Due to all of the other variables involved, the max coil Q will come from a coil about as long as it is wide. You can play with a coil program to get yours in the ballpark. Also, you need to be able to adjust the coupling between the antenna's tuned circuit and the detector, as any loading lowers the Q and hurts selectivity. So you want to have control of that by using a moveable coupling loop. A loop the size of yours becomes the antenna and picks up near-field noise from your home's wiring and appliances unless you put it in a box with aluminum foil on it.
Interesting. Very nice. If I ever get time off in retirement from working part time as a property manager for nearby vacation homes I want to give a nice crystal radio with good selectivity & sensitivity a go before I build either a regen or direct conversion receiver. Most of the fun if not all is finding what works well....
Your big inductor works as classic magnetic loop antenna. Its main disadvantage is internal resistance that directly impacts selectivity and losses. Using litz wire helps to alleviate this problem a little bit. What hams do - they use copper pipe and vacuum capacitors to tune that thing. It allows very good selectivity and significantly improves the losses situation up to a point where the antenna can be used for transmission on high-frequency bands.
Yes. Resistance increases with longer/thinner wires. Then there's the fact that as the core gets bigger, the number of turns goes down until there are too few to work well as a coil.
@@tsbrownie Resistance is futile. At some diameter, the inductance gets to large to be able tune the AM band. I think this happens at about a 50 foot diameter coil. Below that designs can be made to work.
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For some builders, the fact that it receives with no antenna may be a reason to build the thing. Basically my 4x4 foot tuned loop antenna is the same idea only bigger except I didn't do any taps. Unloaded, I get about 2Vp-p on the coil. Loaded with the detector it decreases a lot. I think I may do an experiment of tapping the coil to see if I can get a better match.
Thanks TSB for doing this video, which is of particular interest since I have built a similar crystal radio for my grandson using a 2' X 6' coil. Problem is, I can't get it to receive any stations. I am using a similar circuit with a 40' long wire antenna and 4k ohm magnetic headphones. It works with a signal generator with a -30 dBm signal input. I thought I would try your circuit next to see if there is any improvement.
You're welcome. You have a better antenna than I've ever had. A 2 foot x 6 foot coil? This is a SWAG, because I don't have a square coil calculator. That's roughly equivalent to a circle of 3.9 feet or 1188.72 mm. I will assume a height (length) of 15 mm (roughly half an inch). 10 turns of 1.5mm (34-35 AWG) wire would give you 254 uH. With a 14-365 pF variable capacitor that should get you something. I've never had success with magnetic headphones at all. Yes, I can get a faint whisper from my best radio, but that's it. They can have high impedance, but they are just too power hungry, have too much mass to push around with tiny currents, and still look like a short to the radio. I'd suggest you get a piezoelectric earphone for a buck or two. If you want and can post pictures or a video of it, leave the link.
@@tsbrownie Thanks for your reply! When sifting through the possible problems, looking at each component for weaknesses, the headphones were my first suspect. I originally used a piezo transducer headset, however was shocked when I measured the capacitance, which was 0.1 uF. This is a common problem with piezo transducers because they are actually a disk ceramic capacitor with an extremely high K-factor. I tried canceling out the capacitance with a 2 Henry coil and optimized the detector load resistance, it helped but, the magnetic headset was a noticeable improvement. Mind you, the magnic headsets I am using are probably the highest impedance ones available. My resonator coil has 8 turns (168 feet) using standard 22 AWG enameled wire. I also included 1 turn taps to help match the antenna and detector. Tuning is provided by the standard 25 - 365 pF air variable capacitor. My next steps was to characterize my antenna and detector impedances, then use an L-match to match the antenna to the detector for maximum power transfer but, I don't have the test equipment for that so, have been trying to come up with work-around ideas instead.
@@billharris6886Your coil reminds me of a project I never did from the mid 1980s. It was from an article on how to listen the the song of the Sun. I think the coil was 10 ft x 10 ft and took thousands of meters of wire. It was just that and an audio amplifier. I think the audio had to be recorded and sped up to hear it.
