23-02-2016, 09:56 AM
(22-02-2016, 11:03 PM)Pat Pending Wrote: I must admit Trevor that I'm as mystified as I am impressed with your receiver, normally, you basically have to accept with a crystal set that the signal you get is what you have to work with, and that by the use of transformers and variable couplers you can at best maximise the selectivity trading off sensitivity. I'm currently building a three foot version of my loft mount octagonal loop and yesterday evening gave your idea a try, (it worked well, and repeated your observations, to the extent of receiving radio Scotland (how apt.) with it here in Norfolk using balanced armature phones), the only thing I came up with as an explanation of the improved performance was that the loop aerial can supply more signal current without loading down the "Q" of the tuned circuit than the load needs, and that using this to step up the signal voltage via a transformer allows a greater output to the "load", but then I can't see why a larger turns ratio on the loop coupling won't do the same thing? I've been curious enough about the phenomena that I've done the only thing a lazy thinker can, post a link on other sites to see if anyone starts a dialogue (hope you don't mind!) I'm going to also try a schottky diode if I can find one. One last thing, hope the dreaded MOT goes well!
Regards. Andy.
If I've read it right: As I see it it's all down to transformer ratios, the higher the turns ratio the greater the impedance transformation, eg: say 3k load ('phones + detector) 10:1 ratio = 3k to 300k, 300k would be ok across the tuned loop, no worse than the average input impedance of an RF amp or mixer valve, 5:1 ratio = 3k to 75k, not as good, 1:1 ratio = 3k to 3k, no good, back to where we started.
The only way to maintain reasonable Q on the primary with a low turns ratio would be to reduce the mutual coupling and tune the secondary to help make up for the reduced voltage induced.
Lawrence.
