[TowerTalk] [Bulk] Re: One off balun question

[Grant Saviers]

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 at 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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