"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. The energy from a lightning bolt streams out into space into a region called the "magnetosphere", magnetized plasma created by the interaction of solar wind with the earth's magnetic field. The lightning pulse is reflected or "ducted" back down to earth after a very long trip during which time the frequencies are spread out by a dispersion-like process. Short whistlers might be due to dispersion, but some whistlers last five seconds so ordinary dispersion is probably inadequate an explanation. A radio wave can travel a million miles in five seconds so to accumulate that much difference in arrival times, the signal would have to travel hundreds of millions of miles, assuming a pretty steep dispersion curve. More likely, the whistler is an emission from the magnetosphere triggered by the lightning pulse. (Read Alysson's emails regarding whistlers.) 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.
The ever-present power line hum makes listening to these signals difficult near power lines but modern computers are fast enough to digitally clean up the signal, making natural radio listening a practical home-based activity. I am delighted with a tremendous freeware offering from Wolfgang Buescher (DL4YHF) called "Spectrum Lab" that works in conjunction with the sound card. In addition to oscilloscope and spectrum analyzer functionality, it also provides digital filtering, including a highly effective hum filter module contributed by Paul Nicholson. It does take a pretty quick computer, by the way. This probably is a job for your best computer and not the old one in the closet. 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 the last several seconds (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.)
I just discovered another effective notch filter in the Filter Control Window called "Auto-notch". Set the speed to 0.05 and the FFT size to 4096 or higher and turn off the hum killer. This filter "surges" a bit on big static crashes as it "gates in" noise, making a sound a bit like eating celery, but it doesn't have nearly as much ringing or echo. Here is another very short sound file with another chirp recorded with this filter. Try playing it in a continuous loop.
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I have an unused 900 MHz antenna sticking
out of my chimney that has an ungrounded vertical element. I replaced the 12
inch element with a two yard aluminum rod (see photo). The antenna is
only 75 feet from the power lines so I didn't have much hope for this antenna to
work. The lead-in coax is long so its capacitance (about 1,500 pF) forms a
voltage divider with the capacitance of the antenna so the signal is significantly
attenuated. It is a big gain hit but gain is cheap, as long as noise doesn't become a
problem. The amplifier needs to be high enough in input resistance to give a sufficiently
long time constant to handle the lowest frequency of interest but with 1,500 pF, an input
impedance of 1 meg is sufficient to get a flat response below a couple of hundred Hz. One
should expect to run into troubles trying to amplify a remote high impedance antenna with
a line-powered amplifier and then feeding that signal to another line-powered computer!
The possibility for noisy ground loops is mind-boggling. But ground loops are easily
avoided by simply not grounding the antenna cable at the mast. Now, it would be a safer
antenna if it were grounded! A spark gap lightning arrestor for the center conductor AND
the braid is a good idea for safety. A single-point ground prevents ground currents from
flowing in the shield but it also directs a lightning strike right to your receiver inside
the house! At a minimum, disconnect the antenna when storms are nearby or when you
leave. Don't worry about ground plane, impedance matching, cable impedance or other RF considerations; it isn't a radio antenna; it's just a voltage probe shielded by the braid in the coax. A high antenna seems best (now that I've played a bit). On the other hand, locating the antenna near ground would be more safe but try to stay away from other objects. After playing with a few random antennas, I must say that some locations just don't work well for no apparent reason so try several locations until you can easily detect those transmitters between 17 and 24kHz. Unfortunately, a location where the hum is low is usually a location where the desired signals are low, too.
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Remember to add a neon lamp or, better yet, a Lumex gas tube transient suppressor from the antenna to ground to bleed off static charge.
Here is a "catch-all box that goes between the antenna and the amplifier. It has a Lumex gas discharge tube right across the antenna connector and a 0.01uF, 1,600 volt coupling capacitor in series with the signal. There is also a 10 mH inductor in parallel with an 8.2k resistor at the amplifier connector to block radio stations. 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, add it across the antenna connector to bleed off charge. A lower value is fine if you are using a long cable.
Disconnect the antenna when lightning is nearby or when not in use, regardless of these precautions!
Here is an amplifier 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. Surprisingly, no isolation was needed between the amp and computer once the earth ground was connected!
The more I play with this, the less important the low frequencies seem to be. My version of this amp replaces the 10 uF output cap with the suggested 0.047 uF. The plot below shows how low the line 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. Connecting the shield of the coax to the earth ground at the amplifier's input completely eliminated grounding problems. I can't emphasize the benefits of a good ground at the amplifier input enough! 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.
