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Author Topic: HF Yagi : Coaxial Chokes ???  (Read 3335 times)
N6JSX
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Posts: 217




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« on: April 27, 2013, 03:11:11 PM »

The Cushcraft A3/A4 states/shows these HF Yagi beam antennas require a coaxial choke: 8-turns (close spaced) at 6" diameter using a non-foam coax (like RG-8U).  Shocked

What is does not say or show is if the coax coils can be placed so the boom goes through the center of the coils - allowing very good physical stability mounting location? Or must the Choke NOT be in the same plane as the RF element field?

It also does NOT state maximum pig-tail length between the driven element and choke coil. 

What should be consider when installing this HF choke?

RadioWorks.com has a Yagi-balun?
How about using an UglyBalun?
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W0BTU
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Posts: 1852


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« Reply #1 on: April 27, 2013, 03:31:41 PM »

Cushcraft says that, do they? Well, maybe that works, I've never tried it.

But air core chokes made from coax typically have a much narrower bandwidth than the correct ferrite-cored choke. Take a good look at the Common Mode Choke Impedances chart on http://www.karinya.net/g3txq/chokes, and you'll see.

After I tried some things from K9YC's excellent paper on ferrite chokes at http://audiosystemsgroup.com/publish.htm, I stopped fiddling with narrow-band air-core baluns made from coax. Jim's info there beats anything I've ever read anywhere, either in a book or on the Web.
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N6JSX
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Posts: 217




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« Reply #2 on: April 28, 2013, 10:07:30 AM »

Thanks for your OPINION, but I'd like to get an answer to my 'Coil' questions not a hijacking alternative.  Angry
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KE6EE
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Posts: 455




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« Reply #3 on: April 28, 2013, 02:36:13 PM »

What is does not say or show is if the coax coils can be placed so the boom goes through the center of the coils - allowing very good physical stability mounting location? Or must the Choke NOT be in the same plane as the RF element field?

It also does NOT state maximum pig-tail length between the driven element and choke coil. 

What should be consider when installing this HF choke?

RadioWorks.com has a Yagi-balun?
How about using an UglyBalun?

Coax chokes (or ugly baluns) have inductance/capacitance values based on their cores which are invariably: 1. air or 2. a length of plastic pipe, cylinder or tube with suitable dielectric properties (this means than not just any piece of plastic will work).

A choke with a piece of metal tube going through its center is not going to have the same inductance/capacitance values as a choke with an air core or wound on a suitable dielectric form. Yes it would be mechanically nice to wind the coil around the boom, but the coil must be accurately designed to function appropriately in this form.

The length of connecting cable ("pigtail") is probably not critical until it becomes a significant portion of a wavelength (say .125).

You should consider what the purpose of the choke is before you decide whether you need one and whether to build one. The two references cited above will give you all the information you need to make these decisions. They are superb references.

Form follows function, or should IMHO. Decide what you want something to do and then decide how to build and install it.
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KD8GEH
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Posts: 483




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« Reply #4 on: April 29, 2013, 08:56:41 AM »

Hi Kuby,

Hope your doing okay up there.  I always put mine below the beam on the mast out of the rf plane. No biggie on pig tail length. I made mine from a 8 in piece of PVC to G3TXQ's data. Of course a Ferrite is better...but I have had good luck just following the instructions lol.  No real common mode to speak of.

If you looking to buy one http://www.hy-gain.com/Product.php?productid=BN-86 is pretty okay one packaged. I would stay away from the cheapos.

Email me direct or catch me on the air if you need any help.  PS are you helping the the special events this year?

73  Dave KD8GEH

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KE6EE
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Posts: 455




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« Reply #5 on: April 29, 2013, 09:34:36 AM »

If you looking to buy one http://www.hy-gain.com/Product.php?productid=BN-86 is pretty okay one packaged. I would stay away from the cheapos.

Balun Designs makes high-quality 1:1 isolation chokes. The one below referenced below is $55; the hy-gain is $80.

http://www.balundesigns.com/servlet/the-74/1-cln-1-yagis-dipole/Detail
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K4RVN
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Posts: 812




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« Reply #6 on: April 29, 2013, 05:46:27 PM »

The instructions read to tape the feedline to the boom. Tape the choke to the mast. The braid length feed pigtail is shown to be 4 inches. The other pigtail is not dimensioned but could be determined by measuring after the braid is connected and the coil is attached to the mast. With only 4 inches on the braid pigtail the coil will be very close to the boom and attached to the mast.  I would suggest using nylon or plastic ties in addition to the tape called for in the instructions. Running the coils around the boom is not an option if you are to follow the instructions and would most likely defeat the choke, but don't ever recall anyone who installed one that way, so can't be sure.
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