VLF Natural Radio Reception

Natural Radio

"Natural Radio" is a fancy name for radio noise with natural origins, mainly lightning. At first, you might think that listening to lightning crackles is pretty uninteresting but as it turns out, electromagnetic radiation from lightning can travel great distances and undergo strange modifications along the way. The frequencies of the original pulse can be spread out in time (a process called "dispersion) because the higher frequencies travel a little faster than the lower. The result is that the short impulse from a lightning strike in South America can sound like a chirp in Texas. Slower sweeping tones are called "whistlers" and they are a bit of a mystery. Allyson's explanation is the best I've heard. When conditions are just right, numerous lightning strikes combine with numerous reflections to give an eerie chorus that sounds a bit like a flock of geese.

Hearing these signals requires little more than an audio amplifier and an antenna since they are in the audio frequency range. But the ever-present power line hum makes listening to these signals difficult near power lines. Software is available to digitally clean up the signal, making natural radio listening a practical home-based activity. A tremendous freeware offering from Wolfgang Buescher (DL4YHF) called "Spectrum Lab" works in conjunction with the sound card to display the frequency spectrum and filters are included to remove annoying interference. A hum filter module contributed by Paul Nicholson does a great job on line related noise. Karen, a reader in the U.K. developed the "Humnuller," a simple application that also does the trick. It has the nice feature that you can click a button after an interesting sound and capture a previous window of time (user selected).

Here is what it can do: My antenna without the filter   With the filter

To give you an idea how much work the filter is doing, below is a 'scope view of the hum. It is nearly clipping! (Max is +-32,768.)

Electric Pickup

Be warned that when the Lumex fires, a couple of hundred volt transient will be presented to your amplifier! If you have a high value resistor, say 50 megohms (or a couple of 22 megohms in series), add it across the antenna connector to bleed off charge. A 10 megohm is fine if you are using a long cable like mine.

Disconnect the antenna when lightning is nearby or when not in use!

Note: the circuit below is fine for audio but see the experimental circuit that allows for the reception of a broader frequency range.

Here is an amplifier for use with a long coaxial cable that includes a high-pass filter to help reduce the fundamental line frequency component:

The high-pass frequency is set by the 500k and 1meg resistors, change them keeping the same ratio to change the response, higher values giving a lower frequency response. The values shown will reduce 60 Hz by about a factor of 6 but don't hesitate to use lower values for more attenuation. Another way to reduce the hum if you are willing to sacrifice some low frequency response is to change the output capacitor from 10 uF to 0.047 uF  (adds another roll-off at about 350 Hz). The overall gain is set by the 10k and 180 ohm; increase the 10k to 22k if hum permits. The selected capacitor at the antenna is chosen to give a total of about 2000 pF including the cable capacitance but this value is not critical if the hum is low enough and the gain is high enough. The power supply is an ordinary bench supply with the negative terminal grounded to a good earth ground.

My version of this amp replaces the 10 uF output cap with the suggested 0.047 uF.   The plot below shows how low the hum is before filtering. The system can now handle really big pulses without running out of headroom, as you can see.

Note the little on-off squiggles near the end of the trace. Those are from a transmitter in Hawaii.

Try your amp with just the good old line power ground first but don't be discouraged by trouble. My amplifier happens to be right next to a ground wire that connects directly to an underground copper water pipe. If you don't have a cold water pipe handy try running a ground wire out a window and attach it to a ground rod or an outside water faucet near the water main. Such a good ground is handy for all sorts of radio projects. In this case we are providing a ground for audio frequencies so the main consideration is the resistance of the wire and quality of the ground; bends, turns and length are not important. An isolation transformer between the amplifier output and the computer can help immensely in some cases.

Computer-Powered Receiver

Note: You may need to tie together the "tip" and "ring" of the stereo plug for most soundcards.  Some cards apply the microphone bias to the tip which is also the input but most connect the bias to the pin. Also mute the microphone and possibly the line inputs in the volume control menu. You may find a microphone boost check box that will give you more sensitivity if needed.

