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RX-6 VLF E-Field Receiver     

Contents

History
Environment
Receiver Location

RX-6 Design and Operation
Receiver Performance
Getting The Signal Inside
Powering The Receiver
Construction Tips
Processing The Signal
Results

You can listen to my current receiver while you read.

Some History and Some Thanks. A number of years ago, I heard about these curious phenomena called 'whistlers' on a shortwave science program. I purchased a NASA Inspire VLF receiver kit, but never got much out of it aside from some hiss - which I'm sure was entirely my fault in construction. I put it on the shelf and gave up, figuring that it was just too hard to duplicate what I had heard on the program that day. Quite some time later, I stumbled across Renato Romero's web site, and it re-ignited my interest. I purchased a copy of his book, and after a read and some research on Yahoo's VLF_Group, I decided to take the plunge and build a receiving station of my own.

This page catalogs my efforts to get my original system put together, with some modest success. I document some of my failures, along with bits of advice, to help others not make the same mistakes, provide some humor, and to relieve anyone of the notion that I might actually know what I'm doing...

I owe many thanks to Renato for writing his book in the first place, Paul Nicholson for his patient guidance, encouragement, and help as I experimented my way into operation, and to all of the members of Yahoo's VLF_Group for providing years worth of good reading, analysis, and ideas.
 It is very, very appreciated.

The Marlton Environment and Early Reception Attempts. The part of southern New Jersey where I lived at the time consists of farmland, dotted with housing tracts, industrial parks and small lakes. It was criss-crossed by a number of high voltage power lines - one being about a mile from where I lived. The soil was sandy, with pockets of very dense clay. I lived on top of one of those clay pockets (in fact, Marlton gets its name from 'marl', which is a sedimentary mix of clay and lime.) In my old neighborhood, all of the homes were fed power via underground cables, with transformers housed in small metal cabinets at ground level every thousand feet or so. I had one right in front of my home.

Did I also mention I lived about 11 miles from a city with 1.5 million people?

Not exactly the best place for VLF reception.


I decided to start with Renato's Floating Solar Receiver. I thought I would be clever and start with something that would be somewhat 'independent' of local ground (or so I thought) and nicely side-step all of the classic problems with VLF reception. Renato's design is good - but my implementation was bad. My very first receiver, frankly, was an awful, noisy mess. After more self-induced frustration than was necessary, I scrapped my version of the floating receiver and started over.

Which brings us to our first topic, and my first mistake
...

Receiver Location. It may seem like an obvious statement, but taking the time and carefully selecting a location for your VLF receiver is important. When I get excited about something, I tend to rush. Once I had built my 'cantenna' and original receiver, I wheeled it around in the same general open space in my backyard, and was thrilled to hear some sferics buried under hum and interference. I picked an open spot and set up shop. My initial choice of location actually ended up being one of the worst on my property. My final location, about 20 feet away, had almost 20dB less hum interference.

The AC mains hum, it's harmonics, and interference from other local sources can set up standing patterns with 'local' maxima and minima. Through careful study and observation, you can select a location that minimizes (relatively speaking) that interference.

Take the time to build or borrow a VLF receiver, and survey your area to select the best spot. You will save yourself a good deal of frustration later.

Some tips:

- Build a mobile receiver (RX), constructed in a case that lets you mount a whip, and a ground spike (like a tent stake or long shaft screwdriver.) It makes taking fixed observations much easier.

- If you have a netbook or laptop, don't select candidate spots by ear only. While it may sound like you have found a local minima just by walking around, there are other sources of interference that are at the edge of your hearing that will only show up on a spectrogram. Wolf's Spectrumlab running on a netbook is excellent for this purpose.

- Isolate the rx from your laptop with a 1:1 audio transformer, and several meters of wire (at a minimum). This should keep a lot of the noise generated by the laptop from bleeding into the signal.

- Take samples for each location at multiple times of day (and night.) Hum varies quite a bit, depending on the weather, neighbors and a dozen other factors. A spot that sounds good now, may end up being awful the rest of the time. As an example, here is how the AC mains 3rd harmonic varied with time at my location:

1 Week of 180Hz Amplitude Data
One Week of 180Hz. Click for larger image.

- Take detailed notes. Save spectrum samples with time and location information. You will be glad you did when you go back to analyze your options or get a second opinion.