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.
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.
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 | The 2SK117 is an unusually low noise JFET but other types will suffice. Pick one with an Idss of at least a few mA. If oscillation occurs, try adding a capacitor to ground across the 100 megohm resistor, perhaps 22 pF and add a 100 ohm resistor in series with the gate. |
| The 1k resistor in the source is only a starting value and it should be selected to achieve about 5 volts on the drain (400 uA). With some JFETs with high Idss, this value may be considerably higher than 1k! |
| The 100 megohm bias resistor may be several 22 megohm resistors in series and the value is not critical. A higher value (or longer antenna) will give better low frequency response but that only invites line frequency troubles. Stand reasonably still when listening or changing electrostatic fields will overload the amplifier, especially if you are wearing rubber sole shoes! |
| The 50k pot may be a higher value if desired but don't drop much below 25k or gain may suffer. If the op-amp is fast enough, more gain may be had by increasing the 100k feedback resistor. My brief experiments indicate that the gain is plenty high as-is. |
| The 1uF capacitor in the source and negative op-amp input may be reduced to give the circuit more of a high-pass characteristic to reduce overload from line frequency. The 22uF in the output may also be reduced for the same purpose. In extreme cases, try 0.47 uF caps for these three caps. (The output cap is presently large to accommodate low impedance loads.) |
| The op-amp may be just about any type that will work with a 9 volt power supply including most CMOS types, "single-supply types, and low power types. |
| There are two capacitors indicated across the power. Place the 1 uF near the op-amp and the 47 uF near the FET. If your circuit has stability problems, break the line between the two capacitors and add a small resistor, perhaps 100 ohms. (Keep the op-amp on the battery side of the resistor.) |
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! 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!
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.
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.
Note : A reader has recommended a lower input current op-amp (like the OPA363) for the following two circuits in place of the OP27 used above for less input current noise. For antenna systems with a lot of cable capacitance as above, the low noise bipolar amp is best above a couple of kilohertz but for antenna systems with low capacitance (no cable) a CMOS or JFET amp is best. (The capacitance where the two types of op-amps perform equally is about 500 pF at 3 kHz.) Even more capacitance than the 2000pF used above doesn't hurt with the bipolar amp since the noise current is still the limiting factor; the noise and signal drop together. It takes about 5000pF before the high audio frequencies are degraded (if I did that quick calculation right). So, use as long a cable as you need! But for short or no cables between the antenna and amp, use a FET input op-amp.
If you place the amplifier at the antenna, you avoid all that cable capacitance but your amplifier must have a correspondingly higher input impedance to maintain flat gain at the lower frequencies. For a typical 10 pF antenna (about a meter long) you would want a 100 megohm input impedance to have flat gain down to 150 Hz! One simple solution is to add some shunt capacitance across the antenna, eating into the gain but giving a lower frequency response. Some hand-held designs have a 10 megohm input impedance along with a 50 to100 pF shunt capacitor in parallel. Although the shunt capacitance works great, another elegant approach is to bootstrap a 10 megohm bias resistor so that it has an effective impedance of 100 megohms, or more at the frequencies of interest. The 10 megohm will still discharge static charges quickly, a desirable feature for a portable receiver. These following experimental receivers will probably need a 10 mH choke or some sort of filtering to eliminate broadcast band radio signals.
Experimental circuit (not tested):
This circuit amplifies the voltage on the antenna by a factor of 11. The output signal is divided back down by the 10k and 890 ohms to about 0.9 times the input voltage. This voltage is applied to the bottom of the 10 megohm bias resistor through the .0033 uF capacitor, keeping the voltage across the resistor down to about 1/10 the antenna voltage. Since 1/10 the voltage is across the resistor, it loads the antenna 1/10 as much, as though it were 100 megohms. The feedback signal drops off below a couple of hundred hertz and the resistor begins to look like 10 megohms near DC. Additional gain will be required after this amplifier but too much gain in one box may lead to instability. Make the next stage an inverting stage with a gain of 100 or less and keep the output leads short and away from the antenna. It would be a good idea to use shielded cable to connect the op amp to the output connector. Here is a possible implementation (also not tested):
This circuit could drive an earphone or speaker amplifier but I would highly recommend completely enclosing a speaker amp in its own metal box or metal compartment within the chassis. Otherwise, the large voltage swings will be picked up by the antenna and oscillation will result. This circuit might make a great portable whistler receiver but I'm still sold on the passive antenna/coax approach for permanent installations.
