[TowerTalk] Common-mode choke

[Jim Thomson]

Date: Tue, 20 May 2014 08:02:03 -0700
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Common-mode choke


On 5/20/2014 7:05 AM, Jim Thomson wrote:
> if any choke is high resistance, it will work..provide the Z is sky high.
> Power is still I squared x R.

Yes. I have shown in my tutorial why choking Z on the order of 5K ohms 
resistive is a suitable design goal for most installations, and W8JI has 
reminded us of some where that's not enough. The only string of beads 
choke material that can get anywhere close to 5K on HF is the #73 
material that W2DU chose, and you need several hundred of them to get 
there. You would need that higher value for the high power amps you like 
to use. The largest #73 material bead is just large enough to fit coax 
like RG303. There's a discussion of his choke in the two referenced pdfs.

73, Jim K9YC

##  are you sure 5k ohms is required ??   Steve is showing 1100 ohms resistive for the worse case
scenario   where the coax is a half wave long.   1100 ohms  resistive would result in –30 db shield current. 

##  W8JI designed all the dx engineering baluns..and they all use type 61....go figure.   

##  Im not seeing any drop in local noise when I added a choke balun, consisting of 393 coax wrapped 
around several 2.4 inch od  type 31 cores.   This type 31 balun is in series with my type 43 bead balun. 
Type 43 bead balun is at the feedpoint of the yagi....followed by the type 31 torroid choke balun. 
Im not seeing any improvement in FB  etc, with the addition of the 2nd balun. 

##  has anybody tried measuring the shield current using a clamp on RF ammeter ??    IMO...that
alone would be the real acid test.....  but where do you clamp on ?     All the braids of all my coax cables
get grounded at the top of the tower..via the remote switch box.  Any residual RF that gets past the baluns
is now travelling down the tower legs.  I suppose one could clamp on the coax... on the output side
of the remote switch box.    It could also be clamped on the coax on the back of the amp in the shack etc.

##  Then again, the braid is  grounded to the SPG, on the basement wall.   Some of your choke designs
are optimized  for the low bands, some the upper bands.   I was thinking of using 2-3 chokes...all stagger 
tuned... on  the coax... just b4 the coax gets to the SPG. 

##  how do we know when we have enough choking Z ?? 

Tnx... Jim   VE7RF