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Author Topic: Any point installing ferrite beads on shack end of coax?  (Read 1727 times)
ES1TU
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« on: November 05, 2012, 01:48:24 AM »

Hi there,

I'm doing some coax work in the station and had to cut PL259-s from the coax.
I had an idea that it's a good moment to add some ferrite beads to the coax, before soldering back the connectors.

So my question - is there any point in having ferrite beads around the coax on the shack end of cable? I do have them near the antenna feedline (to prevent coax shield radiation).

Thanks
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AC5UP
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« Reply #1 on: November 05, 2012, 03:45:27 AM »

Common Mode & "RF In The Shack" issues are best remedied at the antenna with a choke Balun at the feed point or a better antenna design.

I can't think of any reason to add ferrites at the radio end of the coax unless you have a specific need for them.
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W5DXP
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« Reply #2 on: November 05, 2012, 05:21:17 AM »

So my question - is there any point in having ferrite beads around the coax on the shack end of cable?

What the beads usually do is cause most of the common-mode signal to be reflected (thus forming a common-mode standing wave minimum current node) so it is best to install them near the antenna. However, beads near the shack can reduce whatever common-mode signal made it through the beads at the antenna end and still exists on the feedline. In fact, one of the most effective common-mode choking functions is to use two chokes spaced 1/4WL apart. And, of course, if it is a ladder-line fed dipole, the ladder-line/coax junction is just about the only place beads can be installed.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
ES1TU
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« Reply #3 on: November 05, 2012, 06:24:39 AM »

Main reason I'm asking this: until now my coax used to run from the roof to the shack (in the basement) via a unused chimney flue (which is more or less in the center of a brick house).
Now that flue will be put to use and I have to remove the coax from the flue and run it down from the roof to the ground on the outer wall of the house.

My main concern is potential RFI to the neighbors.
I have no idea how big issue this is, but I assume that the house was absorbing any potential RFI from the coax, since the chimney flue was more or less in the center of the building.

Now, when the coax will run on the outer wall of the house, the neighbors would probably pick up any RFI, in case there is any. So my only idea was to add additional ferrite beads to the coax.
Not sure how much it would help though. I mainly operate on 20m, 15m, 10m. Output power around 800W.

Maybe I'm worrying too much and seeing the ghost which actually does'nt exist? There has'nt been a single RFI concern from neighbors during my 3 years of QRV.
What do you guys think - is there a reason to worry or not?
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WX7G
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« Reply #4 on: November 05, 2012, 06:40:07 AM »

The neighbors will experience higher RF fields from the antenna than from the coax, as long as the coax shield is choked at the antenna.

I see no harm in placing a ferrite choke at the rig.
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N3OX
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« Reply #5 on: November 05, 2012, 06:57:04 AM »

In almost all cases, it's best to add more choking at the antenna end if you still have substantial current on the coax. 

If you have enough current on the coax to cause RFI to the neighbors above what the antenna causes, you need a better choke at the antenna for sure.  It's different when the coax runs 20cm behind your stereo; there a little bit of residual current can cause a big problem. 

I agree with WX7G that RFI from your antenna should be worse than RFI from your coax unless you have a very pathological situation ... which you, specifically, won't.  The coax needs to be doing a lot of the radiating job to be a worse RFI contributor than the antenna.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
ES1TU
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« Reply #6 on: November 05, 2012, 07:51:32 AM »

unless you have a very pathological situation ... which you, specifically, won't.  The coax needs to be doing a lot of the radiating job to be a worse RFI contributor than the antenna.

Thanks guys! We are very fortunate to have smart people like you in the hobby Wink

Oh, almost forgot - enjoy your Election Day tomorrow.
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K2OWK
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« Reply #7 on: November 05, 2012, 02:28:46 PM »

Just a note, Radio Shack sells clamp on ferrite beads in many sizes and can be used if the  connectors are already installed. They work great. I use them on my computer cables to keep the RF from locking it up.

73s

K2OWK
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