I checked the archives, and I didn't see any discussion of this issue in
the fall, so here goes:
In the November 1998 issue of CQ Contest (OK, I'm behind), Tim Duffy,
K3LR wrote an article about a 3-high remote stack switch.
Tim talks about using current chokes on all his antenna feedpoints,
another choke at the remote antenna switching box, and a third where the
feedline goes into the wattmeter at the shack. These chokes serve to
isolate the sheild of the feedline.
His comment, "It is amazing what the installation of these
high-performance chokes has done to decrease interference and noise
issues at my multi-multi contest station."
The choke he uses is a design from W0IYH. It is about 48 inches of RG-142
(Silver/Teflon coax) and 100 FB-43-5622 Ferrite beads. (For 75 ohm lines,
use RG-302)
If these chokes work, they could be of value to multi- and SO2R operators
with interference problems.
--
Points of discussion:
1) Why would this array of chokes in the feedline system be any more
effective than a single high-performance choke at the feedpoint of each
antenna?
2) Does anyone have any experiance with the W0IYH design? (I did find
some excellent advise from Tom, W8JI to avoid use of type 77 beads due to
"loss tangent" inefficiencies - K3LR noted failures of W2DU baluns using
these beads) Are there kits or premanufactured chokes available?
3) W0IYH supplied me with a July 1997 QEX article detailing his
experiments with balun designs. He details two improved designs over the
W2DU type. One uses 100 FB-43-6301 beads. Another uses the larger 100
FB-43-5622 beads (this being the kind that K3LR noted). An alternative is
to use 36 FB-43-5621 beads, which are longer, but according to the Amidon
price sheet (See http://www.amidoncorp.com/prices/beads.htm), this would
be more expensive than the 100 -5622 beads.
The price sheet also notes even longer FB-43-5623 beads. Only 26 of these
would be needed for the same performance, and they are significantly
lower in price, at $0.55 each:
100 FB-43-5622: 100 * $0.40 = $40.00
36 FB-43-5621: 36 * $1.50 = $54.00
26 FB-43-5623: 26 * $0.55 = $14.30
W0IYH stated these beads weren't available when he wrote the QEX article.
Does anyone have any experience with the -5623 beads, and perhaps it is a
typographical error?
If not, the price of high-quality choke baluns got significantly cheaper
when Amidon introduced these beads.
4) Why bother with Silver / Teflon coax? Why not just use RG-58 / 59? It
should be able to handle legal-limit power on all HF frequencies (given
properly matched loads).
Any comments would be appreciated.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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