Here is a receiver for a laptop or PC that requires no power supply! The receiver is powered by the microphone jack and makes a great dedicated receiver for fixed locations like my outdoor antenna described above. The 10 megohm at the antenna bleeds off any accumulated charge and the Lumex tube hopefully catches high voltage spikes. (Use a bigger resistor for short antennas, maybe a couple of 22 megohms in series.) The voltage rating on my 100 pF capacitor is 30 kilovolts which is unusually high but a 1 kV part will suffice. The 270 pF capacitors and high value resistors form a notch filter for 60 Hz and the frequency may be tweaked slightly by varying the 540 pF. My final value was a 510 pF mica type that gave a deep null at 60 Hz. For 50 Hz environments, increase the values of the two top capacitors to about 330 pF and experiment with a value near 620 pF for the one to ground. The 330 mH choke and 100 k resistor (to kill the Q) block RF signals, as does the 100 pF to ground. My 330 mH choke is a surplus pot core style but most styles will work fine and the value may be somewhat lower, perhaps as low as 100 mH. Much below that and there will be a resonant peak near the broadcast band. The silicon diode may be anything similar to a 1N914 and it just provides a current path in the event the Lumex tube fires with a negative-going pulse.

This is my third design and this time I modeled it - hence all the changes! I used a 1 volt source behind a 10 pF capacitor to simulate a short antenna with no coax and I assumed that the computer contains a 5k resistor connected to 9 volts which is on the high side. Expect something like a 2.5k connected to 5 volts but the results will be similar. (I also used two 2N4401's to form the darlington since I didn't have the model for the darlington handy.)

This is just the response I like for this receiver. It begins rolling off below 1kHz with a deep notch at 60 Hz and on the high side there is a slight peaking at 90 kHz followed by a nice roll-off, reaching over 35 dB roll-off by the bottom of the broadcast band. A 100 mH choke substituted for the 330 mH is flat out to 200 kHz and down only 20 dB by 500 kHz but that might be adequate for most environments. I haven't heard a peep from the radio stations with my 330 mH.

If you would like to flatten the low frequency end a bit, split the 22 megohm bias resistor for the darlington into two, 10 megohm resistors and add a 220pF capacitor to ground at the center point. The response will be flat down to 1kHz and 10 dB down at 400 Hz. Notice that the overall gain works out to 1X  with the simulated short 10 pF antenna. With my 1,500 pF antenna system and the bypassed bias resistors, the gain is about +16 dB and the response rolls off a bit sooner:

This is perfect for my fixed antenna setup! Notice how the bypassing the darlington's bias resistors flattens the lower end of the response. (Your ears won't notice.) On a typical day, the noise of the receiver is about 20 dB below the atmospheric noise at 20 kHz even with 1500pF of additional cable capacity on the input. One can do better but the law of diminishing returns sets in quickly! With a short 2 foot antenna directly connected to the box and held near the ground, the atmospheric noise is only a few dB above the noise floor  so this circuit is best with a longer antenna or an antenna/cable system.

The unit is built in a tiny minibox with terminal strips and point-to-point wiring.

The box is mounted on the wall behind the computer and a ground wire connecting directly to a cold water pipe was added to the coax by cutting through the insulation and soldering a short wire to the braid. This extra ground eliminated all the local hum and only signals picked up by the antenna are present. I only tried this receiver with my outdoor antenna with the long cable that has an estimated 1500pF capacitance. The convenience of no additional power supply is hard to beat. The lack of power supply means there is one less ground loop to add hum. Here is a typical spectrum made with an earlier FET version of the receiver showing the low frequency response and the various transmitters at the high end and this version looks similar only with significantly more gain.

A note on soundcards: I had a bit of trouble setting up my soundcard, mainly in finding all the needed controls and figuring out how to set them. For example, I have the microphone input "muted" even though I'm using it. This keeps the sound from going straight from the mic. to the speaker instead of through Spectrum Lab first. I also had to select a setting called "1 microphone boost" buried in an advanced menu. I have no idea how other cards are configured but you may need to poke around a bit.

Note, you may get better results with an isolation transformer between an amplifier like this and your computer. The problem is that this circuit gets its power from the computer, so connect the transformer as a common-mode choke, with the primary connecting between the amp output and sound card input and the secondary connecting between the two grounds. Don't be surprised if it just makes things worse! Try reversing one of the windings, in case the phasing is incorrect.

