Topband: 10 Turns on FT-140-43 corr. value DE K0FF

Gary Breed
Thu, 2 Nov 2000 14:35:08 -0500

> The actual measured inductances have been running higher, never lower. The
> AL value is supposed to be 952, but has been as high as 1700. No other
> mix has shown variations like this so far.
> I'm getting some more test equipment in here today to look into it  bit
> further but in no case have I seen the AL value drop below the book value.


43 material is commonly used in EMI filtering applications where looser
manufacturing tolerances may be acceptable. It could be the cores you have
were made to meet a spec of "guaranteed minimum" mu of 850. I haven't seen
this variation with Fair-Rite cores, but other companies (e.g. Steward) make
ferrites solely for EMI filters, to the same general spec as 43, but with a
disclaimer that they are not characterized for transformer use. Your
experience points out the need for measurements.

Your experiments are interesting & I've done some myself. My 9:1 xfmrs
use 1.5 inch dia. 43 material toroids -- 8 trifilar turns of #24 telephone
wire put the lowest loss region from about 2 to 5 MHz. Loss is no more
than 0.25 dB.  I have also wound these as a single continuous winding with
a tap 1/3 the way from ground. This is easier for a non-technical ham to
wind and although the loss and mismatch are worse, no one is likely to
hear the difference.

For isolated windings, I use two 1/2" dia x 1/2" long 77 material (mu = 2000)
cores taped together binocular fashion. 9T/3T works OK impedance-wise for
a 9:1 XFMR, but the loss is about 0.4 dB. I also have used these cores for
2:1 xfmrs (7:5 turns ratio) in RX array combiners.

73, Gary

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