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Re: [TowerTalk] One off balun question

To: "Steve Hunt" <steve@karinya.net>, <towertalk@contesting.com>
Subject: Re: [TowerTalk] One off balun question
From: "Jim Thomson" <jim.thom@telus.net>
Date: Thu, 30 Oct 2014 03:22:18 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
##  I realize the uh, measured at low freqs tells me little about the CM Z of a 
choke.  I was just interested to know how much uh you folks are seeing on these 
bigger  CM chokes, where 5-15 turns of coax are used 
on 4-8 cores, that type of scenario.    The only torroid CM choke I have on 
hand is the one described..with 4 turns of 393 on 4 x type 31 cores.   I have 
loads of 393 coax, and would like to optimize a better CM choke
for my 40m yagi......so it only has to perform on that one band.   I will have 
to order some more type 31 or possibly type 43  cores in the 2.4 inch OD 
variety.   I also want to make a better CM choke for my 80m rotary dipole.   
Same deal, that 2nd one only has to be optimized for that one band.    Another 
concern I have is keeping the completed assy  away from the boom of the 40m 
yagi...stray C issues. Ditto with the 80m rotary dipole, since it will be 
mounted to the top of the mast.   I can suspend the assy in mid air, inside a 
nema waterproof box, but don’t know if that will be enough. 

Jim   VE7RF


From: Steve Hunt 
Sent: Thursday, October 30, 2014 2:09 AM
To: Jim Thomson ; towertalk@contesting.com 
Subject: Re: [TowerTalk] One off balun question

Jim,

Yes - I've measured the inductance using a low frequency meter "just for fun"; 
but that's what it was - "just for fun" - it tells you little about the high 
frequency CM impedance of a choke, for (at least) a couple of reasons:

1) The permeability of a ferrite core is frequency dependent and complex. 
Measuring inductance at a low frequency will probably give you a good handle on 
the "base" u' permeability, but tells you nothing about u' and u'' at HF/VHF

2) The effect of inter-winding capacitance will be negligible at 1kHz. But that 
capacitance becomes a very significant factor in a choke's impedance at higher 
frequencies, and it can't be easily deduced from the low frequency inductance 
measurement.

Steve G3TXQ







On 30/10/2014 07:32, Jim Thomson wrote:

After looking at both Steve Hunt and Jim Browns material in depth, I have a 
general question. 
In regard to the many various 1:1 choke baluns on both their sites, has anybody 
actually measured
the uh between the input..and the output ??     IE: center conductor to center 
conductor.... or braid
to braid. Like using a LCR meter at low freqs.      This is more out of 
curiosity than anything.  
I measured the uh on my type 43 bead balun, 17 inchs long, using the 1 inch od 
beads slid over
RG-393 coax, and it measured aprx 38 uh. The 24 inch long version of the same 
balun measured
aprx  57 uh.  Uh appears to be proportional to length.   I also measured a 17 
inch long bead balun, but
using the old type 77 material, and it was 63 uh.   Just for fun,  I also 
measured my torroid balun...
which consist of 4 turns of RG-393 wrapped around 4 x 2.4 inch OD, type 31 
cores.... 126 uh. 

If 4 turns of 393 (.390 inch od teflon coax)  on 4  x 2.4 inch od cores is 126 
uh, then some of the designs
I see on both steve and Jims site must have huge amount of measured uh.   IE: 
5-14 turns on 5++  cores. 
BTW, I used my B+K 875-B LCR meter to measure uh.  It does the reading at 1 
khz. 

Tnx.... Jim   VE7RF
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