## 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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|