Ferrite Beads

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From: chuck adams (adams@chuck.dallas.sgi.com)
Date: Wed Oct 19 1994 - 11:16:06 EDT


Gang,

I mentioned in a posting a while ago that I add a ferrite
bead to the base lead of the final PA in the QRP rigs
that don't already have them or specify them. The
function of the bead to provide a high impedance for
high frequencies such as parasitics and wild oscillations.

The material that I try to use is #43 as specified by
Amidon Associates, P.O. Box 956, Torrance, CA 90508 in
their catalog, which if you don't have you should send them
a request and a couple of stamps or a buck to cover their
postage, etc. The thing is a very large sheet folded to
make up 24 pages of graphs, charts, and text. Well worth
having for any experimenter and ham and you certainly can't
complain about the cost. This document answers a lot of questions
we all see over and over from others who haven't used toroids
or beads before. The last one I have is dated 1990, so I assume
that the address still works, though in the back of mind I remember
something recently about someone buying out someone else. You
guys/gals can straighten me out on this one.

Going back to the #43 material. It has low impedance below 8MHz
then increases to a max around 100 to 200MHz. If you get a bead
made of #73 or #75 material, and it will be difficult to see or
measure differences for the average equiped ham shack, then you'll
probably be in trouble with the resulting PA circuit.

I have an A&A Engineering Spectrum analysizer now and will try
to get around to measuring exactly what the results are by adding
the bead. Others may have already done this or have QUANTATIVE
data to support the effects on spectral purity of the resulting
output, with and without the bead. Milage will probably vary
significantly.

I also add 33V Zener from collector to ground (1/2W to 1W) to
give some protection to final PA if I happen to key into a vacuum
or high SWR where the resulting voltage may puncture the junction
within the transistor and terminate it's life in short order.
Things like the 2SC799 are in short supply. :-)

I never operate a rig without a five pole Cheby filter on the final
PA. A three pole filter just doesn't give enough filtering for
the higher harmonics for me. It's probably legal but I want the
additional filtering and the extra coil and cap are cheap insurance.

dit dit
SIG
Chuck Adams K5FO CP-60
adams@sgi.com


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