- If you are setting up an e-field receiver like this one, avoid the obvious, like nearby transformers, power lines and your home. Also avoid: trees, metal fences, chicken wire around the garden, clothes lines with metal cores, drainage grates, metal lawn furniture, sheds, outdoor cabling and lighting fixtures. Anything and everything metal. All of these enhanced mains interference.


RX-6 Design and Operation. RX-6 gets its name from being the 6th major revision of what amounts to an e-field probe feeding a very low noise op-amp. In reality, there were many more than 6 variations, as I made changes in design and layout, and observed the effects.

RX6 Schematic
RX-6 Schematic. Click for larger version.

Circuit operation (as I see it, anyway): R1 acts as a bias R for the input of U1, an AD823. R2 and C1 form a low pass filter with a cutoff of approximately 630kHz. R2 and C1 also slow incoming transients (to a degree) to allow switching diodes D1 and D2 to clamp excessive input to either the V+ or V- rails. NE2 rounds out the input protection and provides a flash over point between the whip antenna and system ground. U1 is a very low noise op amp, configured as an adjustable gain non-inverting amplifier. First stage gain levels are set between 1 and 100 via RV1 and R3. C2 forces U1 to also act as a high pass filter, with a cutoff frequency around 100Hz. C3 provides high frequency roll-off to help with broadcast band (AM) interference. R4 and C4 form and additional low pass filter with a cutoff of approximately 160kHz. R5 acts as a load for the AD823 to help with oscillation, as the input to the line driver stage (U2, OP27) doesn't present much of load. U2 is a low noise op amp with a low input impedance configured as a non-inverting amplifier with a gain of 2, as well as a second stage of high pass filtering with a cut off frequency of about 100Hz. U2 also acts as a line driver stage for the receiver, and is coupled into a 1:1 matching transformer via C7 and R9.

The unused section of U1 is configured as a voltage follower, with its input pinned to the virtual ground between V+ and V-. This seems to reduce the current consumption of the AD823, and also prevents the unused section from oscillating.

The power supply (shown in the larger graphic) provides the split power supply necessary for the receiver. The current draw on the virtual ground rail (SUP_GND) is minimal, so a simple passive voltage divider will work. L1 and R7 help to decouple the 4 foot cable run from the supply battery on the ground. Despite the good power supply noise rejection of the AD823, leaving them out increases interference in the receiver.

The gain was set at 80 (x40 first stage, x2 second stage) in my implementation. The overall receiver as drawn is capable of a gain of 200.

The input low pass R2/C1, roll-off cap C3, and low pass R4/C4 were all necessary to prevent a local (< 10 miles) 50kW AM station, WCAU 1210, from injecting all kinds of intermod into the VLF signal.

High pass filtering in both stages was necessary to prevent clipping from the local hum, and to reduce the work for digital processing later.

The AD823 has a drive capability of 15ma, and is rated for a 500pF load. Run into a long run of twisted pair cabling with its high capacitive load, the signal became quite distorted. If you examine the parameters for common coax or twisted pair cable, you will see that the total capacitive load of even modest cable runs in in the 1000s of pf. The OP27 is somewhat beefier than the AD823, able to source around 20ma and drive capacitive loads up to 2000pf. It is better suited for driving a longer signal line.

If your environment does not have the intermod problems, you may be able to eliminate some of the low pass filtering, which I did in later versions of this receiver. Likewise, if your situation doesn't need it (shorter cable runs, better cable, etc.) you might also be able to drop the second stage entirely.

Design tip: If you are building an op amp based receiver like this one, be sure to check the voltage and current noise parameters of the op amp you intend to use. Using these you can infer the 'natural' source impedance that the op amp expects.  In an e-field probe, low input current noise and high impedance are important.

Receiver Performance. The noise floor of RX6 is below the VLF spectrum noise floor, allowing relatively clean reception. Overall, the dynamic range is also quite good.

RX6 Noise Floor Analysis
Noise Floor Comparison. Click for larger version.

In the chart above, the red line represents the noise floor of the PC sound card used to interface with the RX-6. The green line shows the noise floor of the RX6 itself, with no antenna connected and a small capacitive shunt across the antenna terminals. The blue line shows a snapshot of the local VLF spectrum from 0-24khz. As you can see, the noise floor of the receiver is below that of the VLF spectrum we're trying to receive.

