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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