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Author Topic: HF antenna experts- Interesting antenna in Diamond  (Read 362 times)
N3MG
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Posts: 27




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« on: May 01, 2004, 11:00:15 PM »

There is a very interesting HF antenna inside Diamond Head Crater here on O'ahu. There is a small military installation (I think part of Civil Defense) that you drive past as you get to the crater summit trailhead.

The antenna looks to be a variation of a log periodic, but instead of cross feeding the elements in the center along the boom, every other pair of elements on opposing sides of the antenna are connected together at the element tips with another piece of aluminum tubing that runs parallel to the boom. Even the "odd" side of the last elements have an aluminum "T" their tips that is not shorted back to the prior element. The antenna is large ~40 foot boom and has about 8 elements.

Here is a link to a photo of the antenna.

http://home.hawaii.rr.com/n3mg/dhant1.jpg

I tried to make a diagram of the antenna, but the posting software does not recognize all of the spaces and clumps togther all of the "1"s I used to diagram the antenna.

The antenna gets more interesting as there are 2 of these antennas mounted together in a "corner reflector" or horizontally polarized "V" configuration. The smaller "director" ends of the 2 antennas are tied together. I could not see exactly where the antenna was being fed. Judging by the size of the longest elements, the antenna it looks like it might go down to the 10 Mhz range.

Is anyone familiar with this antenna design? Curious if this type of antenna has any value in the amateur community. I would suspect the "V”configuration is a means of lowering the angle of radiation short of phasing the antennas on a tower in the classic fashion we are all familiar with. The element phasing appears unusual. I am also a bit puzzled as to which end is the front of the antenna, as one would think the open end of the “V” would be the front, but this is also the end that has the longest elements.

73 to all-

Mark N3MG/KH6
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W3JJH
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Posts: 1421


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« Reply #1 on: May 02, 2004, 12:00:57 AM »

The antenna in your picture appears to be a Collins Radio 237A-2.  These log periodic arrays were build in the '60s for the DoD and NASA.  Very few survive.  They are usually installed at about 65 ft and cover 11 to 60 MHz with 6 dB of gain.  The feed point is at the shortest element.
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K6AER
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Posts: 3525




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« Reply #2 on: May 02, 2004, 12:01:51 AM »

The antenna you discribed is a clamped-mode Log-Periodic. This configuration is to give the antenna additional gain with less elements as formed in a standard LPA. By using two half-wave antennas in a broadside array (upper and lower)you can obtain a much simpler structure by use of the clamp mode technique. This is done for frequencies from 20-60 MHz. At frequencies below that the antenna is to large to be rotated and is generally built in the standard flat configuration.
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OBSERVER11
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Posts: 657




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« Reply #3 on: May 02, 2004, 01:03:12 AM »

I have only seen two of these, one near the opening to the EOC in Diamond Head, the other on I-35 near I-70. Interesting antenna indeed! I would love to play with one.
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K0IZ
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Posts: 738




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« Reply #4 on: May 02, 2004, 09:09:28 AM »

TO OBSERVER11 - I live near I70/I35 and curious as to where you saw this antenna.
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