Portable E-field Receiver

Here is a tiny receiver built into an Altoids tin suitable for use with a laptop computer or portable tape recorder. The prototype antenna is only 1 foot long when fully extended and the receiver fits into a shirt pocket.

The circuit will drive a "crystal" ceramic earphone but the problem with crystal earphones is that they just aren't very loud with a 9 volt signal and outdoor noises can make listening difficult. I had good luck adding a 2k to 10k transformer across the output to boost the voltage but better results are achieved by plugging the circuit into a portable recorder and using the recorder's output earphone jack or speaker for real-time listening. You can just make out the tiny transformer near the output jack. This part will be removed soon and is not shown in the schematic.

I painted my tin with gray hammertone paint and affixed a label to the lid. The circuit is built on a piece of polished copper clad circuit board sprayed with a light coat of clear Krylon to keep it shiny.  All the ground connections are made directly to the copper and a couple of terminal strips hold most of the components.  A battery compartment is formed with two pieces of copper clad board soldered in place as walls and a piece of foam rubber on the lid secures the battery. Just about any assembly technique should work fine so don't feel compelled to copy this approach.

I just upgraded my recorder to an M-Audio Microtrack 24/96 and it is amazing; it easily outperforms my soundcard!

Having battery problems with your Microtrack?

But there is a problem (as usual). The device emits a huge amount of hum and buzz and a receiver can't be anywhere near it. A lot of it sounds just like 60 Hz line noise, too, making it more confusing. I came up with a solution that completely eliminates the interference:

I formed a shield from perforated aluminum, using aluminum eyelets to hold it together. The "trick" is to add a short 1/4" dia. brass standoff on the inside that lines up with one of the 1/4" jacks so that the shield is grounded when the standoff plugs in (second photo). Keep the standoff short, maybe 3/8", so that it doesn't reach the input terminals in the socket. I can use the Super-Tiny receiver below without significant interference, although the gain is a bit low. The Altoids receiver above is a better choice. I cut a hole for the record button since that is the only button needed when "in the field". Here are two spectrum plots with then without the shield with the Altoids receiver about 2 feet away from the recorder. I have the volume down so that the unshielded test doesn't overload the recorder. The difference is night and day (Paul Nicholson's filter is ON in these plots, removing 60 Hz and harmonics):

There is interesting stuff above 24 kHz that I have never seen:

If you want to analyze the files yourself, here are the two files down-sampled to 44.2 kHz:  bad.wav  good.wav  They are still pretty big files, about 1.2 meg, but you won't see the high stuff. You can hear a radio station in the good one so my Altoids receiver needs a little RF filtering. One of these days I'll add a 10mH choke or resistor.

Here is a spectrum from my magnetic receiver below with this recorder without the shield but positioned for minimum interference:

It certainly isn't suffering from a lack of gain or bandwidth!

My simple computer-powered darlington receiver with the rooftop antenna (see above) is my favorite:

No worry about interference from the recorder here; it's in the basement!

Super-Tiny VLF Receiver

Here is a VLF receiver built into an earphone plug! I recently acquired a digital voice recorder to replace the microcassette recorder above and discovered that it is extremely sensitive, easily amplifying the signal from the FET version of the computer-powered receiver above. This new receiver consists of a very low current JFET, a bias resistor (2, 22 megohms in series) and a capacitor; that's all! The recorder is designed to power electret microphones so no battery is needed.

The drain of the FET connects to the center pin of the plug and the source connects to the ground pin. Two, 22 megohm resistors are connected in series from the gate of the FET to the ground pin and a 560pF capacitor connects from the gate to a flexible wire antenna about 14 inches long (not critical at all). The capacitor value isn't critical and a smaller value will work fine. I selected it for its mechanical strength! For the antenna, use wire that is stiff enough to hold its shape but bends easily enough to protect your recorder's jack. The JFET has an IDSS of only about 125uA so power consumption is quite low. (These are the JFETs that I have in quantity. If you want a couple, let me know at charles@wenzel.com.) This thing really works! I just walked out to the end of my sidewalk and made a recording. A car passed near the end and the electric fields produced by the tires make quite a roar. There was no lightning for hundreds of miles but the spherics are easily heard. The hum was removed by Spectrum Lab and the file was down-sampled using dBpowerAMP to make the file small. (I could have used Spectrum Lab for that, too but there is more to the story. See the note below.)

