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Author Topic: Love wire antennas.  (Read 1699 times)
NY0Y
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Posts: 18




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« on: July 16, 2012, 11:02:08 AM »

Alas, tis true...  My all time favorite so far is the Bobtail Curtain.  5+ dB of vertically polarized, low angle gain in a nice figure 8, with little ground radial concern.  I did mine with one 8' ground rod and some chicken wire!

I have just been reading the Antenna Compendium #2, and an antenna reported by K5RP, Russel Prack caught my attention.  It is a magnetic slot (or two vertical radiators) which is pretty low to the ground, and it has claimed ground independence, and good low angle results.  He says that one version for 80 meters is only 14.5 feet tall!  And the 40 meter one is only 15.5 feet tall, with the lower wires 3' off the ground.  It feeds (with a matching section of 75 ohm coax) like a dipole.

Has anyone ever used one of these, and how well do they work?  I have room for an 80 meter one, and I might be able to squeeze in a Top Band one.  Height would only be 40'!!!  Is this smoke and mirrors?  Or do these things really work?  The compendium was published in 1989, but I would have thought I would have heard something more about them.  But nada!

73

Raleigh
NY0Y
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GILGSN
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Posts: 205




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« Reply #1 on: July 16, 2012, 04:17:41 PM »

Hi, do you have a link?
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WB6BYU
Member

Posts: 13287




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« Reply #2 on: July 16, 2012, 04:51:47 PM »

You can find a lot of information about such antennas (part of a family that the late
W4RNL called "Self Contained Vertical" antennas) on his website here:

http://www.cebik.com/content/a10/scv/scv0.html
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KC4MOP
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Posts: 741




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« Reply #3 on: July 17, 2012, 04:18:31 AM »

Cebik is a vault of valuable information, but requires a registration process and continual renewal and endless email.
The guy must have dedicated his life to antennas.
I guess they are protecting the intellectual knowledge base contained in their site.

Fred
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K1CJS
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Posts: 6034




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« Reply #4 on: July 17, 2012, 05:07:09 AM »

Any antenna will 'work'.  The question is--how well will they?  Usually the claims made are for the antennas being used under ideal conditions--something that we know just doesn't exist.

The same thing that is said for these 'deals' that we sometimes see is true for antennas--but far more so.  If it looks too good to be true, it more than likely is.
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K4SAV
Member

Posts: 1842




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« Reply #5 on: July 17, 2012, 07:47:43 PM »

I remember reading about that antenna many years ago when I bought that antenna compendium, and found it interesting.  I remember analyzing it but I couldn't find the file, and that was a long time back, so I modeled it again.  The article doesn't say what the gain of this antenna is, and being so low to the ground (bottom wire at 2 ft, top wire at 23 ft for the 80 meter version) I was interested to see what the gain was. 

I did a model of the 80 meter version (3.55 MHz target).  Using the formulas provided it came out resonant on 3.41 MHz, so I adjusted the horizontal wires to bring it up to 3.55 MHz.  That required shortening the horizontal wires by 4.7 ft.   The feedpoint impedance was 128 ohms. The 2 to 1 SWR bandwidth was 110 kHz after matching.  The radiated pattern is all vertically polarized.  The azimuth pattern agreed with the figure in the book.  The take off angle was 27 degrees and the gain over average ground was 1 dBi.  Not too bad for an 80 meter antenna with the top wire at 23.1 ft.  That's about 1 dB better than a 67 ft vertical with 8 ohms of ground loss, although this antenna is not omni-directional.   The gain off the ends of the antenna is down about 10.6 dB from broadside.   At 15 degrees elevation it's also about 6.1 dB better than a dipole at the same height of the top wire.  Of course the dipole wins big time for high elevation angle signals because this antenna has a null overhead.

The 80 meter version takes up a significant amount of real estate (112 x 24 x 23 ft high) but so does a vertical, counting the radials.  A 40 meter version wouldn't be too bad.  At 12-13 ft high, that could be hidden, for those that have a problem with antenna restrictions.  A 160 meter version is going to be big.   An inverted L would be a better choice for 160.

Raising the 160 meter version by 6 ft (which puts it out of reach of curious fingers) only improved the gain by 1 dB.

Jerry, K4SAV
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LA9XSA
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Posts: 376




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« Reply #6 on: July 20, 2012, 03:16:33 PM »

By your simulation results it sounds like a pretty good DX antenna, if you can rotate it, since you can null out undesired noise sources and local signals, and get a good take-off angle.
Is it comparable to a vertical phased array?
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K4SAV
Member

Posts: 1842




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« Reply #7 on: July 20, 2012, 04:03:09 PM »

It acts like two very short verticals separated by 0.4 wavelengths and fed in phase.   Radiation from the horizontal wires cancel.  Those verticals are very short (about 21 ft on 80 meters) so there isn't much gain (maximum about 1 dB more than a 1/4 wave vertical).  A phased array of 1/4 wavelength verticals would have more gain.  It does have a low take-off angle, same as a 1/4 wave vertical.  Rotating an 80 meter version would not be practical.   

The advantages of this antenna are that it can be built without tall supports and it requires no radials.  The disadvantages are that it takes up a lot of horizontal space and it isn't omni-directional.

Jerry, K4SAV
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