wrote:
John Popelish wrote:

rocky wrote:

To sort of sum things up, then what should I use for a choke balun on
75 meters, 77, 73 or 43 mix? More the better? I read the W2DU article
and he used 73, but I thought 77 may be better?

If you are going to have to go out and buy the cores, type 43 (and its
competitors) is way more common and cheap than any of the others. It
is also readily available in long form toroids called shield beads.


You are going to get into trouble with the W2DU balun at high power
levels or with high commonn mode voltages across the balun. Each bead
suggested is only good for about 1/2 watt dissipation in open air
during long duty cycle.

I assume you are talking about the beads mentioned in this document:
http://www.w2du.com/r2ch21.pdf

At the bottom of page 21-6, he says that the beads have a .197 inch
hole and are .190 inch long.

The beads I am suggesting are over an inch in diameter and over an
inch long. A half watt would barely raise their temperature
noticeably above ambient. And they will fit over coax that is a half
inch in diameter. There are other shapes that are more cost effective
for smaller diameter coax. 28B0591-200 has a .23 inch hole, but is
1.38 inches long, with a .591 inch OD. A half watt won't warm that
one very much, either. And it costs only $.76 each, if you buy 10 of
them.

My point was that there are cheap long form factor toroids available
now, that weren't available when W2DU designed and described his BalUn.

Also, a string of beads is a terribly inefficient use of ferrite
materials. If you double the expense of the balun by doubling the
number of beads, you only double the common mode impedance.

But it spreads the heat out, and doesn't require you to kink the coax,
which makes it last longer in the weather. If you want to wind large
diameter toroids, they now make low cost versions of these for EMI
purposes, also. For example Steward # 28B2400-000, outside diameter
2.4 inches OD, 1.4 inch ID, .5 inch thick costs only $3.36 at Digikey,
quantity 1. Again, Steward type 28 is roughly similar to Fair-Rite
type 43.

If you use a less resistive material with lower loss tangent and
multiple turns through the core, you can greatly increase power
capacity and choking impedance with much less increase in cost.

Yes, the turns squared effect makes better use of the mass of the
ferrite to produce impedance.