As is often the case with quickie projects like this, a problem arose. The darn thing picks up the LCD mux frequency. I tried shielding the connector and lower portion of the antenna and it helped greatly but it didn't look very nice. The solution is quite simple; just use an earphone extension cord. The cord lets you easily hold the antenna over your head while keeping the recorder comfortably low, anyway. Yes, that's the ticket; its a feature...  Or, find a metal box that the recorder fits inside and connect the box to the recorder's ground, perhaps at the earphone jack.

This setup will easily catch whistlers and other audible phenomenon, it fits in your pocket easily, and the Olympus is very sensitive, but the recording quality doesn't hold a tiny birthday candle to the Microtrack.

Note: This recorder only makes WMA files and a converter is required to use the data with Spectrum Lab. After a little panic attack, I found www.dbpoweramp.com. With the addition of a CODEC, this program can convert WMA files to WAVE format (see the website for instructions). It's easy to use, once you get it installed and running; just right click on the file and select "Convert To", a new entry in the menu. All sorts of options are available. The program is free except that the mpeg portion expires after 30 days.

Another Note: The audio input on these recorders can be "tricky". Mine looks at the impedance of what is plugged into the earphone jack to decide whether it is a microphone or earphone and whether it has stereo capability. It seems to do a test at the instant of initial connection and remembers the results until the plug is removed.

Dual-Mode VLF Whistler Receiver

See the note above for connecting to a computer soundcard.

This portable receiver may be completely powered by a recorder or other device designed to use button microphones or it can use its internal battery to provide headphone listening. I decided to see if I could increase the values on the notch filter like that used in the computer-powered receiver above by nearly a factor of 10. Since this receiver will be able to run completely on the power supplied by a soundcard or recorder microphone jack, there isn't any power to spend on a source-follower. So, the notch filter isn't buffered and must operate at a very high impedance to avoid loading the antenna. This circuit probably pushes practical values to the edge, however! Although the very high value resistors exhibit a correspondingly high noise voltage, the improved sensitivity from this impedance increase "outruns" the noise and the performance is pretty good. (The noise voltage of a resistor only increases as the square-root of the resistance.) This is about it, however; the capacitors are already getting close to the capacitance of even a short antenna. A 32" antenna seems to bring the atmospheric background noise above receiver noise adequately, even on a fairly quiet day. (The circuit noise was observed by substituting a 15 pF capacitor for the antenna.)

The resistors were made by hand-selecting 22 megohm resistors and soldering 2 or 4 in series to get precise values. (The resistors are in white, green, and red heatshrink tubing. The green and red ones are the carefully selected values for the filter.) 30 pF disk capacitors were the perfect value for these resistors to get a deep 60 Hz notch and, surprisingly, no tuning was needed.  The 300 mH consists of three, 100 mH chokes in series with a 270k resistor to kill the Q. Just about any value of choke from 100 mH to 1 Hy will work here for most purposes but such chokes aren't particularly common. The wires on these chokes is extremely fragile and bending the leads near the body can easily break the fine wire.

The response is flat to above 100 kHz but is down over 20 dB by the bottom of the broadcast band and dropping fast. The simulated response is shown below.

Instead of the darlington transistor used earlier, I tried a JFET that is intended for such a job. They aren't easy to find, however! Look for a JFET that has an Idss of a few hundred microamperes max and a pinch-off voltage near one volt or lower.  Not too many FETs fit the bill. I included the Lumex tube in the event I use the receiver with my rooftop antenna which can pick up a charge. It probably isn't needed for portable whip antenna use. The silicon diode on the gate could be left out, too since it is only there to prevent a high reverse voltage when the Lumex tube fires.

Note: FETs  with a higher pinch-off voltage, say 2 volts, will work with soundcards that supply a higher bias voltage like 5 volts. The soundcard voltage and current may be directly measured with a DC multimeter. My laptop supplies 2.25 volts and only 0.75 mA but one desktop I measured supplies 5 volts at 2 mA. The bottom line is that you want the drain to be at least a volt above the source.

