Jim,
IIRC, my 40m Moxon only is 8 turns on 3x 240-31 and the 80m Tornado
tuned rotary dipole only is 10 turns on 4x 240-31 both of RG142 double
silver braid TFE coax inside a Home Depot plastic junction box and
mounted on the boom or mast respectively. I think the R in each case
was more than 4k ohms at band center on my VNWA. Both in QRO service
for 3 years w/o issues. Not sure how I would know if I had stray C
issues or not.
Grant KZ1W
On 10/30/2014 3:22 AM, Jim Thomson wrote:
## 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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