I knew I should have kept my mouth shut!
To ease MY burden I am sending this to the reflector....everyone man your
Form: 1' diameter x 5.5" long ( 25 x 138 cm)
The choke is wound in 3 sections. All sections are #28 close wound....I
dont know what that wire equates to in other civilized countries.
Starting from the top:
5/8" (17 cm) of winding totalling 42 turns
27/32" (21 cm) open space with 2 1/2 turns spread to cover
13/16" ( 22cm) of winding totalling 55 turns
3/4" (18cm) open space with 2 1/2 turns spread to cover
1 3/8" (30cm) of winding totalling 82 turns
There is about 3/8 " (13cm) of open space at the top before the actual
winding and 3/4" (18cm) at the bottom left for a clamp and spacing from
If I'm off an RCH dont worry...this is not 100% rocket science but I did
clean my bifocals and use a magnifying glass to count turns.
This is the current version of the choke, earlier ones had slightly
different windings and were known to do nasty things.
As Rich mentioned this type of design...actually 3 chokes in series...can
exhibit multiple series resonances due to individual sections plus
coupling between sections. The GDO confirms this in out of circuit
measurements. For the homebrewer I wonder if it would not be advantageous
to install a parallel aluminum plate ( grounded of course) say 1" from
the choke and bypass each section to the plate? Values of .001, .0047
and .01mF in descending order come to mind.
Any comments out there?
I still prefer a single section solenoid wound choke and a 3/4" form
appears to offer an excellent freedom from the nasties and yet allow
plenty of inductance in a reasonable length. If 160M is not used a 1/2"
diameter form will show even less stray C. The Heath SB200/220/230 series
is an interesting example of an "adequate" choke on a 1/2" form...only 50
uh but it is clean from 3.5 to 30+ Mhz...the first hot spot is well
over 40 MHz.
When I first started getting serious about choke designs...back at Wang
Labs in the 80's...one of the more intelligent engineers put me on to
"form factor" as a key to performance. That info plus a great HP Network
Analyzer in the lab did the rest.
73 Carl KM1H
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