SID Receiver
Unstable Sunspot Group
First though, an explanation of why a SID occurs in the first place is needed. SID receivers are built to operate in the VLF range. This frequency range is from 3-30KHz and possesses a number of unique characteristics. VLF signals are capable of traveling as powerful wave fronts which follow a trough formed between the Earth's surface and the Ionosphere. These signals are able to travel large distances and I have monitored NWC in North West Cape, Australia from my location here in Florida a number of times in the past. VLF signals follow the curvature of the Earths crust and can also penetrate a ways into the crust. This is why the United States and other countries operate VLF stations. These stations are used to communicate with submerged submarines.
Now, SIDs come into the picture with the fact that VLF signals are capable of being reflected from the Ionosphere due to the creation of a wave guide between the Earth's surface and the D layer. It is this wave guide and the various electron density variations that are present in the D layer and E layer that cause the variations in signal strength we see in SID receiver plots. During quiet solar days the D layer is more or less in equilibrium, but when there is a solar flare event the extra energy impacting the Ionosphere serves as an additional ionizing source which causes an increase in the density of free electrons throughout the D layer. The increase in free electrons makes the D layer a better reflector to VLF signals and also lowers it's height. The end result is a significantly stronger VLF signal thanks to the sudden ionosphere disturbance. For a more in-depth explanation on VLF propagation read this discussion.
I have for some time been interested in building a SID receiver to monitor Solar activity. I first tried using an HF receiver and a VLF converter to do the job, but because the receiver had no way of turning off the AGC (Automatic Gain Control) I was not able to detect any SID activity. I was able to detect the daily pattern in the VLF signal strength though, so that did give me hope. I then decided to just build a simple SID receiver from scratch. The place to go for all your SID needs really is AAVSO. Those guys have their stuff together and is where I got most of my SID information from. I decided to go with the "Simple, Easy-to-Build, SID Receiver" for my receiver design. I ended up using two OP27 operational amplifiers in place of the TL082 as I had these on hand and they are very low noise devices anyway. The TL082 is a Dual JFET OpAmp and is handy for this job, and Radio Shack even carries it! My point is that there is a lot of experimentation room when it comes to SID receiver design and nothing is set in stone. Experiment with the design and try using what you already have if possible. Click on the SID Receiver Schematic to see my slightly modified version of the receiver. I made the second OpAmp capable of variable gain for easy control of over driving issues. I'm also using 1N34A germanium diodes as opposed to the 1N914 diodes as these did not work well for me. I'm still using the telephone cable to feed the circuit with power and for getting the highly amplified signal to the recorder. In other words, I'm not using a feedline between the loop and the amplifier and the amp is mounted on the loop itself as per the directions in the AAVSO post above.
SID Receiver Schematic
This receiver relies on the antenna entirely for its selectivity. It
is the antenna which tunes in the VLF station. The receiver simply
amplifies and demodulates the VLF signal. The type of antenna needed for
this job therefore is a loop. There are a lot of options when it comes
to building a loop antenna, but I opted for a somewhat larger design. I
used two 3 foot wood dowels about 1x1 inch in size and created a cross
by drilling a whole in the middle of each dowel and fastening them with
a bolt. I then used 14 gauge insulated stranded copper wire which I
purchased from Home Depot to wind the loop. I used hot glue to keep the
wire in place as I wound it around the form. If this size wire is used
its desirable to get around 24 turns for the loop. I got 31 turns out of
mine. The exact number is not important because capacitors will be used
for tuning the antenna and not exact number of turns on the loop. People
like to get really technical with the building of loop antennas, but I
have found that so long as you have a lot of wire wound around a form
the antenna will work just fine. I used 14 gauge wire because it does
give your antenna a higher Q value which is useful for tuning tighter
portions of the band. But again, use what you have at hand. Magnet wire
works just fine as well...
Now comes the fun part, tuning the
loop antenna to the VLF station you want to monitor. This is done by
inserting capacitors in parallel with the loop output wires. As a guide,
I had to use about 27pF of capacitance with my 31 turn three foot loop
to tune in NAA at 24KHz. This will vary depending on the size and number
of turns used for the loop. What VLF station you use will depend on your
location and what stations are about 1000 miles away. In my case,
NAA in
Cutler Maine is a perfect candidate and it doesn't hurt that the station is a
monster putting out 1Megawatt of power! Here's a list of
VLF stations to choose from.
It's important to try to use capacitors that also have a high Q when tuning the loop. Preferably try using Polypropolene capacitors or at least Mica capacitors. This is because depending on what quality capacitors you use you could end up with a bandwidth of say 500Hz or 2KHz! You obviously want a narrow bandwidth so you can null out nearby stations, so a 500Hz bandwidth is ideal.
