27-06-2020, 08:42 PM
You don't need a receiver for less than 48 kHz, certainly not for less than 24 kHz. Just a band pass amplifier and plug it into sound card sampling at 48 kHz or 96 kHz. Plenty of free software on Android (use 4 pole plug to use the mic in connection on a phone/tablet), Windows and Linux.
Search for VLF or Whistlers. Many of the circuits are old. You can use an LM358 opamp. There is not really any advantage in using a lower noise part unless you have a big loop aerial. Either have a filter specifically for Alexanderson or a good 22 kHz low pass and 12 kHz high pass. If you can do 96 KHz sampling then you can have a higher low pass. It must be a bit less than 1/2 the sample rate as filters are not cliff edge. You might want 2 x LM358 for four stages of filter. Even my Android Tablet does a quite good waterfall spectrum display.
You can use Cat5 cable with the pairs shorted to give four wires. Then connect the "loops" in series to give x4 turns per loop. Around a window frame or three shelves of a bookcase with as many turns as there is wire. You can use twin bell wire and then connect the two wires in series. Or 4 core alarm wire. Treat any twisted pair as one wire, shorting both ends. Shielded Cat 5 may work with the shield earthed at only one end.
It's not critical.
You can calculate or measure the inductance and "resonate" it with a capacitor which might 10nF to 100nF depending on the number of turns.
There is also ELF. Same techniques but below 50 Hz using spikes in the ground.
Search for VLF or Whistlers. Many of the circuits are old. You can use an LM358 opamp. There is not really any advantage in using a lower noise part unless you have a big loop aerial. Either have a filter specifically for Alexanderson or a good 22 kHz low pass and 12 kHz high pass. If you can do 96 KHz sampling then you can have a higher low pass. It must be a bit less than 1/2 the sample rate as filters are not cliff edge. You might want 2 x LM358 for four stages of filter. Even my Android Tablet does a quite good waterfall spectrum display.
You can use Cat5 cable with the pairs shorted to give four wires. Then connect the "loops" in series to give x4 turns per loop. Around a window frame or three shelves of a bookcase with as many turns as there is wire. You can use twin bell wire and then connect the two wires in series. Or 4 core alarm wire. Treat any twisted pair as one wire, shorting both ends. Shielded Cat 5 may work with the shield earthed at only one end.
It's not critical.
You can calculate or measure the inductance and "resonate" it with a capacitor which might 10nF to 100nF depending on the number of turns.
There is also ELF. Same techniques but below 50 Hz using spikes in the ground.
