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Author Topic: Converting a circular polarization antenna  (Read 10097 times)
AF5CC
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Posts: 1018




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« on: June 14, 2013, 08:26:37 PM »

I am looking at a Cushcraft 416TB circular polarized antenna for UHF terrestrial SSB/CW use.  If I were to remove the one set of elements, and only attach the feedline harness to the one driven element, how bad might it affect the pattern of the antenna and the feedpoint impedance?  Anyone been successful in converting a circularly polarized antenna into just a horizontal polarized one?

73 John AF5CC
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N4UFO
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« Reply #1 on: June 15, 2013, 06:44:37 AM »

I just bought a a KLM that was converted. He removed one set of elements and changed the feedpoint. Apparently it worked fine for him. But I have to put it all back.   Undecided
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AF5CC
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« Reply #2 on: June 15, 2013, 09:41:25 AM »

Any idea as to how the feedpoint was changed?

73 John AF5CC
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N4UFO
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« Reply #3 on: June 19, 2013, 07:03:52 PM »

Well, with this antenna, there are two feedpoints... one for each polarization. He just hooked up the feedline to one feedpoint only I believe. It needs the matching section left on through... the loop of coax from one side of the feedpoint to the other.

Kevin

( you know, it's too bad I already unboxed this KLM... I'd have offered to trade you. I have a Cushcraft 2m to go with it just like on the cover of the 416tb manual.)  Cheesy
« Last Edit: June 19, 2013, 07:12:22 PM by N4UFO » Logged
N7TEE
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« Reply #4 on: July 11, 2013, 05:46:59 PM »

John.

If you are running more than about 250 watts into the antenna you will have to change the feed point to accept the higher wattage.  We did just that so that it would let us run about 1000 watts into it for EME.

Dave
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N6JSX
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« Reply #5 on: July 28, 2013, 07:06:26 AM »

If I read your question properly, you just one to use one side of the antenna. Making the antenna into effectively a Horz/Vert polarized like the Cushcraft 144-20T antenna.

You do not need to remove the elements as cross-polarization has ~18db of difference so the parasitic elements will not have much of an affect.

The 416b uses a 1/4wave 'offset elements' to create circular polarization with a equal-phase coax impedance splitter. Do not use the splitter for discrete polarization OPs.

To create a dual polarized antenna - mount the antenna putting the cross in the vertical & horizontal. Run TWO coaxes (one to each driven element/polarization) or obtain a UHF mast mount relay switch to select what ever polarization a single coax feed you want to connect too. (FYI, I put 12VDC into the coax to energize the relay, like the Ameritron 4X tower antenna switcher does).

With this configuration you will have a vertical FM repeater beam and horizontal SSB beam.

[FYI, another item to consider that WILL affect the vertical polarization pattern is if a vertical metal antenna mounting mast is used. To negate skewing the pattern due to the extra metal, consider using PVC w/wooden stiffener or fiberglass mast.] 
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