Receivers and Antennas
In typical radio listening (like AM, FM, and shortwave), you tune the receiver to a radio frequency that is much higher than the human hearing range. The receiver demodulates this high-frequency signal to recover the sound that was embedded in it. VLF receivers are much simpler, and not really “radios” at all. They simply translate electromagnetic waves into sound … literally a bare wire feeding a high-gain audio amplifier! (Actually there are 2 types of VLF receiver: E-field and B-field. I’m mostly discussing E-field.)
The INSPIRE project offers a VLF receiver in kit form, with field-proven amenities like filters and a built-in headphone amplifier. (They also have great general info on VLF phenomena.) Others are available either pre-built or as kits.
The Bike Loop is a (b-field) antenna and receiver combo built by Roberto and Claudio Parmigiani (IW2FER), Gabriele Seleri (IW2DWN). The antenna is made of bicycle rims!
I built a stereo pair of the “Super-Tiny VLF Receiver” designed by Charles Wenzel (www.techlib.com). It’s incredibly simple (3 parts!) and runs on “plug-in power” provided by most small flash recorders.
I assembled mine in “dead-bug” style, super-sloppy with parts substitutions based on what I had lying around. It works wonderfully. (My FETs were 2SK170′s.)
A metal box would be ideal, but plastic with foil-tape shielding worked OK.