The BAA SID VLF Receiver ProjectBackI am involved with Beta testing the BAA Radio Astronomy Group SID receiver designed by John Cook. These notes document my findings and observations setting up and using the receiver. BackgroundA SID (Sudden Ionospheric Disturbance) occurs when the atmospheric 'D' layer is degenerated following a solar flare. The receiver is tuned to a distant (approximately 1000km) Very Low Frequency (VLF) Radio Station. In my case, I have selected a station in Northern Germany transmitting on 23.4kHz.
The BAA RAG plan to offer a completed VLF Receiver module that will eventually plug into the Starbase system. The notes below document my observations integrating the unit into my own observatory The Receiver ModuleTypically, the receiver module will be supplied in its own RF shielded tinplate box. The interface to the outside world is via a 64 pin Eurocard connector and a BNC connector to the aerial. See the BAA RAG circular for a photo of the completed module. My early Beta was supplied without the tinplate box and has been inserted into my receiver system as supplied. 
Boxing it upI managed to 'acquire' a 1U rack mount box from an IT installation that was being decommissioned. The box was originally used as Fibre Optic Patch Panel. I stripped out the connectors and filled in all the holes with Plastic Padding. A couple of coats of paint and it looks almost presentable. The three Printed Circuit boards were simply fastened inside the box using M3 screws and 12mm spacers. Special effort was made to ensure that everything running at 0V was earthed to the case of the box. 
Power SuppliesThe unit requires +15, -15 and +5V DC supplies. Current consumption is fairly low, so a 250mA transformer was purchased from Maplin Electronics and a simple Power supply was assembled on Veroboard. The circuit diagram below provides a fairly smooth on-load unregulated supply of +/- 17V and a regulated supply of +5V. 
Mains input is fused inside the box with a 100mA fuse and the case is earthed. I have placed a ferrite block over the mains cable to try and suppress any mains bound interference. Initial tests suggest that more mains suppression may be required. ADCThe output from the receiver provides upto 5V DC output. The reality is that this will be dependent on the efficiency and gain of the aerial. To interface to your computer, you will need to convert this DC output to a digital stream that the computer can understand. An Analogue to Digital Converter (ADC) performs this task for you. Radio Sky supply a very nice piece of software called Radio Sky Pipe that is available as a download from their web site. It is available in two variants - a single channel restricted (free to use) variant or a fully featured 8 channel version available for a (relatively) low cost. Also available from their site is a circuit for an ADC based on the MAX186 ADC chip. See here
The AerialThis is the fun bit! Getting the aerial tuned is a bit tricky and involves some maths. But use web resources where they are available. 
The tuning of the aerial is dependant on three things. The frequency that you want to tune the aerial to, the inductance of the wire forming the aerial and the capacitance to bring onto resonance. Tuning is very sharp which means that the bandwidth is very narrow, so that you will need be quite sure of the variables. 
The result is an inductance of 23mH Now we have to calculate the required capacitance to tune the aerial to 23,400Hz. Yet another online calculator exists here 
To physically achieve this, we have to wire the capacitor in parallel with the coil we have painstakingly wound. It is very unlikely that the exact value capacitor will exist anyway so a variable capacitor will be required. Air spaced capacitors normally have a maximum value of about 400pF, so fixed value capacitors will be required to be wired in parallel to build up to the total capacitance required. See the diagram below. Use polysyrene capacitors for this job, they are much more efficient than ceramic capacitors 
This is what is really looks like. Note the two small polystrene capacitors across the capacitor. Both of the variable capacitor halves are in use. The capacitor is glued down to the baseboard using a dollop of Plastic Padding.
Getting it all workingCable it all up, check all supplies are present and correct and you can start fine tuning. Install the software as described on the RadioSky website. Select ADC Channel 1 and check that the PC is recording. You will need to carry out this bit during daytime hours (not sunset or sunrise). Carefully tune the variable capacitor and watch the output. The receiver has a delayed response to changes in signal strength and it will lag tuning by a few seconds. Look for a peak. Once the peak has been identified, (adjusting the capacitor by about 5 degrees either direction will see the signal fall rapidly) then you can start aligning the board. There are 4 trimmer potentiometers to adjust. Two optimise the Frequency, one sets the 'Q' or magnification and one sets the output gain. Overdriving these potentiometers will cause the output of the receiver to take off. When this occurs, back the potentiometer by about 1/2 a turn and the level will drop again. Once all have been optimised, simply case it up and enjoy your instrument. The coaxial feed cableOne other thing you will need to consider is the characteristic impedance of the coaxial cable feed from your loop aerial to your receiver. Use a good quality cable, not the rubbish that you get for TV aerial downleads. I used RG58. This has a characteristic impedance of 50 ohms. I am not particularly worried about matching - this is running at almost DC, but the cable does have a capacitance that needs to be considered. RG58 has a capacitance of 100pF per metre. For a short patch lead, this is fairly insignificant, but when you get bored with your object d'art or your partner demands that you put it out of sight or you want a cleaner signal and you put it in the loft, that 10 metres of cable now has an additional capacitance of 1nF. If you don't allow for this, your receiver will suddenly stop working. Satellite TV coaxial cable may also be used. This has a characteristic impedance of 75 ohms and a capacitance of about 56-66 pF per metre depending on the type. Orientation of the aerialThe maximum signal strength will be when the loop of the aerial is pointing in line (not broadside) to the signal transmitter. In this case, for my aerial located in Northwest England receiving a signal from the North German transmitter, the frame is aligned E-W.ResultsEarly days yet, but at least its working. The graphic below shows a 14 hour sample, clearly showing how the 'D' layer forms at dawn and disappears again at dusk. Note that dawn is UK time and dusk is German time. 
It can also be seen where the transmitting station is turned off at dawn. I appear to have a few glitches as well that need to be resolved, but it is working. The aerial will be placed in the loft to try and keep it away from as much man made noise as possible. It is very susceptible to noise and would easily detect when the flourescent lights were switched on and off. I may also build a larger aerial, perhaps 1m square. This will increase the capture area and by maintaining the same number of turns allow me to use lower value capacitance. Updated 2006-02-28 |