The receiver can also run on an internal battery and an audio amplifier is included to drive headphones. Notice that when a recorder (or soundcard) is plugged into the top jack, the internal 10 k resistor is disconnected and the JFET gets its power from the recorder.  Most earphone jacks have that little switch built in. The internal headphone amplifier may be operated with or without a recorder connected and the power switch may be left in the "off" position to save the internal battery if the recorder or computer has an adequate sound output capability. The amplifier has more gain than is needed and a resistor may be added in series with the 10 uF between pins 1 and 8 to reduce the gain a bit (see the manufacturer's data sheet).

Construction is mostly point-to-point wiring using terminal strips and isn't particularly critical. The headphone amplifier is built on a little piece of perfboard that is supported on one end by a short length of PCB card guide glued to the side of the case. A little drop of wax keeps the board from sliding out. It's just typical prototype construction. A PCB would be nice!

I like this receiver. It is fairly small (4.5" x  2.3"), works with my computer and various recorders without using the internal battery, operates as a stand-alone receiver, and works well with a short antenna or my rooftop whip. It probably isn't a project for everyone, however. The FETs are hard to find and I don't have my typical endless supply. The one I used is a house-numbered part that has the right Idss and gate cut-off voltage determined by experimentation! (The electrometer FETs that I have in the hundreds will work but the gain is just a bit low for the typical recorder or soundcard.) Possible number include:  2N4338, 2N4302, E201, 2N3370, 2N3438, 2N3460, and maybe a VCR4N. The chokes aren't much easier to find ( I used three in series) and the resistors are a bit of a pain in the neck to match. The matching may not be necessary but I wanted a really deep null and I wasn't disappointed. The 60 Hz is virtually invisible.

Remember, direct connection to the sound card can cause excessive hum problems, depending on you setup. A 1:1 isolation transformer can help immensely, but the receiver will need to run on the internal battery. Or, as an experiment, try connecting the isolation transformer as a common-mode choke, with the primary connecting between the amp output and sound card input and the secondary connecting between the two grounds. Don't be surprised if it just makes things worse! Try reversing one of the windings, in case the phasing is incorrect.

Baby's VLF Monitor

While staring at an otherwise useless 900 MHz baby monitor, it occurred to me that the circuit to operate its microphone is perfect for operating a transistor for a VLF receiver, something along the lines of the Computer Powered VLF receiver or the Super Tiny VLF Receiver. I chose the Super Tiny version and added a 330k resistor in series with the gate to cut down on intermodulation products from radio stations. Here's the contraption:

The baby monitor is removed from its case and mounted in this larger one along with two D-cells, a power switch, and the Super Tiny VLF receiver circuit. The original baby monitor antenna is the short gray one and the VLF antenna is the longer black telescoping type. The close-up of the VLF receiver below shows a 363k resistor and 750 pF capacitor from the antenna (Fahnstock clip) to the gate of the J201 JFET (or any type suitable for low voltage application). A 47 megohm resistor connects the gate to the source and ground (and also the microphone cable's shield) and the drain simply connects to the microphone cable center conductor.

The range of the baby monitor is great (using an old scanner), and the fidelity is surprisingly good. I'll be keeping my eye out for more of these in the garage sales! The purpose of this gadget is to listen to unusual local VLF signals like passing cars, buzzing bees, etc. I rushed it together to try to catch a hoard of bicycle riders that occasionally pass my house in the middle of night. (They're an odd bunch that go for all-night rides on the occasional full moon!) Unfortunately, their route didn't pass the house this time. But I did catch a couple of other sounds during the day. A couple of notable examples below were recorded with Karen's Humnuller program.

Bug buzzing the flowers by the roadside.

The VLF antenna was near the ground and sloping over the flowers, adjusted for minimal spherics.

Car passing and a few tweaks

A couple of loud tweaks can be heard just as the car passes, not to mention an unusual whine. That car must be a hybrid! Most of them just roar. The antenna was horizontal and near the ground under my truck for this recording. That turned out to be a questionable location, by the way! Not only did several neighbors become alarmed by it, but a couple of police officers also spotted the thing. I must admit, it does have a bomb-like appearance! It spent the rest of the night under an overturned flower pot with the VLF antenna sticking out the little drain hole in the bottom.

