I recently tried to make a DIY foxhole radio. Althogh I made it according to the manual (140times coil rotated, ground(220V), blued razor blade, pencil core and antenna (just a steel stick about 1 meter), it doesn't work. And I'm guessing that the problem is the manual's AM RF(radio frequency)and my region's one don't match, so i've concluded that there's some realtionship between the RF and coil rotation times and i should know it. What's direct interaction between the coil rotation times and RF? And secondly, i learned that when you wish to receive radio signal, the LC circuit's resonance frequency should accord with the radio's one. But there's nothing seemingly similar to capacitor in the foxhole radio! I'm complicated, so i want to know how it works without the capacitor.

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  • $\begingroup$ Would Electrical Engineering be a better home for this question? $\endgroup$ – Qmechanic Aug 15 '17 at 17:59
  • $\begingroup$ Aha, coil has its own capacitance. Then is it enough to make a resonance to receive radio signals? $\endgroup$ – Physik matters Aug 16 '17 at 9:21
  • $\begingroup$ I used to make those, back when they were new technology. The output is quite high impedance, so you need high impedance earphones (much higher than modern earbuds) or use an audio transformer. Or try connecting the output to an amplifier. I got good reception with a 10 m antenna wire. A 1 m stick won't give you much signal. $\endgroup$ – Keith McClary Aug 16 '17 at 19:02

If the bandwidth of the LC tank is large enough it will catch all local radio stations that are strong enough. The design is probably meant to use stray capacitance. The problem is more likely in the demodulation. Try replacing the razor blade / pencil demodulator with a germanium signal diode or metal contact diode to troubleshoot. If the circuit works with the germanium diode then you can fiddle with the pencil / razor blade until you get diode functionality. If it doesn't work with the germanium diode then you need to check the rest of the circuit and make sure to have a good ground.

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  • $\begingroup$ You mean the coil rotation times have little to do with malfuctioning? Our region often uses 90~100Mhz FM channel. So is it okay to use 140 times coil to receive it? And about ground, 220V plug has two ground tips. Then is it proper to attach the line with just one tip? $\endgroup$ – Physik matters Aug 16 '17 at 9:11
  • $\begingroup$ It's not going to work for FM. FM requires a different type of demodulator circuit. And the coil would also have been designed for 500 kHZ to 1.5MHz range since that is the AM band. There should not be 2 grounds as far as I know. If it is a four plug 220V then it should have 2 live (X and Y), 1 neutral (W) and 1 ground (G) pins. $\endgroup$ – mmainville Aug 16 '17 at 16:51
  • $\begingroup$ I'm going to buy a p-n diode(usually sold in market) but the details it has are different. There are various kinds of diodes-6A,10A,etc. Then what should i buy? Even I cannot calculate the total impedance, for it is too complex. $\endgroup$ – Physik matters Aug 18 '17 at 15:01
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    $\begingroup$ Even a diode will not work for FM demodulation. It will only help to troubleshoot if you are picking up a local AM radio station. You would want a small signal diode for this. These ones ( 6A, 10A even 1A are diode rectifiers used mainly for power supply rectification )are not good choices for AM demodulation. You want to use a diode with a low forward voltage drop like Germanium or schottky or metal contact. 1N34A is very common for this purpose. $\endgroup$ – mmainville Aug 18 '17 at 22:07

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