Gary's SRF-39 with the big loopstick


sloshatron <wa1gwh@...>
 

Hi Folks;

Very interesting antenna transplant that Gary performed on the SRF-
39. Any special reason that the coils are way out on the ends of the
ferrite bar? I presume the original antenna and the big transplant
needed the coils separated to allow movement for alignment.

The reason I ask is that on the crystal set discussion group that I
participate in, Ben Tongue (of Blonder-Tongue fame) has pointed us to
an article of his where he examines the efficient construction of
ferrite bar inductors. He has concluded that best performance occurs
with the coil centered and occupying no more than about 1/3 of the
bar's length.

I wonder if the total inductance required for an SRF radio could be
brought about with the two coils closer together and more or less
centered? Perhaps a few turns would have to be removed from one or
the other coil in order to counter any possible increase in total
inductance by having them closer together.

I can't see why an air core loop could not be used as a transplant
antenna. Except for the distributed capacity differences (and I'm
not sure how a ferrite inductor vs an air core one varies) I think
that an inductor is an inductor. The one difficulty I can think of
is how to tune the darn thing for alignment purposes. I guess you
could measure signal strength with a meter at the audio output, and
then remove turns as you go through the same alignment procedure as
with a ferrite bar. It would be a slower process and might involve
adding back fractional turns. The meter would enable the alignment
scheme to work in though you have to turn off the radio and remove
turns.

The reason I bring up air core loops is that crystal set
experimenter's have pointed out quite a bit of variability of
performance in ferrite bars. An air core loop would eliminate this
problem as well as finding a suitable bar.

I have copied Gil Stacy's idea of a variable capacitor and an
inductor as a booster antenna on my SRF-49. Except that I used a 4
1/2 inch air core coil of #21 wire basket wound (left over from a
crystal set project!). I gives a very prominent boost in
performance, so I presume one like it or bigger, of suitable
inductance, might work very well.

Comments?

Thank you,

Garry Nichols

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