So how do you go about tuning a loop antenna? The simplest way if you have an Oscilloscope and a signal generator is to loosely couple the signal generator to the loop by using a few turns of wire wound around the loop and attaching the Oscilloscope to the output of the loop and parallel connected capacitors. Now sweep the signal generator from 0Hz up until you see a peak in the amplitude of the signal an then back down again. The peak in amplitude seen on the scope is the resonant frequency of the loop antenna with the corresponding capacitors in place. The lower in frequency you want to go the more capacitance you will have to add to the loop. This process took me two days to complete so be patient and take your time. If you don't have an Oscilloscope and signal generator it's still possible to tune the loop so don't worry. I also experimented with Spectrum Lab by DL4YHF as a tool for tuning the antenna. With Spectrum Lab I connected the loop directly to the sound card and was able to see not only the resonant frequency of the loop but also the VLF stations I was trying to tune! A typical sound card is able to sample up to 24KHz so NAA will just be visible at the top of the spectrogram. I then experimented with different capacitance values until I was able to put the passband of the loop right at 24KHz. Again, I did this before I even had the OpAmp section completed! If you have any questions on the process please don't hesitate to send me an email.
The output of the receiver once everything is tuned and constructed properly will be a variable DC signal. A number of different data logging devices can be used for gathering SID events. I have used a few different approaches in the past. I have used a digital multimeter capable of connecting to a PC as well as paper type stripchart recorders. Recently though, I purchased a DI-194RS data acquisition starter kit from DataQ. This ADC is a simple and inexpensive device ($25) that does a great job at gathering data unattended for days on end. It attaches to a PC via a serial port and comes with gathering and playback software.
This completes the SID monitoring station. The monitoring station consist of the tuned loop antenna, the OpAmp amplifier/diode demodulator, and the analog-to-digital converter attached to a PC for unattended monitoring. So what can you expect to see on a quiet solar day? Well, below is a two day plot during a typical quiet solar period. This particular plot is of NAA in Cutler Maine at 24KHz.
The plot starts at around 10:00 p.m. local time here in Florida (EST). During nighttime hours the VLF signal drastically increases in signal strength and then begins to vary due to fading. Presumably the propagation path during these hours is via the E layer. These are the hours where the receiver is not capable of detecting SID events from solar flares. After all the Sun is on the opposite side of the planet. Right around 6:00 a.m. local time the signal begins to drop in strength until at round 7:00 a.m. it is at the lowest level of the day. This is the transition between E layer sky wave propagation and D layer creation/propagation due to the Sun's ionizing effect on the Ionosphere. During daylight hours and quiet solar periods the plot will be pretty close to a straight line. This is because during these hours the D layer is more or less at equilibrium. Now, if there was a solar flare of sufficient size during the daylight hours the plot would show a sharp rise in the signal strength of the VLF station and then a slow decay back down to the previous level. It's possible to tell how strong the solar flare was just by looking at how much the signal increased before going back down again.
I should again stress that this is a very effective way of monitoring the Sun for solar flares, and many Universities and scientist use this technique as a supplement to the more advanced techniques such as satellite monitoring of the Sun. Furthermore, SID event monitoring is used by scientist to study the D layer itself. The electron densities in the D layer are not high enough (typically < 10^3 cm^-3) to allow ionosondes and incoherent scatter radars of even the highest power to receive significant echoes from this layer. Also, the D layer is too low in altitude for conventional satellites to make in situ measurements yet too high for balloons. Therefore, the D layer is very difficult to study by scientist. SID monitoring is one effective way of studying this poorly understood layer of the Ionosphere.
The VLF range is very susceptible to thunderstorm induced noise. During the summer, Florida experiences daily evening showers and thunderstorms and these can clearly be seen on the plot as the noise towards the end of day two.
As an experiment I decided to plot the SID receivers sunrise and sunset patterns against my local and NAA transmitter in Cutler Maine sunrise and sunsets. The two graphs below cover seven days beginning 10/22/08 through 10/28/08. I used this USNO Navy web site to get the exact sunrise and sunset times for each location. The times are in UTC.
As can be seen, the data sunrise and sunsets are more closely related to the local sunrise and sunsets as opposed to the transmitter sunrise and sunsets. These were the results I expected to get. Actually, on the sixth day (10/27/08) NAA went off the air during the daylight hours. On that day I plotted the sunrise and sunset times for the much weaker NLK VLF station on 24.8KHz out of Jim Creek Washington state. I picked this station up off the side of the loop. Nevertheless, the data sunrise and sunset times were similar to the other six days.
I hope the information in this page might be of use to experimenters. I have also built a simple compass magnetometer that is a great companion to this SID monitor. Together, one could detect the arrival of X-rays due to a large solar flare at the Sun along with the accompanying CME as it impacts the magnetosphere. All this and a cloudy day too!
Good luck, and please email me if you have any question.