The upward swing of the RX-6 noise curve below 5kHz may be able to be improved by raising the value of the bias resistor R1. It's on the list to try, anyway.

The slight upward tilt of the VLF spectrum above 23kHz in this graph was an artifact of the sound card I was using (Sound Blaster!), and not from the receiver itself.

Average current consumption for RX-6 is relatively low, around 12-15ma, with upward spikes during sferics and other activity.

Getting The Received Signal Somewhere Useful. Getting the VLF signal from an RX-6 to the PCs I'm using for processing the signal proved to be one of the single biggest challenges I faced. While it sounds relatively straight forward, like stringing some coax or twisted pair from point A to point B, there are a number of issues that adversely effect the quality of the signal and the performance of the receiver.

Tip No. 1: A difference in ground potential between your receiver and where you process the signal. Try this experiment: Place a ground rod in the spot you've selected for your receiver, and add a run of hookup wire. Use this to measure the difference in potential between your domestic ground (which feeds your PC) and the earth. If you see a 10mV or 100mV difference, you'll probably be fine with transformer isolation. However, in my case, there was over 1V difference, in the form of a nasty, spiky triangular waveform. After some investigation (turning off and disconnecting different loads in my home, checking ground resistance for my breaker panel, etc.) I came to a preliminary theory that the differential voltage probably came from the placement and grounding of the in-ground transformers in our old neighborhood, causing ground currents. The best above ground e-field "hum minimum" on my property was on the general axis between two of the transformers - which places the ground rod along a ground current path.

Tip No. 2: Long cables, even on/in the ground, can act as a very nice antenna for interference to enter your receiver. Be careful where you run the signal line for your receiver - avoid running parallel to power lines (inside and outside), near signal lines for TV antennas/satellite dishes, in-ground accent lighting, etc. Unless trees or other obstacles prevent it, shorter direct runs are best. I have had good success burying my signal cable 1-2 inches underground.

Tip No. 3: Even if you utilize 1:1 transformers on your signal line to provide 'galvanic isolation', you still aren't truly 'isolated'. Why? Every transformer has a (usually) small but finite capacitance between the primary and secondary windings. That capacitance acts as a path for AC signals. In this case, it can act as a path for that nasty 1V sawtooth. I ended up using a series of 3 transformers in my signal line, and I am contemplating adding a fourth. Fortunately, adding the transformers didn't introduce very much attenuation for the desired signal.


Signal Cable Drawing
Signal Cable Configuration. Click for larger version.

Tip No. 4: You may have to ground and/or shield different parts of your cable run differently. Somewhat counter-intuitively, grounding one side of the first segment of my signal line helped to reduce hum by another 3 to 5dB. Go figure. Not a recommended way to do things, as it somewhat defeats the isolation you're trying to achieve in the first place.

Tip No. 5: Large capacitive loads frighten most op amps. If you take a look at the pF/foot(or meter) specifications for common coax or twisted pair cables, you will soon discover it doesn't take a very long cable to produce a large capacitive load. As I stated in my analysis the RX-6, some op amps are better suited for driving these types of cable. However, you can help the situation by careful consideration of the cable you have on hand or are about to purchase. Try to minimize the C per unit length.

One of my current experiments tries to side-step some of these factors by sending the VLF signal optically via low cost fiber optic cable. The challenge I'm currently working on involves flattening the response curve as much as possible from 1-100kHz. Check back for updates.

Powering The Receiver For Extended Observation. Connecting a mains powered DC supply to the receiver defeats the galvanic isolation you just worked so hard to achieve. My initial attempts at resolving this and powering my receiver used a solar panel with a basic current limiter and a sealed 12V gel-cell battery. The first few iterations of my receiver seemed to do fine with this arrangement. However, as the noise floor lowered and the system improved, I ran into a problem: hiss from the Sun. I removed the current limiter, double checked the panel to make sure that it was only a series of Si solar cells (and no active circuitry), and added filtering - but I was still unable to eliminate the hiss completely. The solar panel was fairly close to the receiver and antenna, so it could also have been capacitively coupled. Either way, I ended up dropping the solar panel altogether to eliminate the noise. I was later able to work around this problem by moving the panels a sufficiently large distance away from the receiver when I built my Wifi VLF Receiver.

