VLF Receiver

In messing with radio receivers over the past few years, I often saw people mention low frequency receivers for listening to "the Earth's" radio  transmissions. this includes radio signals given off by lightning strikes, and various solar/magnetosphere sorcery.

As is often the case, Charles Wenzel's web page is a good place to start (and stay). He has several VLF receiver designs that range from a simple JFET dongle you plug into a recording device (or even your phone's microphone input) to fancier schematics that use somewhat specialized op-amps with ultra high impedance JFET inputs.

The high impedance input part is fairly important, since many of these designs use a small whip antenna that we don't want to load down too much to diminish the radio signals we're trying to receive, amplify and hear.

I built one of the simple JFET "dongles" and it works pretty well for what it is. Google's hearing assistance app provides pretty good amplification and the sounds can be recorded with another recording app. I used Caustic (an impressive synth app). A cheap set of dollar store earbuds with builtin mic works well -- replace the tiny circuit board with your new JFET circuit, route the JFET output to the mic pin on the plug, keep the earbuds routed to the phone plug's headphone output left & right. (Hold on to the board you removed for a future project that needs a tiny electret mic!)

Simple JFET antenna amplifier for smartphone mic input.

Next I built a standalone receiver, adding a JFET input stage to an LM324 opamp loosely based on Wenzel's design for a two-stage op-amp receiver. It also worked halfway well, but got crowded in  the Altoids tin I insisted on using as a housing.

Receiver with discrete JFET input stage and two-stage LM324-based amplifier (2 op amps unused...)

How high of an impedance do you need?

I had also salvaged some older BA4558 audio op-amps from old radio equipment. In reading and refreshing my understanding of operational amplifiers, I noticed that even BJT-based op-amps have quite high input impedance depending on the external components you use and the particular configuration you design.

Would a basic dual op-amp like the BA4558 make a decent VLF receiver if you use one amp as a high impedance voltage follower stage and the other as a high gain  inverting amplifier*?

* Note: Charles Wenzel points out that it's a good idea to have the VLF circuit have an overall inverted signal at the output so that the amplified signal going to headphones, etc. doesn't cause feedback in the nearby input stage circuitry.  

The answer to the above question is yes, a basic audio amp like the BA4558 is enough to make a decent VLF receiver!

Below is an example of what you might hear: pops and crackles that some say sounds like bacon frying. That's the radio signals given off by fairly "nearby" lighting strikes. The radio waves from more distant strikes sometimes travel much further around the globe in the Earth's natural waveguide formed by the ground and charged particles in the ionosphere, with different frequencies traveling a bit faster than others. So the pops become "tweaks" -- little chirps that sound like the lightning is into synthesizer music. You can hear a couple of those "tweaks" near the end of the audio recording. 


Source for cheap whip antennas

Dollar Tree sells a little mirror with a telescopic handle. Remove the mirror (or keep it attached!?) and use the telescoping handle as a whip antenna. Note that the metal is in  theory solderable, but can be tricky -- you may have to crimp some copper or brass around the base tightly to make electrical contact and then solder to that.