Gary's SRF-39 with the big loopstick
Thanks for your comments on the SRF-39FP mega-antenna transplant, and welcome to the discussion. The crystal set group and the ultralight radio group seem to have a lot in common (interest in maximum performance from minimal equipment), so we are especially happy to welcome enthusiasts from your side of the AM-DX hobby.
My interest in huge transplanted loopsticks grew out of a fascination with the SRF-59's stock DX performance, with only the tiny 1.75" ferrite bar. After receiving three TP's with this miniscule antenna, I began to seriously wonder what the amazing CXA1129 chip could do with a generous DX antenna, and whether such a combination would outperform the classic DX portables (ICF-2010, E1, ICF-S5W) in sensitivity.
Alignment of about 25 SRF-59's (for myself and others) gave me a good idea of how the stock loopstick works, i.e. a fixed longer coil with a tap point, and a smaller, movable coil used for alignment. Around February, in an experiment documented on dxer.ca., I transplanted my first 6.25" ferrite bar into an SRF-39FP "prison radio," which is basically an SRF-59 with a bigger tuning dial, and better components. The first "Super Prison Radio" was constructed without an LCR meter, and essentially was an exact copy of the stock loopstick, although on a larger scale. It had definite alignment peaks at 600 kHz and 1400 kHz, and had sensitivity roughly equal to the ICF-2010 (better on some frequencies, slightly worse on others).
Using an accurate LCR meter, further experiments were conducted on three other (6.25" and 7") loopsticks, using recycled coils with inductances carefully matched to the stock coils. After careful alignment, these units had sensitivity generally superior to the ICF-2010 on all frequencies. They deadlocked with the ICF-S5W on the lower and middle frequencies, but these "SPR2" units were more sensitive on the high end than the S5W.
The recent experiment, using two 7.25" ferrite bars tightly taped together, resulted in an "SPR3" unit with a total antenna length of 14.5", and sensitivity clearly superior to any stock portable on every frequency. Concerning the separation of the larger and smaller coils on the loopstick, Garry, this was primarily a convenience for construction, since the loopstick mounting post is attached to the center of the loopstick for secure mounting, and also the smaller coil needs about 2" of space to find the 600 KHz alignment peak, even after the optimum inductance of .598 mh is obtained across both coils.
Concerning Ben Tongue's impression that ferrite bar loopsticks have the best performance when the coils are centered (and occupying no more than 30% of the bar's length), I have constructed SRF-39FP loopstick transplants where the both coils were near the center, and found performance identical to the "separated" orientation. We must remember that Sony engineered the SRF-59/39 loopstick with separated coils (presumably for alignment), and that the stock loopstick has fantastic DX performance for its size, in the "separated" orientation. If a hobbyist wished to have both coils near the center of the bar, he would have a tricky alignment process, and would also need to construct a support framework that avoided the coils (making the support look something like a slingshot). IMHO, such additional hassle wouldn't give any real DX improvement, Garry. Thanks again for your interest in the mega-transplant!
73, Gary DeBock
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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
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
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.
John H. Bryant <bjohnorcas@...>
Garry,toggle quoted messageShow quoted text
It would be great (and helpful) to see a picture of your air-core experiments.... including the current one. If you have any problems at all uploading to the Photio files area, let me know and I'll be glad to help out.
At 08:24 PM 4/4/2008 +0000, you wrote: