[TowerTalk] One off balun question

[Steve Hunt]

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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