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[TowerTalk] W0IYH Feed line Choke Performance

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Subject: [TowerTalk] W0IYH Feed line Choke Performance
From: (Jim Smith)
Date: Wed Aug 20 01:03:12 2003
The WA2SQR info is great.  However, it seems to me that one can take 
this further.  

In the original work one is cautioned not to "bunch wind" the coax. 
 However, if you look at the data for the 8 turns bunched, starting at 1 
MHz and stopping at 6 you see typical parallel LRC behaviour below the 
resonant frequency with ever increasing impedance and phase angle 
getting closer and closer to zero.  Somewhere between 6 and 7 MHz the 
phase angle goes through zero and becomes negative.  i.e. resonance is 
somewhere between 6 and 7 MHz with consequent very high impedance.  To 
see how high the impedance can be near resonance, look at the 8 turns 
single layer data for 12 MHz.  Here the resonant frequency is very close 
to 12 MHz as shown by the phase angle being quite close to zero.  The 
impedance is a dizzying 15,480 ohms.  (Take that, you $0.40 each ferrite 

So, it makes sense to me that for single band antennas one should bunch 
wind the coax and resonate it for the band in question.  Admittedly, you 
will need some way of determining the resonant frequency as bunch 
winding, by its nature, doesn't lead to easily predictable values of L 
and C.  However, for a given frequency, it looks to me as if a bunch 
wound choke will be physically smaller and lighter.

As not everyone has the equipment or skill needed to determine the 
resonant frequency of a bunch wound choke, maybe someone would take the 
trouble to find out how many turns on the 4-1/4" or 6-5/8" form, as 
appropriate for the band, it would take to produce a resonant choke for 
each band.  Maybe this will be me as I'm going to want to put chokes on 
a bunch (maybe I should have used a different word here) of feed lines 
in the (hopefully) near future.  Don't hold your breath though.

I suppose that for a multi band antenna if you want the ultimate in 
choke impedance you could wind one for each of the bands and cascade them.

Of course, if 500 ohms really is enough impedance to reduce the common 
mode current to a negligible amount then none of this matters.  Is a 20 
to 1 reduction of coupling of noise picked up by the coax and fed into 
the antenna on receive enough?  I don't know but 20:1 is 26 dB (4 S-Units?).


73 de Jim Smith    VE7FO

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