VLF Natural Radio

Being a fanatical radio hobbyist, I'm interested in about every single aspect of radio.  It stands to reason then that I would become interested in natural radio signals, or signals that emanate from the earth or space and not made by man.  I had read about these signals years ago but never really got serious about trying to listen to them until I read Kevin Carey's "Below 500 kHz" column in the March, 2006 issue of Monitoring Times. Kevin provided a schematic and plans in the March & April columns on how to build a VLF natural radio receiver called the BBB-4, or "Bare Bones Basic" receiver.  The plans on how to build one of these are readily available on the internet, and are fairly simple to build.  I built mine in less than a week.  The plans can be found at http://www.auroralchorus.com/bbb4rx3.htm.

My Receiver:

This is my version of the BBB-4.  I built it Manhattan style completely from parts I had on hand, and housed it in an old A-B switchbox case.  Instead of the 2N3819 JFET transistor called for in the original plans, I substituted an MPF102 JFET of which I had several, and which is almost totally identical to the 2N3819.  For L1, the plans specify that a Radio Shack audio transformer (part number 273-1380) can be used, but I used an audio transformer I had which has about the right amount of inductance. I connected the tape out jack to C8, but instead of using an external audio amp I added one on the board using a Motorola MC34119 audio amp chip and a few extra components.  This  amplifies the audio enough to drive a set of headphones or a speaker.  I installed a flat laptop speaker in the top of the case and connected it to the board using a set of jumper pins, which the speaker plug slides onto nicely.  This way I can easily disconnect the speaker to remove the top half of the cabinet. I attached a BNC jack on the rear of the case and use a long telescopic whip with a BNC elbow adaptor.  I also constructed a 10' wire antenna connected to a BNC plug for hanging from trees in electrically quiet locations. I spray painted the bottom half of the case white, and printed a label using my computer.  I then glued the label to the painted case, and then oversprayed it with clear spray paint to keep it from rubbing off or getting wet.  For the rear of the case, since it had large, elongated holes for three DB25 jacks, I cut part of the top of an old CD-ROM case to fit the back of the case.  I painted this strip of metal white to match the back of the case, then held it in place by one screw and the BNC jack.  I cut seven large holes in the top of the case for the speaker, and then painted it black with textured paint.  Before installing the speaker in the top of the case, I placed some black felt over the holes to keep dust out. Since completing the BBB-4 and taking it out to a local park far away from power lines for its first test listen, I made a couple of modifications.  First, since I seemed to be troubled by a large amount of feedback in the receiver when I had the tape recorder connected and running, I soon learned that I needed to ground the receiver.  I installed a binding post in the rear of the case which is connected to the radio's chassis.  I then soldered a wire to a large nail which I can push into the ground and attach the other end to the binding post making a good ground.  This got rid of the squeal and also the whine from the tape recorder. Secondly, since I tend to listen with the headphones connected to the tape recorder and the recorder connected to the audio preamp output, I changed the standard 10K pot I had installed with another 10K with a switch.  I connected the +9V to the Motorola audio amp chip to the switch on the pot so that I can turn it off and save battery power. Listening with the BBB-4 I'm still new to natural radio listening, and I never seem to find the time to get out away from power lines, so I haven't managed to hear a lot of different natural radio signals yet.  However, once I did complete construction I decided to test out the receiver.  I walked out to the back yard to get as far away from power line noise as I could.  I was listening to some "sferics," or static from distant lightning, when my dog came up to me to see what I was doing.  I reached down to scratch his head and was surprised to discover that I could hear it in the receiver.  It was the static electricity which I was generating that I could hear.  I never thought that I could use a dog as a transmitter.  petting_the_dog.mp3 The above file is a recording I made of the audio from the receiver as I scratched the dog's head.  You can plainly hear it over the power line hum. I plan to take the BBB-4 out to different locations far from power lines in the near future.  I'll post audio files of different natural signals here as I hear them. May 19, 2006 - Last night I head my first "tweeks."  I took the receiver out into the back yard after sundown to see if I could hear anything (after reading that whistlers were more easily heard at night than during the day) and in with the sferics I managed to hear a few tweeks.  I was happy that these were the first natural radio signals (aside from the sferics) that I heard.  I did record them but I won't put the recordings up here because there is tremendous power line noise on the recording and the tweeks are hard to hear.  I'm still looking for a nearby quiet location to listen from during nighttime hours.  I hope to find a quiet spot to visit early in the morning to try to hear the dawn chorus. Links: Auroralchorus.com Lots of good VLF natural radio information here, plus audio files www.altair.org/natradio.html More information on natural radio signals, plus natural radio projects McGreevy ground-based VLF recordings has many recordings of natural radio signals. The Longwave Club of America (LWCA)'s natural radio page Whistlers and Related Ionospheric Phenomena is a detailed technical book all about natural radio.

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