I've constructed a choke balun from a big toroidal core I had lying
around, abt 3.5 inch O.D., 2 inch ID, unknown material, measures about
125 ohms pure inductive at 7 MHz with 3 turns on it, so I figured that
10 or 12 turns would be an effective 7MHz choke*.

The coax is RG-55 double shielded RG-58 size, solid center conductor,
solid polyethylene dielectric.

I wound the coax pretty tightly for neatness; each turn is probably
only 1 inch inner diameter.

Is this too tight? With a 47 ohm resistor in series with the output
wires (I figure about 0.05 microhenries there, it's a few inches of
wire), I get what I think are more or less expected results right now.
I've been playing with one of Reg's programs to see if the impedance at
the input side of the balun jives with the termination, and I think it
does, but I can't just go up up up in frequency with the MFJ-259 to see
if I've affected the *cable* impedance unless I cut off my weather
sealed, heat shrinked, and Anderson Powerpoled coil tap wires and put
on a BNC and terminator.

So I wonder about the longevity of the thing. How tight do you coil
your toroidal choke baluns?

*Another question, figured I'd put it at the end. What's the
impedance/length of the 43 material core that fits over RG-58 down at
7-10MHz. Amidon and Fair-Rite sites have some spot measurements higher
up that I've found, but I'm not turning up charts for all of HF.

I know 43 material is not used so much for low-power HF choke baluns
because of reduced per-bead impedance, but I got a big box of them for
free, so it's just stringing effort on my part to make a bead choke for
40m.

73,
Dan
N3OX

wrote:
I've constructed a choke balun from a big toroidal core I had lying
around, abt 3.5 inch O.D., 2 inch ID, unknown material, measures about
125 ohms pure inductive at 7 MHz with 3 turns on it, so I figured that
10 or 12 turns would be an effective 7MHz choke*.

The coax is RG-55 double shielded RG-58 size, solid center conductor,
solid polyethylene dielectric.

I am assuming that this means that the O.D. is about .2 inch.

I wound the coax pretty tightly for neatness; each turn is probably
only 1 inch inner diameter.

Is this too tight? With a 47 ohm resistor in series with the output
wires (I figure about 0.05 microhenries there, it's a few inches of
wire), I get what I think are more or less expected results right now.
I've been playing with one of Reg's programs to see if the impedance at
the input side of the balun jives with the termination, and I think it
does, but I can't just go up up up in frequency with the MFJ-259 to see
if I've affected the *cable* impedance unless I cut off my weather
sealed, heat shrinked, and Anderson Powerpoled coil tap wires and put
on a BNC and terminator.

So I wonder about the longevity of the thing. How tight do you coil
your toroidal choke baluns?

I recently wondered about how such tight turns might affect the
longevity of cable in sunlight, but my concerns were dismissed.

*Another question, figured I'd put it at the end. What's the
impedance/length of the 43 material core that fits over RG-58 down at
7-10MHz. Amidon and Fair-Rite sites have some spot measurements higher
up that I've found, but I'm not turning up charts for all of HF.

Fair-Rite has an impedance versus frequency curve for most of their
shield beads.
http://www.fair-rite.com/newfair/pdf...log.pdf#page=1


I know 43 material is not used so much for low-power HF choke baluns
because of reduced per-bead impedance, but I got a big box of them for
free, so it's just stringing effort on my part to make a bead choke for
40m.

What are their dimensions?

Size 2643012702 (.38" O.D., .25" I.D., .25"L) has a Z of about 12 ohms
at 7 MHz, 15 ohms at 10 MHz.

Size 2643625103 (.625" O.D., .312" I.D., 1.125"L) has a Z of about 80
ohms at 7 MHz, 100 ohms at 10 MHz.

John,

Thanks for the info.

What I've got are the .38 OD .25 ID ones. Thanks for the info. Seems
like I've been overkilling the VHF stuff. I think my 2m beam has 50 of
them on it, and if they're 12 ohms down at 7MHz...

I'll save that fair-rite link for future reference, but right now it's
locking my browser!

I guess the bead balun would be more broadbanded. I've not yet
measured the self-resonant frequency of my toroidal choke, but I'd
imagine it's somewhere in mid-high HF.

I think I'll string up a hundred beads and use that as my choke.

Thanks for the input es 73,

Dan
N3OX
www.n3ox.net

wrote:
John,

Thanks for the info.

What I've got are the .38 OD .25 ID ones.

How long?

Thanks for the info. Seems
like I've been overkilling the VHF stuff. I think my 2m beam has 50 of
them on it, and if they're 12 ohms down at 7MHz...

The impedance of type 43 beads keeps climbing (though not in
proportion to frequency) to well above 100MHz.

I'll save that fair-rite link for future reference, but right now it's
locking my browser!

It will unlock after a significant fraction of the 8 Meg file loads.
At least, that is what it does with mine.

I guess the bead balun would be more broadbanded. I've not yet
measured the self-resonant frequency of my toroidal choke, but I'd
imagine it's somewhere in mid-high HF.

I think I'll string up a hundred beads and use that as my choke.

Way more than needed. :-)

They're the 1/4" long ones.

I dunno, I like 1200 ohms... 24 times my feedpoint impedance?

I'm sure it's overkill given the induced currents that will flow
anyway...

I've got the beads, though, and I could use it on 80m too...

I can only basically have one antenna up at a time so I've gone over to
modular antenna parts and so one lower-HF bead balun can be used on
whatever I'm playing with at the time. I've only got one permanently
installed antenna, and that only until the wind blows...

Ah, apartment life.

73 John..