That's the completed unit with the cover installed. It should be quite weather resistant and the batteries will last for several days of continuous operation. This battery-powered receiver can be placed quite a distance from the house and overhead wires yielding some of the cleanest reception I've heard. I might add provisions for a ground wire to clip to a metal rod, car chassis, etc.

Any audio gadget that uses one of those silver can microphones can be modified in this manner. Whatever the device is, it already has plenty of audio gain for the microphone and it supplies the required bias current.

ELF Monitor with AGC and Hum Removal

This highly specialized ELF/VLF receiver is for continuous monitoring of ELF signals primarily in the audible frequency range. The receiver is intended for a fixed installation in a location plagued by excessive line-related interference. Many components are selected for the local conditions and an optimized system can be difficult to achieve but the results can be well worth the effort. The unit is mounted on the back wall of a basement laboratory (photo to left). For experimentation purposes, the circuitry is built on a white protoboard permanently mounted in the case (right). The final unit has several variations from the picture and is easily reconfigured for additional experiments.

The circuitry consists of several sub-systems shown in the above block diagram. Each will be described separately.

Surge Arrestor and Preamplifier

The first component is the antenna interface and preamplifier stage. The antenna is a single element about a meter long at the end of a fairly long cable exhibiting about 2000 pF capacitance. Despite the high capacitance, the preamplifier is designed with a high impedance to accommodate shorter cables in the future. The antenna connects directly to a Lumex surge arrestor rated at 230 volts breakdown. This arrestor is paralleled by a 51 megohm resistor that slowly dissipates any charge on the antenna that has not reached the Lumex threshold. The antenna is coupled to the preamplifier FET via a .01 uF, 3000 volt ceramic capacitor which limits the energy that can reach the preamplifier without significantly attenuating the desired signals. A NE-2 neon lamp is connected to ground on the preamplifier side of the capacitor to further suppress voltage transients. An 18 volt zener diode is connected to the positive supply voltage to clamp any voltage transients generated by the firing of the Lumex tube, possibly eliminating the need for the NE-2. The 9 volt reverse bias on the zener ensures that it has little to no effect on the received signal. This surge suppressor system survived a severe storm where charges were great enough to cause the Lumex tube to repetitively fire in a relaxation oscillator fashion, causing quite a squealing buzz. No circuitry was damaged as a result. The FET is additionally isolated from the surge protection circuitry by a 10 k resistor that was actually added to reduce the tendency of the FET to demodulate AM radio stations. The resistor works in combination with the FET capacitance to effectively roll off frequencies above a few hundred kilohertz.

A 10 mH choke and two capacitors near 253 pF were added in series to ground to eliminate 100 kHz interference from Loran-C transmissions. Loran is off the air, so just leave these components out.

The FET is an E113 but many substitutes are possible; choose one with an IDSS of a few mA. The 2.5 k source resistor was selected to bias the drain to about 5 volts, leaving about 3 volts across the FET, so just select a different value for your JFET, making sure to achieve at least a few volts across the FET and the drain resistor. A good choice of FET will have only one or two volts on the source.

The source resistor is bypassed by a 22 uF capacitor in series with a 330 ohm to give the stage some gain. The optimum gain depends entirely on the level of AC hum at the receiving antenna and this gain should be high enough to drive the AGC amplifier into significant rollback. Otherwise, the AGC amplifier will be operating at such a high gain that the bandwidth of the system will be limited by the gain-bandwidth of the AGC op-amp. As a point of reference, this installation has a 60 Hz signal of about 3 Vp-p on the drain and, after passing through a "bridged"-twin-T filter, the voltage is about 400 mV p-p which is large enough to drive the AGC amp into a nice gain region. Yes, it is a bridged-twin-T in that the twin-T filter shown below is bridged by a resistor to let a little of the hum through for the digital filter to use for locking. The amount of gain in the FET and the amount of bridging are somewhat arbitrary but lower gain in the FET seems best. No bridging resistor would be used for external processing with a computer filter like Karen's PC Humnuller or Spectrum Lab. Remember, this circuit is specifically designed to deal with a large line frequency signal and the AGC will crank the gain "way up" without a large AC signal present. A simple fix for environments where the hum is low or varies a lot would be to bridge the photocell in the AGC circuit (below) with a smaller fixed resistor to keep the gain below some reasonable value (perhaps 4.7 meg instead of 22 meg).

