Some Thoughts
on
E-field Whistler
Receiver Design
Scott Fusare N2BJW
My introduction to natural radio listening came on a midsummer’s eve
this past July. The solder was likely still warm on my McGreevy BBB-41
as I listened in on the VLF cacophony accompanying the major magnetic storm
that raged overhead. Nearly continuous multiple hop, diffuse whistlers
mixed with chorusing and tweeks competed with the constant background crackle
of strong sferics. I was so taken with these unusual sounds that I began
an immediate search for alternative receiver designs incorporating more
elaborate filtering. Although hum free locations are plentiful here in
Northern Vermont, they all require either a short hike or a long drive
to access--not conducive to pre-work sunrise listening.
After reviewing the various E-field receiver designs found on the web I settled on one in particular that offered more comprehensive filtering, in addition to higher gain, than the BBB-4. My first tests of this new receiver were disappointing, all signals where greatly attenuated (as compared with the BBB-4). Additionally, what seemed to be a RTTY signal was present, stronger than the barely audible natural signals save the sferics. After unsuccessfully troubleshooting this receiver, I replaced its higher gain front end with the single FET BBB-4 front end--leaving all other filtering, pre and post, intact. Signal strength was depressed even further! But the RTTY signal was no longer present. Intrigued as to why this, apparently, more robust design would not outperform (or even equal) the BBB-4 I began to do a bit of research on E-field VLF/ELF receivers. Per the literature2,3 an electrically short E-field antenna
(in our case vanishingly short) may be modeled as a voltage source, as
given by signal field strength times the equivalent height of the whip,
in series with the reactance given by the isotropic capacitance of the
whip, the radiation resistance and the loss resistance at the frequencies
of interest. For our case I will consider the frequency range from 500
Hz to 20KHz and a 1-meter long vertical whip. Given these parameters, the
radiation resistance is almost nil and the loss resistance so many orders
of magnitude below the capacitive reactance that the antenna can be modeled
as a capacitor in series with the signal source. The isotropic capacitance
of a long thin antenna is approximately 10 pF per meter. Given the above
criteria, this results in source impedance that ranges from a low of 800K To further explore this idea, I constructed a “simulated antenna”2,3
to aid in bench testing and help give evaluations that were more quantitative
and less subjective. This antenna consists of nothing more than a small
value capacitor housed within a driven shield to mitigate stray capacitance.
I choose 12 pF to approximate a ˜1-meter whip. Using a 10 mV RMS, 50
At peak frequency response (~3 kHz) my BBB-4 is capable of greater than
40 dB of actual gain. When sourced as it would be in normal use (with a
short vertical whip) the effective gain drops to 20 dB at peak. The above
is not meant to in any way denigrate Mr. McGreevy’s elegant receiver, but
rather to demonstrate that E-field natural radio receivers, designed to
be used with short whips, must have extremely high input impedance. 300
M This is starting to sound more like the requirements of an electrometer
than a radio receiver! A 300 M My solution to this conundrum has been to use a boot strapped source follower in front of any filtering or voltage gain stages. This allows all manner of filtering to be driven from the low impedance output of the follower prior to any voltage gain stages. The front end’s dynamic range can then be made much larger than is normally encountered in natural radio receivers. As the follower has no voltage gain, strong out of band energy does not drive the (follower) FET into a non-linear region--no mixing of NAA down into the audio pass band as occurred above. In fact, tight filtering can now be used to remove the majority of the unwanted crud before so much as a dB of voltage gain is used without killing the desired signal. The following circuit is a modified version of a source follower lifted from the pages of Nuts and Volts magazine. Testing it with the above mentioned simulated antenna yields >85% signal recovery, the 15% being lost to strays and junction capacitance I believe. At DC, the input impedance is ~3.3 M The only shunting capacitance appearing at the input is that of any strays occurring in the wiring to the gate and the internal junction capacitance of the FET itself. As the source “follows” the gate, the capacitance of this junction does not charge and is therefore not an issue. The drain is fixed, so the Miller effect is also not a problem. This holds the input capacitance of the FET to a couple of pF. There are techniques that will further reduce the apparent junction capacitance7,8 but I have found the residual capacitance to be more of an aid than a hindrance, as explained shortly. R9 and the MPF102 should have the bare minimum of lead length needed to reach the input