[TowerTalk] Ladderline - what are the facts??/

[Dan Zimmerman N3OX]

>
> Unfortunately, that's taking the macro view when what's happening is micro.
> AND,
>
losses are sufficient that the magnetic circuit isn't very close to ideal.
>

Well that particular case was just talking about how it's a bad idea to wind
your common mode choke using widely seperated windings on either end of a
bar.  That would apply for even very low loss materials as far as choking
impedance went.  Core heating would be less of a problem.  But it'd be a
terrible choke.  It'd be a couple of inductors in series with the
differential mode with a little tiny mutual coupling.  That was my point.


> N6RK has suggested
> that in addition to the choke under test, a coaxial choke wound according
> to my
>
guidelines should be added to the line to insure that the common mode
> voltage/current is small.


I'd think the common mode impedance of one of your chokes would be hardly
changed by the substitution, though I guess you wouldn't necessarily be
sure.

I'd think it would actually reduce inter-turn capacitance effects to switch
from coax to twisted pair.

But common mode effects would cause a problem, and it's probably best to
keep all other things equal.  Wouldn't it just be easier to test into dummy
loads though?

For what it's worth, I just did a quick check with a bead balun I have
laying around made with 50 #43 beads strung on a twisted pair of #14 Teflon
and  14MHz.  My 100W Bird slug in a thruline fixture reads within a
needlewidth of the 94W mark on both transmitter side and dummy load side of
the balun.  No idea of the absolute calibration but the relative change is
nil.  No warmth detected.

A single bead on a single wire on my MFJ-259B reads 11+j26 where the loop of
wire reads 0+j14.  11+j12 around 14MHz would be about right for a single
small 43 bead (these are the  ones that juuuuuuuuuuust fit over RG-58 and
are about as long as the ID).

And anyway, that would fit your definition of "lossy" ferrite I suppose?
I'll try later with some other stuff I've got laying around.  I've got some
73 binoculars that should heat up pretty good with even a couple watts.

I think I have some big 31 clamp ons even.

Anyway, I look forward to hearing what others have to say on the matter.

Another measurement that might be interesting would be the change in input
resistance of a properly terminated twisted pair as you wind it on a core.
The differential mode core losses should show up as a change in the input
impedance of that line, right?     Depending on the relative sensitivity and
calibration of your impedance measuring gear vs. your power measuring gear,
one approach might be better than another.



Dan