Currently, the RX-6 is powered via a 115AH deep cycle marine battery. A charge lasts for months, and the batteries have two important properties: cheap and simple.

Another option to consider is high-frequency (100kHz or higher) AC fed via twisted pair and isolation transformers.


Construction Tips. If you build one, I recommend trying the RX-6 out on a breadboard. The RX-6 is not a high frequency design, so in general layout is not that important - to a point. The gain of the receiver is not insignificant, and coupled with the high impedance input stage it can tend to oscillate if you don't take some care in construction and signal routing. The first few versions were on a small breadboard with a very messy layout (a result of many revisions and experiments.) Cleaning up the layout and spacing things on a larger breadboard has completely eliminated the early oscillation problems. My long term goal is to make a PC board for this receiver, which should be relatively painless with something like KiCad or gEDA.

The receiver body itself was housed in a Carlon outdoor plastic junction box purchased at a local home improvement center, with a weep hole and some dessicant.

Be sure to put drip loops in your wiring before it enters the box!

The whip antenna is approximately 8 feet tall, and made from 3/4" PVC pipe. Internally, it is 18ga hookup wire glued to the top cap, run through the length, and brought out through a small hole in the bottom cap. The body is also filled with clean, dry sand. This arrangement provides some weight to the antenna, and seems to have eliminated the majority of noise from wind and rain.

It may also be necessary to 'sound proof' the receiver enclosure itself. For a while I could clearly hear individual raindrops hitting the enclosure. Adding some light padding around the receiver in the enclosure solved the problem.

The enclosure and whip are attached to wooden stake with some 'give' to it, and the stake itself is secured to a large steel spike in the ground. Experience has shown that a somewhat flexible mounting method and whip tend to be less noisy than a firmly mounted one in strong wind. You want something that flops very gently from side to side in a strong wind - not something so flexible it flops around wildly, or so stiff it sings like a guitar string.


Processing The Signal. Once you get the signal indoors, there are quite a number things you can to do with it. Just to name a few:

- Catching Whistlers
- SID Monitoring
- VLF DXing
- Ionospheric Research

To accomplish any of these, however, you will need to be able process and store the signal in various ways. The two primary software packages available for doing this (that I'm aware of, anyway) are fortunately both very good, and free:

Windows: DL4YHF's Audio Spectrum Analyser. If you're a ham, you're probably already familiar with Wolf's software in other contexts.

Unix/Linux: Paul Nicholson's vlfrx-tools.

I utilize vlfrx-tools for initial processing and storage, as well as streaming and post processing (SID detection, etc.) I also use Spectrumlab on the same signal, forwarded from vlfrx-tools, for it's enhanced real-time display.

For information on my vlfrx-tools setup, please see this page.


Results. Overall, I was pretty pleased with the results. The receiver was quite reliable, and sounded quite nice. I frequently listen to it in the background as I do other work. Having completed the receiver in the early winter, I didn't expect to hear much - but I did expect to hear at least something. Quite some time went by, and while the background noise and spectrum looked and sounded like it should, I didn't hear any whistlers. No chorus. I heard tweaks, but that was about it. Then, early one January morning, I got 4 surprises - one of which you can see here:

Spectrum Of A Whistler
Spectrum of a whistler. Click for larger image.

Whistlers! (Finally!) Since that time, I have received more, along with a nice 6 hour run of chorus on 2012-02-15 (see below.) It was certainly a start.

Here are some samples. You may need to raise the volume a bit to hear the whistlers.

A whistler received at 2012-02-15 06:38:04 UTC.
A whistler received at 2012-02-05 22:18:07 UTC.

A 3.5 minute compilation of whistlers and chorus, received early in the morning 2012-02-15. I've cranked the gain quite a bit, so that you can more easily hear the chorus starting about half way through the clip.

I hope this page encourages you to take the plunge. You'll be glad you did.

Handy VLF Links

http://www.abelian.org/vlf/
http://www.vlf.it/
http://www.abem.se/products/wadi/vlf-freq.pdf
http://sidstation.loudet.org/stations-list-en.xhtml
http://www.spaceweathersounds.com/

http://www.backyardastronomy.net/vlf_receiver.html






Copyright (C) 2010-2017, Mike Smith

The content presented here is the original work of Mike Smith unless otherwise shown. Please contact me via email for comments or errors. Electronic Projects, Circuits, VLF Radio and More.