I'm looking to build an antenna to research the 5-500 Hz range. I already had decent success with just a simple square loop 1.5 ft per side (air core), about 12 turns. Despite having an estimated resonant freq. approx. 4 MHz, inside the house, it does easily pick up all the odd harmonics of the power line frequency. But weaker signals are extremely faint and below 60 Hz the spectrogram is all noise.

I will be using a ferrite rod for an improved loop antenna (mu-metal is too expensive for me). I found an excellent tutorial here on building an induction coil antenna (http://www.vlf.it/romero3/ics101.html) that rivals far more expensive commercial antennas. I will be constructing a "poorer-man's version" of his design.

Can anyone recommend (or, even better, supply a schematic for) a simple but reasonably effective pre-amp for my "poorer-man's version"? The Romero design has a 3-stage pre-amp + 1-stage supply and I'm looking for something simpler, even if performance is less. My main concern is decent dynamic range in the 5-200 Hz segment (areas above this seem easier), keeping in mind the power line harmonics can be quite high.

  • \$\begingroup\$ Toroids are closed magnetic field devices - they let very little in and very little out. If you use a ferrite material it will be a bar shape i.e. not a closed magnetic circuit. I don't see any reason why you shouldn't use transformer laminate either - it'll start to become lossy at 500Hz but considering it's not very lossy at 50Hz it aint gonna be a showstopper at 500Hz \$\endgroup\$ – Andy aka May 8 '13 at 22:02
  • \$\begingroup\$ Thanks, Andy, I've confirmed you are correct. "Toroids cannot be used as receivers as they have closed loops. Rods, bars and plates have open loops and are thus suitable as radio receivers." \$\endgroup\$ – Max May 8 '13 at 22:08
  • \$\begingroup\$ I've also confirmed that the optimum length/diameter ratio is from 10 to 25 for a magentic-core (rod) loop antenna. I will be refining my question shortly. \$\endgroup\$ – Max May 8 '13 at 22:10
  • \$\begingroup\$ If you had the circuit specified by romero, what would you connect it to? \$\endgroup\$ – Andy aka May 9 '13 at 8:14
  • \$\begingroup\$ Andy, I've been using my soundcard since it's all audio frequencies and it's worked well thus far. You could assume about a 10 kΩ impedance on the input port. I've done some tests and the card appears reliable down to about 5 Hz, at which point the AC coupling capacitor takes out the signal. \$\endgroup\$ – Max May 9 '13 at 10:16

The circuit from romero is this: - enter image description here

Salient points that are noteworthy are the balanced input amplifier which uses a virtual earth thus relying on the DC resistance and inductance of the coil for dictating the maximum gain limit. The OP-07 is a decent choice - it's fairly low noise device and it has a 200Hz low pass filter built in courtesy of the 200pF (C8) across R1 (3.3Mohm).

Note about it being a balanced amplifier - Electric field pick-up will be common mode to both ends of the coil so you need a balanced input. A balanced input ignores (to a great extent) common mode signals.

You can optionally bypass the Low pass filter stage and drive directly onto the final gain stage so I would consider using the front end of this circuit up to the R17 and the switch and inputting that output via a 4k7 to your sound card. This is the simplest circuit that would be effective.

For a PSU you should be able to run it from any split supply from +/-3V to the +/-9V romero uses. At a push you could run it from a 9V battery with a potential divider (like R30 and R29) but use lower values of resistance (1k won't upset the input balance too much) but put 100uF across each resistor OR just copy romero's PSU.

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