WiFi VLF E-Field ReceiverContents
The Reception Environment
Receiver Design And Operation
Processing The Signal
A note before you begin: This is not a configuration I currently use to receive the VLF band, due to limitations I discovered after its' construction. Instead, this is a record of the challenges and design choices that happen with this type of remote receiver. Hopefully, these notes will help someone else considering a similar project.
You can listen to my current receiver while you read.
Motivation. After relocating to a spot southwest of Lynchburg VA, I began to consider the design limitations of my original VLF receiving setup, shown here. One of the primary limitations of a VLF receiver in a suburban or semi-rural environment is the presence of power lines and transformers above and below ground. Aside from direct near field reception of AC hum and harmonics, one of the biggest sources of domestic interference is carried into the receiver via the cable that connects the receiver to the building that houses the computer and soundcard or A/D converter. The cable acts as an antenna, but also carries noise current produced by the (likely) different ground potential between the receiver and the building. You can read more about that effect here.
One way of working around this is to feed the output of the VLF receiver into an FM transmitter. The problem with using an FM modulator is that strong sferics can tend to over-modulate the transmitter, and the FM link itself can add noise and intermodulation. Also, the bandwidth and dynamic range of the FM link can be a problem.
One potential way around the limits of FM modulation is to digitize the VLF signal prior to sending via radio. After some promising discussions on Yahoo's VLF_Group, I decided to build a new setup that digitized the VLF, and transmitted it via 802.11 wireless IP link.
The Reception Environment. The part of central Virginia in which I live is located in the foothills between Lynchburg and the Blue Ridge Mountain Range. The area is mostly forest and farmland, woven with many streams, rivers and several large lakes. It is dotted with low density housing and sparsely placed small cities. It has the occasional high voltage power line, but most of the power lines follow streets and highways. The soil is predominately a red clay, with granite, limestone and volcanic formations underneath.
The location in which we were living had two runs of power lines, separated by about 500-600 feet, running parallel to the front and rear of the property.
Receiver Location. 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. Following the technique outlined here, I found a location not quite equidistant between the power lines that was my best possibility for reception: a trade off between being within a local hum minima, but well within the electromagnetic shadow of a row of 100+ foot pine trees. It is interesting to note that the AC mains were only one of two significant problems: many samples showed I had equally strong interference between 40-47Hz, mirrored rather nicely around 60Hz, re-appearing at approximately 100Hz. The harmonics from this source did not extend nearly as high as those from the AC system, but they were rather strong in the low end of the band. I have a preliminary theory that it has to do with a railroad yard about 1 mile from my location, which I based on hearing activity aurally and correlating it with what I saw on spectrograms. Regardless of the source, it was a strong mess, and I needed to address it.
Receiver Design and Operation. The additional low-end interference prompted me to develop a "split" version of my original VLF receiver, which includes enhanced low-end filtering, and is detailed here.
System Description. Based on bench experiments, I discovered that putting the VLF receiver in close proximity to the GPS board and embedded PC (a modified ALIX 2d3) injected all sorts of digital hash into the receiver. I found that moving the receiver at least 10 feet from the ALIX and GPS and including a little extra power filtering eliminated that problem. Based on my previous experience with solar panels, and advice from members of the VLF_Group, I also decided to place the charge controller and solar panels some distance from the receiver - which brought the total number of enclosures to three. The result is shown in the diagram below.
WiFi System Diagram. Click For Larger Image.The ALIX 2d3 was installed with a version of Voyage linux, running Paul Nicholson's vlfrx-tools. The 'sound card' I chose was an E-MU 0204. The 802.11n connectivity was provided by a $5 dongle purchased on eBay. The GPS was the Mikroe Smart GPS module I had used in my prior installation.
Here is a small gallery showing the waterproof tubs with the various components in them (click for larger images):
System Performance. The noise performance of the split version of the receiver is very good, and below the desired VLF signal. You can see more details about it's noise performance here. As part of re-designing the system, I no longer wanted to be constrained by a 48kHz sound card, so I opted to run the E-MU 0204 at a 96kHz sample rate. I was able to see a number of interesting VLF stations that I never been able to observe before - which was quite nice. However, there were a number of disadvantages to the system that eventually caused me to switch back to a hard-linked system:
USB Soundcard Support. While there is full ALSA support for the E-MU 0204, the USB hardware and drivers for the ALIX (and the Raspberry Pi, and the Beaglebone...) don't seem to be quite up to the task of continuous, high-speed use. I had frequent vtcard reset cycles and lost samples. Others have had similar experiences with the Raspberry Pi. Some of the problems can be eliminated by careful pairing of the USB soundcard and embedded PC and using specific, lower, sample rates. In the end, I was interested in expanding my reception beyond 48kHz, and this setup was simply not up to the task.
Energy Equilibrium. When you bundle a receiver (12-15ma), a GPS (~90ma), and an ALIX (300-550ma, depending on processing), your power budget easily exceeds half an amp or more. It takes a significant investment in solar panels to make the system completely independent. Frequently, when we had extended runs of cloudy days, I would have to swap batteries when the one in use went flat. I could have added more panels, but by that point I had already decided to abandon this approach.
Crowded 802.11 Bands. One of the side effects of enabling 802.11 connectivity on phones, set-top boxes, cordless phones, etc. is that the 802.11 bands have become crowded. It can be difficult to achieve consistent, high rate data flows (especially at the rates required for 96Khz, stereo, 16bit data). I forced the use of 802.11n, did a site survey and selected an unused channel, turned off 40Mhz channels (an 802.11n enhancement), and tried various other forms of creating a dedicated virtual circuit. In the end, the interference and lack of reliability was too great to overcome. I was baby sitting the system entirely too much.
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.
End Notes. Overall, developing and deploying this type of reception system was a good learning experience. I'm sure that with a lower sample rate and very careful tuning, the system could be made more reliable. My interests and needs, however, ended up ruling this system out.
Handy VLF Links