 14 feet wide by 30 feet depth. There are no structures in the roof main area, but there are several C-band mesh satellite dishes (for domestic satellite TV reception) along one edge of the roof.

Radials are probably not an option - for reasons that I stated.  I was wondering if I mounted a hex-beam antenna on a mast in the center of the roof, then installed an inverted V ax for 40 meters that is supported by that mast.

... or somehow use the hexbeam mast as a radiator for the lower bands - but I'm going to install radials for that option?

I'm down on the 4th floor of the building, but have a clear run up the rear of the building for my feeder cables


Simon

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KC9Q
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Posts: 49




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« Reply #3 on: August 15, 2012, 11:18:43 AM »

Don't know how much room you have on the roof, but you could look at a full-wave loop.  No radials required at all, very low noise, does not attract lightning as it is at DC ground.  An 80 Meter loop is about 96 ft per side (square); a 40 Meter loop as about 48 ft per side (square).  If possible it would be easy to use an external auto tuner to feed the loop, and have coax down to the station.  Loop calculation is 1005/F(MHz) for loop size in feet.  Nice thing about a loop is it will operate at ALL harmonics (both even and odd).  Just a thought.

73,
Mike
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W5WSS
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« Reply #4 on: August 15, 2012, 12:25:55 PM »

You could do a trap dipole made of tubing  for 40 60 80m along with and affixed underneath the hexbeam on an adequate vertical support, centered and located middle of roof with a rotor to turn them. The two horizontally oriented horizontally radiating antennas would be very effective from 70ft rooftop height.
Affix them mechanically synchronized so that both point to area of interest.
Also a 14X25 wire rectangle shaped closed loop fed with ladder line to a tuner would work reasonably.
One could install short vertical PVC pipes even 5ft lengths at each corner of the roof then drill small holes through the near top of each to pass the wire like #14 thhn stranded copper wire thread through the holes feed at any convenient location but I like to feed where I can route as straight and stood off as possible.
A loop such as this will work dx nicely from 70ft.
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WB6BYU
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« Reply #5 on: August 15, 2012, 12:37:18 PM »

One solution for radials is to attach them to the roof with tape or caulk, so they can be
walked on rather than tripped over.  You can use fairly thin wire for this.

The roof isn't quite wide enough for a 40m inverted vee with a reasonable included angle,
but you can shorten the elements with loading coils to make it fit the space available,
or run the wires to the far corners of the roof and then bend the ends around to follow
the edge to use up whatever length is needed.  Try to keep the included angle to 120
degrees or more if you can, which may require mounting the feedpoint at less than
maximum mast height unless you can put up support posts in the corner to tie it to.

For 80m or 160m I'd see if there is some way you can run a sloping wire down from the
roof to a nearby tree, or down the side of the building to your window.
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N4UM
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« Reply #6 on: August 15, 2012, 02:04:17 PM »

14 by 30 is a pretty small roof! Assuming you mount the Hex Beam in the center of the roof at a height of 10 feet above the roof, you should be able to support a shortened, inductively loaded 40 M inverted vee about 20 feet on each leg just below the Hex Beam.  I would suggest a pair of 25 uH coils inserted about 6.5 feet in from each end of the inverted vee.  You'll have to play around with the coils and their placement but these figures should get you in the "ballpark."  The Hex beam should work well if the proximity to the 40M antenna doesn't mess it up.  Your call will be exotic enough to work stuff on 40 but I'd forget about 80 till I saw how 40 worked out first.  Keep us posted on what you do and how it works out.
« Last Edit: August 15, 2012, 02:06:49 PM by N4UM » Logged
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