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 Post subject: ferrite beads on coax
PostPosted: Apr Mon 01, 2019 4:48 pm 
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Location: Bath, Maine 04530
What type and size and number ferrite beads would you use on the outside of a coax going to a dipole antenna to stop RF coming back into the shack? Using RG 8X. 80 through 10 meters. Thanks, Carl

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Mon 01, 2019 6:57 pm 
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Joined: Aug Thu 20, 2015 3:09 pm
Posts: 1262
Location: Albion, CA, USA, 95410
I prefer an at the feed point balun for choking coax fed dipoles but beads work as well. This page will get you something you like and Palomar makes great products.

https://palomar-engineers.com/antenna-p ... balun-kits

Jonathan


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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Mon 01, 2019 7:54 pm 
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Joined: Jun Mon 22, 2015 6:21 pm
Posts: 395
Location: Bath, Maine 04530
madlabs wrote:
I prefer an at the feed point balun for choking coax fed dipoles but beads work as well. This page will get you something you like and Palomar makes great products.

https://palomar-engineers.com/antenna-p ... balun-kits

Jonathan

Thank you Jonathan. Just what I want to use. I am trying to keep the feed point of the dipole as lite as I can to reduce the sag. A coax wound choke balun would be much heavier. Carl

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Mon 01, 2019 8:30 pm 
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Joined: Aug Mon 09, 2010 2:57 am
Posts: 565
Location: York PA.
http://www.k9yc.com/2018Cookbook.pdf

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Mon 01, 2019 8:34 pm 
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Joined: Aug Thu 20, 2015 3:09 pm
Posts: 1262
Location: Albion, CA, USA, 95410
Glad they had what you needed. Have fun and maybe I'll meet you on the air someday.

73,

JOnathan


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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Tue 02, 2019 12:16 am 
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Joined: Jun Mon 22, 2015 6:21 pm
Posts: 395
Location: Bath, Maine 04530
k3msb wrote:
http://www.k9yc.com/2018Cookbook.pdf

Mark, They did a LOT of testing. Thanks, Carl

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Tue 02, 2019 2:55 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 3848
Location: Lincoln City, OR 97367
Greetings to the Forum:

I had a popular balun for this purpose made by Unadilla (W2AU). I was not impressed with the weather sealing, and I was VERY not impressed by the fact that there was insufficient ferrite in the core to enable the thing to work on 80 meters. it fell apart in the middle of the band and refused to pass much energy below 3700 KHz or so.

I myself use ferrite choke beads on the coax jacket; you can keep adding them until you have enough to accomplish the decoupling at the frequency of interest.

Just my $.02 worth; your mileage may differ.

Regards,

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Tue 02, 2019 12:22 pm 
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Joined: Jun Mon 22, 2015 6:21 pm
Posts: 395
Location: Bath, Maine 04530
Jthorusen wrote:
Greetings to the Forum:

I had a popular balun for this purpose made by Unadilla (W2AU). I was not impressed with the weather sealing, and I was VERY not impressed by the fact that there was insufficient ferrite in the core to enable the thing to work on 80 meters. it fell apart in the middle of the band and refused to pass much energy below 3700 KHz or so.

I myself use ferrite choke beads on the coax jacket; you can keep adding them until you have enough to accomplish the decoupling at the frequency of interest.

Just my $.02 worth; your mileage may differ.

Regards,

Your two cents well taking and I agree about adding more beads if needed.

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Fri 12, 2019 6:16 am 
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Joined: Aug Fri 29, 2014 6:17 pm
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Location: Vincennes Indiana
It'd be nice to see someone make coax that had ferrite in the jacket, dunno if that'd do much good for sure tho.


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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Fri 12, 2019 5:46 pm 
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There is some science to this... best information was posted by the late G3TXQ.

http://www.karinya.net/g3txq/chokes/

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Sat 13, 2019 3:11 pm 
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Joined: Aug Thu 20, 2015 3:09 pm
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Location: Albion, CA, USA, 95410
Peter, good link, lots of info and I like the mix/turn chart. Bookmarked.

Jim, I have one of those baluns and had the same thoughts about the weather proofing and overall construction. However, it has been in the air for some years at my second QTH and seems to be holding in there. It works on 80 for me.


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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Mon 15, 2019 4:53 pm 
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Joined: Jun Wed 08, 2011 2:33 am
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Location: Ohio 45177
MFJ makes an inline ferrite choke with connectors on both ends, I believe. It is just the same thing as their current balun with a different configuration of connections, apparently. Think it is teflon coax with ferrites on the outside. I have one of those baluns on my 20M antenna and it works fine. But is heavier than a W2AU balun, which I use in a dipole that is end supported and fed with Belden RG 58 to save weight at the center.

