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[TowerTalk] 1/4w matching, Common mode chokes and BAL-UN

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Subject: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN
From: "Jim Thomson" <>
Date: Fri, 31 Dec 2010 07:11:41 -0800
List-post: <">>
Date: Wed, 29 Dec 2010 20:52:22 -0800
From: Jim Brown <>
Subject: Re: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN

On 12/29/2010 7:35 PM, wrote:
> I think a more accurate description of several coiled turns of coaxial
> cable would be a RF Choke, serving to limit common mode currents on the 
> outside
>   of the coax cable.

That's an Alice In Wonderland view of things. Yes, the coil of coax is 
an inductance in the common mode circuit, but the common mode circuit 
also includes the series impedance of the transmission line. If that 
line is less than 1/4 wave, it is a CAPACITIVE REACTANCE, so the 
inductive reactance CANCELS all or part of it and the common mode 
current INCREASES rather than decreases.  The same thing happens if the 
line is between 1/2 and 3/4 wavelengths. Further, the electrical length 
is based on a Vf of 0.98, NOT the Vf of the coax (because the common 
mode current flows on the outside of the shield).  The 0.98 factor 
accounts for the slight shortening effect of the outer jacket.

###  OK... I'll  buy that.    OK, now what exactly happens  IF the coax is
exactly  1/4 wave long.....or  3/4 wave long.....or  5/4  wave long ???? 

##  W8JI  sez if you use a 1/4  or  3/4  wave of coax...  on a yagi... and
bond the braid to the top of the tower.... you will end up with a HIGH Z   at 
DE. [he's talking abt several thousand ohms].  If the coax is taped along the 
then you may well have to factor in the C..between the outer braid and the boom.
That stray C could easily be measured  with any LCR meter.  It could easily be 
and MAY affect the overall length of the 1/4  or 3/4  wave of coax.   Your .98 
VF  may have
to be tweaked slightly... if applied in the above case... with coax taped to 
boom...and  braid bonded
to top of tower. 

##  The only  easy way to see if his concept works..would be to use a clamp on 
RF ammeter... and run
it along the coax.... BELOW the  top of the tower.   I'd also bond the braid to 
the base of the tower. 
W8JI  sez  that using a balun in this case..[with a 1/4 wave of coax..with 
braid bonded to top of tower] 
will make matter's  worse..and  will not supplement the high Z  of the  1/4 
wave of coax.   I find that
hard to believe, I think they would supplement each other.   It seems to me his 
concept  hinged  on
the actual  height of the tower   VS  the band in question. 

Later.... Jim   VE7RF  

The only effective common mode choke is a very lossy parallel resonant 
circuit, which is what you get if you wind multiple turns of coax around 
a lossy ferrite core (like #31 or #43). All you have to do is use the 
number of turns that puts the very broad resonance where it covers the 
bands you work.


73, Jim K9YC


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