On Fri, 20 Nov 2009 09:10:27 -0800, Jeff Liebermann
wrote:

You lost me here. Just what are the beads suppose to do if not block
the reflected signal?
Presumably, they do serve some useful purpose.

They block common mode current. That is their purpose as a CHOKE. A
good BalUn contains choking, but a choke is not always a BalUn. For a
quarterwave BalUn, the beads serve to isolate each end which is both a
choking AND a BalUn necessity. As the loop is isolated from the sheet
metal, isolation is preserved (allowing the section to "possibly"
serve as a quarter wave choke - neglecting the geometric mean of Zs).

Now, you're suggesting that they could make the VSWR worse?

That is only a possibility, not a necessity.

I've never seen a Yagi-Uda antenna with a 50 ohm coax hung
directly onto a folded dipole because there are usually easy ways to
do the matching and balanced to unbalanced conversion.

Yagis are often balanced by design. You should have seen 50 Ohm coax
coiled nearby the feedpoint as a choke (an alternative method to using
beads). If you have not, then you have probably seen an installation
that suffered degraded performance (and either to the operator's
unending grief, or to their total unawareness). You give some sense
of already knowing this with your comment that follows:

Digging a deeper hole, I've been assuming that if the ferrite beads
were not there, the coax cable will radiate. After all, that's one
purpose of a balun, to prevent coax radiation from mangling the
pattern. That's still a dubious proposition due to the large length
of exposed center conductor at both ends of the coax piece, which
certainly will radiate some. I can add that to the model, but I don't
know how to model the ferrite beads.

I've written to that where I took exception to you modeling the Z as
series, and I pointed out it was shunt. Apparently you didn't follow
that comment.

You model the beads as either creating an open circuit, or adding a
huge resistance in-line. The net result to current is the same. The
long and short of it is that to model the exterior of the coax, you
add a wire of same length back to the choke section and call it a day.

73's
Richard Clark, KB7QHC