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Fri 19, 2019 3:14 am 
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Reading Dr Jerry Sevick on baluns and ununs, he dismisses the idea of putting beads over coax as it is just adding cost unnecessarily. Wrapping coax through a ferrite toroid increases inductance by the turns squared. More bang for your buck and only one device needed.
Personally, I've never found it necessary to put a choke(aka Balun) on a dipole, unless there is some unbalance going on somewhere. If you have rf in the shack then there definitely is an unbalance. But that's an unusual situation. Don't waste your money.

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Fri 19, 2019 10:55 am 
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Joined: Jun Mon 22, 2015 6:21 pm
Posts: 395
Location: Bath, Maine 04530
Phil Coe wrote:
Reading Dr Jerry Sevick on baluns and ununs, he dismisses the idea of putting beads over coax as it is just adding cost unnecessarily. Wrapping coax through a ferrite toroid increases inductance by the turns squared. More bang for your buck and only one device needed.
Personally, I've never found it necessary to put a choke(aka Balun) on a dipole, unless there is some unbalance going on somewhere. If you have rf in the shack then there definitely is an unbalance. But that's an unusual situation. Don't waste your money.

I agree with you and never had a problem with dipoles. I am going to add some ferrite when I change the coax as a preventive maintenance step if they were cheap and light. I found some at very low cost. Once the antenna is up it's there to stay ,I HOPE. I am to old and crippled up to do much antenna work. AKA very bad back. Carl

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Sat 20, 2019 1:10 am 
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Joined: Nov Mon 02, 2009 7:01 am
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Location: Lincoln City, OR 97367
Greetings to Phil and the Forum:

You wrote:

Quote:
Reading Dr Jerry Sevick on baluns and ununs, he dismisses the idea of putting beads over coax as it is just adding cost unnecessarily. Wrapping coax through a ferrite toroid increases inductance by the turns squared. More bang for your buck and only one device needed.


As usual, the devil is in the details. It is difficult to find a single toroid that is rated for RF frequencies and has a large enough center hole to accommodate several turns of cable and still allow the connector to pass through. If you do find one, it is both heavy and expensive. If you are using a feed line with connectors on both ends, this means that if using a smaller torroid, you then have the tricky job of installing a coax connector very close to the toroid physically, possibly working on the roof.

The snap-on beads from Amidon are a lot simpler and easier to implement.

As far as not needing an RF choke, you probably don't as long as you are close to the natural resonant frequency of the antenna. Depart from it very much... to the point where you need an antenna tuner in the shack... and the RF on the outside of the shield will be considerable. Since I run a multi-band antenna, a choke is a pretty good idea.... at least for me. Your mileage may differ.

73,

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 Post subject: Re: ferrite beads on coax
PostPosted: Apr Sat 20, 2019 7:37 pm 
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Location: Bath, Maine 04530
Hay Jim, I am running a fan dipole and have ten snap on ferrites. I don't know how many of them I will use. I am changing the coax and retuning the three bands it is cut for. 40, 20, and 10. 15 might get close enough to use but I do have tuner. Work bands with tuner. I am looking for as little weight as I can work with to reduce the sag in the center. I may not have any hair left when the antenna comes in tune, HI HI. Carl

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 Post subject: Re: ferrite beads on coax
PostPosted: May Fri 17, 2019 3:09 pm 
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Quote:
As far as not needing an RF choke, you probably don't as long as you are close to the natural resonant frequency of the antenna. Depart from it very much... to the point where you need an antenna tuner in the shack... and the RF on the outside of the shield will be considerable

Sorry but I disagree completely with that premise. Deviation from resonance can have no effect on common mode currents on the shield. Only unbalance will cause it. I have never experienced rf in the shack by operating off resonance.

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 Post subject: Re: ferrite beads on coax
PostPosted: May Sun 19, 2019 1:46 am 
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Location: Costa Mesa, California
When I am camping, I run an 11’ whip with a loading coil and sliding tap. This clamps to an adjustable metal pole about 4’ off the ground. There is a ground plane made by 8–12’ wires run to metal stakes. I use 50’ of coax to my tent. Between the coax and the antenna, I have a common-mode choke made with 18’ of coax wrapped on a 4” ABS pipe form with the ends secured using wire zip ties. The transceiver is a Yaesu FT 897D running 100 watts. It works very well. The transceiver does have an auto tune attached and the antenna must be tuned with a RigExpert AA 54.

Norm

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 Post subject: Re: ferrite beads on coax
PostPosted: May Sun 19, 2019 2:44 am 
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That may be a waste of 18' of coax. Have you tested the effects of not having the choke in place?

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 Post subject: Re: ferrite beads on coax
PostPosted: May Sun 19, 2019 3:14 am 
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Common mode current is pretty hard to measure. The coax choke is easy to make and cheap. 8 turns for 20 meter, 14 for 40, 24 for 80– according to the “experts”.

Norm

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