On Thu, 30 Aug 2007 22:40:58 -0400, "Rick (W-A-one-R-K-T)"
wrote:


The Carolina Windom uses a coax feedline from a 4:1 balun at the feedpoint
to a 1:1 current-type unun at the 22-foot point.

Do you guys think that some number of snap-on ferrite chokes of the type
Radio Shack sells at

http://www.radioshack.com/product/in...entPage=search

Of undetermined parentage.

(Radio Shack part number 273-105), wrapped around the RG-8X feedline at
the 22-foot point, will be suitable in place of something like the MFJ-915
"RF Line Isolator" (unun)?

Could be. (There is every possibility that the UnUn that comes with
the commercial unit is not suitable - in spite of testimonials to the
contrary.)

If so, how many should I use? I have four of them now and the local Radio
Shack claims to have some more in stock.

Put a short length of wire through it, use an antenna analyzer to
measure the Z. Now, divide that number into as much Z as is necessary
to isolate the end of the 22-foot feed point. That is how many you
will need.

Each of those ferrite cores is about 1.25" long by 1" square. I have no
idea what type or grade of ferrite material they're made out of.

The previous suggestion renders that moot.

The real question devolves to the parenthetic statement made earlier:
How much Z do you need? It should be 3 to 10 times the Z found at
that drive point. However, with a wild card of the extra radiator
found in the 22 foot drop, it is no longer a simple 4:1 relationship
found at the upper end.

So, we again enter into the exciting world of amateur discovery
through experimentation. Keep choking up until you get the
performance you desire - then stop choking up.

This, then, raises the next real question: What performance are you
expecting?

73's
Richard Clark, KB7QHC