As I mentioned earlier, I'm not convinced that flat audio response down to a couple of hundred hertz is necessary or even desirable. Try adding a switch in series with the bootstrap 10k resistor (the one that goes to the .0033 uF cap). With the switch open, the input impedance will drop to 10 megohm and the frequency roll-off will jump up to a couple of kilohertz. You will still hear lower frequencies, just somewhat attenuated. I suspect that switch position will be your favorite when in town. (I haven't tried this circuit yet, by the way.)
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!
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The plot below shows that there is really a lot of activity right now. The 17.8 kHz signal has retired but several others are quite strong. A quick web search suggests that the 24 kHz signal on the right is from a transmitter in Maine. |
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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.
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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.
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:
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There it goes: |  |
Here is a spreadsheet that can be used to compare various op-amps for use as VLF amplifiers:
| Openoffice.org |
| Excel |
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.
Alysson sent a couple of compelling emails regarding whistlers that I have combined into one below. Although I have no scientific basis for my thoughts, her explanation seems on target. It is easy to imagine that a batch of high-velocity ions from the sun arriving near a pole will spiral in the earth's magnetic field and will radiate energy in the process. As they slow down it makes perfect sense that the frequency will drop. Diffuse whistlers might be due to a spread in arrival times for larger clouds of ions. I personally don't see the connection to lightning (that's not saying much) since the ions will travel in spirals simply due to their kinetic energy and the magnetic field. Also, since all the "whistlers" I've picked up in Texas have been motors, I wonder if the radiation is more of a near-field phenomenon. Anyway, enough ranting by an uninformed hack; on to the excellent emails:
Hi,
I was just browsing your site, and found your VLF Whistler Detection article (http://www.techlib.com/electronics/VLFwhistle.htm)
In that page, you state that:
Short whistlers might be due to dispersion, but some whistlers last five seconds so ordinary dispersion is probably inadequate an explanation. A radio wave can travel a million miles in five seconds so to accumulate that much difference in arrival times, the signal would have to travel hundreds of millions of miles, assuming a pretty steep dispersion curve. More likely, the whistler is an emission from the magnetosphere triggered by the lightning pulse.
You may be interested to know that when I was at University College of Wales, Aberystwith in the late 1970’s, the Physics department research budget was spent almost entirely in the pursuit of this phenomenon. The research was led by my tutor, Dr A.D. Maude at that time (sorry, no citation, just my memory of his lectures, but you might like to try http://www.google.co.uk/search?hl=en&q=Aberystwyth+Ionospheric+Whistler&meta= ). What was found was that these were generated by the entrapment of high-energy ions in the Earth’s magnetic field. The ions originate, reasonably enough, at the solar surface.
At the relatively low field densities of the temperate latitudes in the high ionosphere, these particles will penetrate deep into the field before losing enough energy to become trapped. They are then deflected along the field, but due to their momentum, travel in a corkscrew orbit. This acceleration of the charge results, quite naturally in the emission of radio frequency radiation.
In shedding further energy, the helical orbit decays – becoming more linear (a longer orbital period), and thus a change in the frequency of the emitted RF. The period of decay for these high energy particles is of the order of 5 to 12 seconds, but the VLF signals become difficult to detect without a high-altitude detector. (We sent up rockets regularly).
I went to look at what remains of my old notes last night …. From what Dr Maude said, the lightning whistlers are probably caused by ions generated within the storm itself undergoing the same process, but since their energies are only a few MeV and in (relatively) low atmosphere, the paths tend to be short (maybe 2-3 seconds only).The best whistler times are during periods of intense sunspot activity – when large volumes of solar ions are being pumped out with energies in the GeV and possibly TeV ranges. My own theory (unsupported) is that the thunderstorm RF discharges cause a resonant oscillation in the solar ions which, as you say, will kick them into their helical orbit. The solar whistlers are, though, observable at any time albeit rather less frequently.
Dr Maude’s rockets were built using (then) the highest sensitivity receivers available (it was a very generous budget), the whistlers only being detectable from stratospheric altitudes or above. I find it amusing that it is now possible to put together a receiver sensitive enough to make observation of this phenomenon for “pocket money” prices.
I hope this is of interest.
Regards,
Alysson Rowan
