Hello, and thank you MrChips.
First post, really, apart from this, which I link here to shorten the introduction to the present one. I apologize beforehand if this post is long, or is a well covered subject here, but I am still finding my way around. They won't be so long in the future!
So, crystal radios. In brief, so far;
Mk 1 did not work. As expected.
Mk 2A did not work, either. New coil, 2" diameter with 26 gauge wire, using a biased LED as a diode. Made an earphone from a small, salvaged piezo speaker out of a junked appliance, and used non-polarized ceramic capacitors to attempt to tune. I think I heard some static at one stage, but not sure.
Mk 2B worked wonderfully. I found some idea out on the internet about the Mystery Radio that uses two coils, so I reworked the Mk 2A coil to include a second, coupling coil on the same diameter (note, no taps). By this time my germanium diodes had arrived in the mail, also, as had a small variable capacitor and a "proper" piezo earphone. Reception and volume of my targeted AM station were good, showing that the math was working (I prefer doing the math manually rather than using the abundance of coil calculators I have found on the internet, otherwise I am not learning anything). However, there was some lack of selectivity, as I could hear some other stations simultaneously in the background. Whatever the case might have been, it convinced me the Mystery Radio was the way I wanted to go.
But I wanted to do it with a difference; concentric coils. I later found that this idea has been applied with success before. Here are the specs of the Mk 3A;
Coil 1;
71 winds of 22 gauge on a 3" diameter, with 4 intermediate taps. Originally, I wanted to make this around a 350 uH coil, but there was some imprecision in the wire gauge - it was salvaged from a defective starter motor solenoid and may be British Standard gauge (when I measured it later with a micrometer it turned out to be 28.7 thou)- and this resulted in the coil being a bit longer than desired, lowering the calculated inductance to 315 uH, exactly, funnily enough.
Coil 2;
50 winds of 26 gauge on a 2" diameter, with 2 intermediate taps. Wanted 140 uH, got 136 uH on calculation of the finished product, for similar reasons as above.
I assembled these two coils concentrically. I was thinking sort of "transformer", if that applies. See the diagram. Apologies for that schematic, I have no circuit drawing software and did it on Inkscape. The antenna, incidentally, is 55 feet of 18 gauge, insulated, solid copper wire. Straight, tapped from one end.
Okay, results and questions;
On first testing, I got a slightly weaker reception of the signal than the Mk 2B, but much better selectivity, especially when using the diode taps. Strength was increased to a par with the Mk 2B by adding the 47K resistor to earth before the earphone (I have no idea why 47K appears to be the magic number, but all references I have looked at that have a resistor seem to point to this value). Some signal "saturation", if that is the right term, was improved by using the antenna taps. I worked around the limited selectivity of the variable capacitor by placing different value ceramic capacitors in parallel with it, and can get the whole AM range this way.
Then, as an experiment, I hooked it up using only the outer, 3" diameter coil as the sole source, disregarding the inner coupling coil altogether (I will call it the 3B). The reception volume was phenomenal, but the strangest thing is that it works better (better selectivity) without the ground connected at all! Connecting the ground results in many stations being received simultaneously, with no apparent improvement in strength. Weird and unexpected.
Why can I do away with the ground in this configuration? It makes no sense to me.
Finally, though I do not want to fully spoil the joy of experimentation, I want to improve my Mk 3A. Would, perhaps, closing the gap between the two coils produce better reception strength? For example, if I wound a 140 uH coil on a 2.5" former and placed this concentrically within the 3" coil?
My thanks again, wonderful forum, some excellent reading material that I am endeavoring to read piecemeal, and not least, my thanks in advance for any responses to this post.
First post, really, apart from this, which I link here to shorten the introduction to the present one. I apologize beforehand if this post is long, or is a well covered subject here, but I am still finding my way around. They won't be so long in the future!
So, crystal radios. In brief, so far;
Mk 1 did not work. As expected.
Mk 2A did not work, either. New coil, 2" diameter with 26 gauge wire, using a biased LED as a diode. Made an earphone from a small, salvaged piezo speaker out of a junked appliance, and used non-polarized ceramic capacitors to attempt to tune. I think I heard some static at one stage, but not sure.
Mk 2B worked wonderfully. I found some idea out on the internet about the Mystery Radio that uses two coils, so I reworked the Mk 2A coil to include a second, coupling coil on the same diameter (note, no taps). By this time my germanium diodes had arrived in the mail, also, as had a small variable capacitor and a "proper" piezo earphone. Reception and volume of my targeted AM station were good, showing that the math was working (I prefer doing the math manually rather than using the abundance of coil calculators I have found on the internet, otherwise I am not learning anything). However, there was some lack of selectivity, as I could hear some other stations simultaneously in the background. Whatever the case might have been, it convinced me the Mystery Radio was the way I wanted to go.
But I wanted to do it with a difference; concentric coils. I later found that this idea has been applied with success before. Here are the specs of the Mk 3A;
Coil 1;
71 winds of 22 gauge on a 3" diameter, with 4 intermediate taps. Originally, I wanted to make this around a 350 uH coil, but there was some imprecision in the wire gauge - it was salvaged from a defective starter motor solenoid and may be British Standard gauge (when I measured it later with a micrometer it turned out to be 28.7 thou)- and this resulted in the coil being a bit longer than desired, lowering the calculated inductance to 315 uH, exactly, funnily enough.
Coil 2;
50 winds of 26 gauge on a 2" diameter, with 2 intermediate taps. Wanted 140 uH, got 136 uH on calculation of the finished product, for similar reasons as above.
I assembled these two coils concentrically. I was thinking sort of "transformer", if that applies. See the diagram. Apologies for that schematic, I have no circuit drawing software and did it on Inkscape. The antenna, incidentally, is 55 feet of 18 gauge, insulated, solid copper wire. Straight, tapped from one end.
Okay, results and questions;
On first testing, I got a slightly weaker reception of the signal than the Mk 2B, but much better selectivity, especially when using the diode taps. Strength was increased to a par with the Mk 2B by adding the 47K resistor to earth before the earphone (I have no idea why 47K appears to be the magic number, but all references I have looked at that have a resistor seem to point to this value). Some signal "saturation", if that is the right term, was improved by using the antenna taps. I worked around the limited selectivity of the variable capacitor by placing different value ceramic capacitors in parallel with it, and can get the whole AM range this way.
Then, as an experiment, I hooked it up using only the outer, 3" diameter coil as the sole source, disregarding the inner coupling coil altogether (I will call it the 3B). The reception volume was phenomenal, but the strangest thing is that it works better (better selectivity) without the ground connected at all! Connecting the ground results in many stations being received simultaneously, with no apparent improvement in strength. Weird and unexpected.
Why can I do away with the ground in this configuration? It makes no sense to me.
Finally, though I do not want to fully spoil the joy of experimentation, I want to improve my Mk 3A. Would, perhaps, closing the gap between the two coils produce better reception strength? For example, if I wound a 140 uH coil on a 2.5" former and placed this concentrically within the 3" coil?
My thanks again, wonderful forum, some excellent reading material that I am endeavoring to read piecemeal, and not least, my thanks in advance for any responses to this post.
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