60 Hz Twin-T Notch Filter

The twin-T notch is built from old-fashioned mica capacitors and inserted into a small plastic box (simply for convenience). Any capacitor type will work as long as the values are precise. Two, 270 k resistors are placed in series to achieve 540 k and two, 0.005 uF capacitors are paralleled to achieve 10 nF so only one value of each type part is needed. The bridging resistor, about 1 megohm, that lets a little hum pass this filter is located on the protoboard and is not shown in the schematic. The dedicated protoboard is beginning to earn its keep!

AGC Amplifier

The AGC amplifier employs a CA3140 as the gain block with its gain controlled by a resistive type optoisolator. The LED/CdS optoisolator used in the prototype is a Raytheon CK2143 acquired from my vast inventory of really old stuff (charles@wenzel.com). Other similar parts or even an LED glued to a CdS photocell will also work. The signal level is determined by rectifying the audio with two ordinary silicon diodes. In this implementation, the diodes are referenced to the power supply and the rectified voltage drops down from there, requiring an op-amp that can "look at" the positive rail, typically a JFET type like the LF356 or some more modern rail-to-rail types. The diodes could be reconfigured to operate near ground, requiring an op-amp that works well with near-zero volts on the inputs. The LF356 compares the rectified voltage to the voltage on pin 3 and slowly increases or decreases the current in the LED to achieve balance. The 4 uF combined with the input 1 megohm give this AGC a very, very slow response and it can take a minute or two for the amplifier to stabilize when power is first applied so be patient. This very slow response prevents impulses from significantly changing the gain which would cause the hum filter to pass a bit of hum for several seconds. The 220 ohm resistor in series with the LED may need to be significantly increased, perhaps to several thousand ohms, if the AGC "hunts" which will appear as a slow increase in gain followed by a sudden decrease in a periodic manner. Increasing the LED resistor reduces the gain of the LED/photocell pair.

The values shown may be varied quite a bit, depending on available components but a few precautions should be observed. Bias the first op-amp (resistors on pin 3) to a voltage that is well within the common-mode input and output voltage range of the amplifier; the output will be swinging several volts. The AGC level is set to achieve about 4 volts p-p since the PIC Humnuller cannot tolerate signals above 5 volts on the input. The following block includes a rail-to-rail amplifier that runs on 5 VDC that further protects the PIC from excessive inputs but you don't want that amplifier to be clipping as a normal part of operation.

Limiting Amplifier and Differential Amplifier

The limiting amplifier is simply a rail-to-rail voltage follower running on 5 VDC (TLC2272). The input of the follower comes from the audio output of the AGC amplifier directly above. This follower protects the PIC from excessive input voltage swing. The PIC provides a filtered version of the hum that is input into one side of the differential amplifier with the buffered signal being applied to the other input. The output of the differential amplifier is therefore the desired ELF signal with most of the line-related noise removed.

The 1 uF on the output is intentionally on the low-value side to roll off the tiny residual 60 Hz signal.

The tan board contains Karen's  PIC Humnuller.

This receiver works even better with a computer filter program. To leave out the PIC Humnulller, don't bridge the bridged-t filter with a resistor (not shown anyway), leave out the TLC2272 and drive the sound card line input from the AGC amplifier's output through a 1 uF coupling capacitor. Use Karen's PC Humnuller program or Wolfgang's Spectrum Lab to filter out line noise.

A sample of the filtered audio during an unusually inactive period demonstrates the effectiveness the PIC Humnuller. Half way through this recording, the white wire from the PIC is pulled out, eliminating the filtering. The difference is dramatic as is the horrible level of hum in the air around this facility! Karen's PC filter does work better but this unit does not require a computer.

Note that the filter is called a "bridged-T" in the photo but that is incorrect. I'm so used to using the amazing bridged-T filter in RF work that the name stuck in my head!

Like a broken record: in many instances you will get much better results with an isolation transformer between the audio output and the sound card.

Direct questions and comments to charles@wenzel.com.

Here are a few recordings:

Loud Lightning Crash

That motor starting (Not as loud as the magnetic pickup, but still clear.)

Little Chirps from distant lightning.

