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Author Topic: How does this antenna work or is tuned?  (Read 841 times)
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Opcom
Patrick J. / KD5OEI
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« on: June 02, 2024, 07:10:05 PM »

This odd military antenna is an AT-438/GR, for 154-176MHz. Maybe it can be modified for ham use but I have not been able to find this design in the antenna books here, or find an explanation online matching this arrangement. It's listed as a folded dipole 15".

The west coast antenna manual shows folded dipoles, but they are shown as a center-fed  regular dipole, with an unbroken parallel wire running from one end to the other. So yes it is shorted just like this vertical ground plane, but the AT-438 is apparently not a balanced antenna. Or is it?

I'm missing something LOL.


* AT-438_GR.png (27.29 KB, 1095x709 - viewed 115 times.)
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Radio Candelstein
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« Reply #1 on: June 02, 2024, 07:42:31 PM »

It's a Folded Monopole.  It amounts to half of a folded dipole, similar to the way a quarter wave ground plane is half of a plain ole dipole.  The idea is to raise the resistive part of the impedance up from the theoretical 36 ohms of a ground plane.
There's some bandwidth advantage due to the thickness of the structure.  But, top to bottom it should be around a quarter wave at frequency of interest, as with the radials.
Assuming it's not some magic military input connector...what's it look like on an impedance measuring setup?  Or an SWR sweep.  One nice thing, in some environments, is that it sits at DC ground and may be less susceptible to precipitation static.  I've mostly seen them on roofs at airports, so look for a match down in those frequencies if you can't find happiness in the ham bands.
73 de Norm W1ITT
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WD8BIL
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« Reply #2 on: June 03, 2024, 08:21:46 AM »

Two ways to achieve a match.

1) Add to the coil
2) Lengthen the radiating element.

#2 is preferred since it maintains the efficiency.
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W2JBL
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« Reply #3 on: June 03, 2024, 08:50:44 PM »

That design is common in the two way industry mostly on VHF-Low 30-50 MHZ. Made by Celwave and others. They are very broadband and yours will likely work without mods on 2 meter CB. I have one that was cut for 31 MHZ and it's flat at 29.0
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Opcom
Patrick J. / KD5OEI
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« Reply #4 on: June 05, 2024, 05:01:32 AM »

The light just went on! Folded monopole. Of Course! I guess it was assumed a natural variant of the foded dipole in many books and not mentioned.

It's on a crank-up tower on a ranch in rural AR.

After applying an MFJ-259 to the end of the cable at terra firma, I told the gent he'd better get it and the cable down for inspection. The trouble is that it has poor VSWR throughout the stated VHF range and just as bad on 2M. I did sweep it from HF through 175Mhz just to see what the whole mess looked like.

Either something's wrong with it, or the 75FT of cable itself being very old is decayed and might be flattening it out around an average of 3:1 to 6:1.

I only judged the height of the loop above the ground plane from the earth using a sextant, but it'll be more precise once it's at hand. The total length of tubing would be twice that, plus inverted U-shaped 180 degre curve at the top.

The gentleman has an interest in monitoring 2M, the sheriff, and VHF air band. I keep after him to get licensed and talk to me on AM HF using NVIS antenna, which is real easy to put up and he could also use for SWL.

I don't care for 2M much. The constant social media-esque prattle and/or clique-ish behaviour on some of the local repeaters have turned me off to it. The only time I ever use it is if I volunteer for something.

For what he wants to do, a discone would be better.
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Radio Candelstein
Steve - K4HX
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« Reply #5 on: June 05, 2024, 10:37:07 AM »

Attached is a pic of this type of antenna.  I saw it on the top of a fire station recently. Looks like it has been there for a long time.


* IMG_2977.jpg (1015.91 KB, 4032x3024 - viewed 86 times.)
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