connector. I have taken to mounting mine “flying lead” style on the antenna connector, a technique used in ultra high impedance instrumentation (well, in my Keithley electrometer anyway). Alternatively, a piece of coax could be used but the shield must be driven by the output of the follower--not grounded. In short, whatever technique that works5 should be used to keep any stray capacitive coupling with the gate at a minimum--a few pF here and there is a killer when you are looking at the signal source through only 10--20 pF. This is, in my opinion, the major reason that E-field receivers are so sensitive to nearby trees and other vertical conductive items. The world is full objects (including the ground) that are more than willing to be the “other plate” of a capacitive voltage divider. Absorption of the wave front energy by these objects obviously also plays a role, but it is likely minor compared with this loading effect. Without R8 some peaking will occur just before the corner frequency. This may or may not be desirable depending on your application. I included it, as my intentions were to have as flat a frequency response prior to the -3 dB point as possible. Its exact value will vary depending on the size of your whip. You can use a signal generator and a “simulated antenna” sized to approximate your whip to set it up, or the lower tech, but very effective, cut and try and listen method. Alternatively, it can be left out altogether; the peaking may be desirable. C1 should be between .001 and .1 µF, this gives a -3 dB point that varies from ˜1.5 kHz down to near 300 Hz. Be aware that using the larger values also brings up any hum nicely. Nothing besides care in avoiding stray capacitive coupling to the input gate is at all critical about this circuit. All parts are available at Radio Shack and it will happily run on anything from 6--18V. I live within 1 mile of 2 powerful AM transmitters and as such was pleased to find that auto-rectification was almost non-existent with this circuit, but it was still detectable way down in there. Adding R9 forms a low pass filter with the residual junction capacitance of the FET, neatly taking care of my BCB interference problems. Any other practical method of keeping the BCB energy low enough to prevent self-rectification would lower the input impedance. That last few pFs comes in handy. If strong medium wave signals are not an issue, R9 can be eliminated as it just provides more opportunity for stray shunting capacitance to sneak in. I have had much success with this little circuit, both as an impedance converter for the front of my BBB-4 and as a front end for my ever-changing home brew receiver. There are obviously other ways to accomplish the same thing, an op-amp will work fine in the boot strapped follower configuration (but much worse auto-rectification in my experience). Should you choose this route, Burr Brown manufactures low input capacitance (1 pF) op amps. Look for electrometer grade devices such as the OPA129. Alternatively the antenna can just be made larger, increasing its isotropic capacitance and lowering its source impedance. This will inevitably lead to long antennas if the rule of keeping the source impedance to 1/10 of the receiver impedance is held. If the input contains just a few hundred pF of shunting capacitance, the antenna must then have a few thousand pF--that’s BIG. I want to mention that Helliwell6 reports medium latitude whistler field strengths that range from 4 mV/meter to 5 µV/meter (unfortunately he doesn’t give the distribution). Although not the local BCB station, neither are these really weak signal strengths. I would propose that the idea that hunting whistlers is weak signal work has grown out of the fact that many receiver designs are disposing of the majority of the energy prior to amplification, giving the impression that the signals we are chasing are very weak. I don’t think this is necessarily the case, we just have to keep from squashing them down to nothing prior to trying to detect them. In conclusion I want to say that I have no particular expertise in this area and am completely prepared to be wrong on all of this. I would welcome any correspondence on the matter and would particularly like to hear from anyone that tries this idea out. Scott Fusare N2BJW
____________________________ 1 Steven McGreevy, "McGreevy BBB-4 Natural Radio Receiver," http://www.triax.com/vlfradio/bbb4b.htm 2 Toshimi Okada and Akira Iwai, Natural VLF Radio Waves, Research Studies Press 1988 3 Arthur Watt, VLF Radio Engineering, Pergamon Press 1967 4 Paul Horowitz and Winfield Hill, The Art of Electronics, Cambridge University Press 1989 5 Keithley Instruments, Inc., Low Level Measurements 5th Edition 6 Robert A. Helliwell, Whistlers and Related Ionospheric Phenomena, Stanford University Press 1965 7 Larry K. Baxter, "Capacitive Sensors Offer Numerous Advantages," http://www.planetee.com/planetee/servlet/DisplayDocument?ArticleID=1843 8 U.S. Patent #4390852, Buffer Amplifier, http://www.uspto.gov/patft/index.html |
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