First of all, after having talked to Earl Cunningham K6SE,
my suggestion is to use the exact measurements and techniques
described by him for the Pennant transformer.
There IS some variation in the permeability factor in the FT-140-43 core and
I'm working
with the factory on that right now. You would expect around 950 uH from 10
turns spaced on
the form, but actual samples are coming in at higher values. In a cross
sample of almost 100 cores that I've tested, I've seen 2 distinct groups,
but within the group, the tolerance is close. It seems that the batch to
batch mix is variable though, and I will get that resolved soon.
Back to the original design:
(using 26ga enamel wire)
FT-140-43 core 8T primary, 28 (for 75) or 34 (for 51) Ohm feedline.
MAKE SURE the windings both have the same "sense" or "hand", in otherwords,
make sure they point the same way on the core.
Freq 8T (56uH) 28T (672 uH) 34T (991 uH)
12.25:1 ratio= 18:1 ratio=
918 Ohms 918 Ohms
R*L R*L R*L
500 kHz 177 Ohms 2111 Ohms 3113 Ohms
1000 kHz 355 4223 6227
1800 kHz 639 7600 11200
3500 kHz 1242 14780 21800
These number have been rounded off a little, but it matters not.
All that is important is :
A) Balance (and sense)
B) Ratio
C) Primary initial impedance at the design lowest design freq.
The inductance remains the same (more or less) across the spectrum for
which the core is designed. Most cores are good way below the broadcast
band.
Of course the reactance (R*L) changes according to the frequency.
Changing the turns count (as long as the ratio remains constant) will only
shift the
operating range. As long as your desired frequency is still within the
operating range
it will work the same, but again I suggest go with Earl's design so we are
all reading
off the same sheet of music as far as the transformer at least.
As far as the Bev transformers are concerned, I continue to recommend the DC
connected transmission line type over the others. I do agree that an
isolated shield
can (possibly) improve noise coupling, but the DC shunt in the grounded
design
seems far safer and I have not had trouble with the noise coupling at all
here.
Pennant Transformers according to Earl's design will be available
shortly from Array Solutions. They are in a gasket cast aluminum box with
a brass weep hole, and have a stainless steel rope eye, and gold
plated binding posts. The coil is wrapped in fiberglass tape, and
dipped in liquid polystyrene for stability.
The Beverage transformers according to my own design will be there also,
and have Cast aluminum housing, gasket sealed, groundrod clamp and
aux. brass ground screw, DC grounding of the antenna wire with a Kynar
fusible link, and an air gap, choke shunt to ground, gas tube protected, and
on the coax side it is DC blocked,and drained.
Taps allow 50 or 75 Ohm feed and 4:1 9:1 and 16:1 winding ratio selection.
Fiberglass wrapped coil with a 140 size core for low loss, dipped in liquid
polystyrene.
Teflon connector and gold plated binding post.
Hope this helps
Happy Building, Geo KØFF
http://homepages.dstream.net/K0FF
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