There are some interesting carriers that come and go, too. A narrow carrier just appeared at 17.8 kHz, dropped in amplitude, then reappeared as a spread-spectrum signal. I caught a second burst of the spread-spectrum signal starting up (and ending as I type):

Here's a signal starting on 21.4 kHz that may be from Hawaii. It is big enough to see on the 'scope, despite the AC hum. Now that tells me the antenna/amp are working!

Magnetic Pickup

There are two approaches for receiving the signals, a loop or coil antenna that picks up magnetic fields or a whip or wire antenna that picks up the electric field. (Nature manages to make electromagnetic waves at these frequencies but you cannot make an "electromagnetic" or tuned antenna, it would be too big.) The prototype magnetic pickup coil shown below is made with a common 120 volt valve coil. Its 1/2" dia. hole perfectly accommodates the larger Amidon ferrite core (R33-050-400).  (These removable coils slip over an enclosed solenoid and are commonly used in industrial equipment valves. Make sure to take them out of the metal housing and remove any metal parts.)

The core is simply secured with a cable tie on each side of the coil. The resulting inductance is about 900 mH. The loopstick is hot-melt glued into the case as is the circuit board and battery holder. The front panel was designed with a CAD program, printed on glossy report cover stock and sprayed with a clear coating. Spray adhesive holds the label to the Bakelite cover. The second op-amp is not visible in the photo below; it was added "dead-bug style" near the pot.

The spectrum exhibits plenty of activity down to 60 Hz and up to the top of the hearing range but, presently, I am not sure of the actual bandwidth. The black coil probably wasn't wound with picking up submarine communications signals in mind! I suppose I could disconect it to determine the resonant frequency but it seems plenty adequate for the task. (See the spectrum chart made with my new Microtrack above.)

This is a spectrum with the antenna on my desk in a less-than-perfect orientation. Despite all the noise in this room, the VLF transmitters can be seen at the high end (along with a few computer monitors). Let me reiterate that this environment is awful!

The 2.35 meg resistor  is made from two 4.7 meg resistors in parallel.  Most sound cards and recorders will not need the additional gain provided by the second op-amp and the volume control will need to be set very low but the extra gain is nice for listening with a crystal earphone. Also, this amplifier rolls off low frequencies below several kHz. In my limited experience, that gives a fairly flat spectrum (see the plot above the schematic). But, for better low-frequency response, lower the 18k resistors to 1.8k and reduce the4.7 and 2.35 megohm resistors to 820 k and 410 k, respectively (or something close). The feedback resistor should be 1/2 the voltage divider resistors. That will move the response down to several hundred Hz.  Lowering the input resistors improves the low-end frequency response, but also increases headaches from the hum!

My version is slightly different than the schematic. My second op-amp is a CA3130 with a 56pF compensation capacitor between pin 1 and pin 8. Also, pin 3 of mine connects to the wiper and pin 2's 10k connects to the bottom of the pot. These differences shouldn't matter. I just walked out to the end of the sidewalk with the antenna and my laptop and made a short recording with the antenna sitting on the ground. It leaves a lot to be desired compared to my e-field whip but I have strong magnetic fields out there; a fair test will require some effort. Notice the predominance of little chirps. The magnetic pickup seems to pick up less of the local static crashes and more of the distant signals than the whip antenna.  I think this thing will be a real contender away from power lines.

Old spectrum snaps:

Typical Day

Lots of activity! What's that at 19,460?

I think I'll unplug my antenna now!

Look at the amplitude and bandwidth of this signal!

Here is a spreadsheet that can be used to compare various op-amps for use as VLF amplifiers:

The spreadsheet allows the user to adjust the antenna length, add shunt capacitance, vary the bias resistor and noise voltages and currents for a particular amplifier. The OP27 and OPA363 are the default amplifiers. The default setup is similar to my 2 meter antenna with the long coax cable.

Warning: I just threw this spreadsheet together and there may be an error or two! If you try to figure it out, I converted noise sources to Norton equivalents to make the math easier (except the op-amp noise voltage which just adds).

Note that the OP27 is a good choice for my long cable length but it would be less than optimum for a short cable. Also, try a JFET like the older LF355.  It is only 11dB worse than the OP27 at the floor with my long cable and better through the audio hearing range for as much as 100pF shunt capacitance.