When I decided that SID monitoring would be an activity that might be interesting and worthwhile pursuing, I looked for information on the technique in general, and equipment in particular. I soon found that most of what is available is often very outdated and mostly put together by the observers themselves. I therefore decided to use whatever guidelines seemed sensible, and design my own system.
The first step was to design the antenna. Since loop antennas seem to be the norm in SID monitoring, I considered several ideas for a loop antenna which, besides the obvious need to do what it is supposed to do, would also be robust and look professional. It would appear that many SID observers construct their loop antennas on wooden frames, square frames being the easiest to construct. I took a different approach and constructed the loop antenna shown in the photo on the right.
The ring has a diameter of 1 metre and contains the loops of wire. For the ring I used a length of a new type of plastic plumbing pipe which is very strong. The wall of the pipe has two layers, and in between these layers is an aluminium foil shield which serves to prevent heat from being radiated outward through the pipe when it is used for hot water installations. This shield is a bonus as far as the SID antenna is concerned. Since small loop antennas respond to the magnetic field of received signals, it is desirable to shield the antenna from the electric fields generated by local man-made noise sources.
My idea is to place the electronics at the antenna feed point in a suitable weatherproof enclosure. For this purpose holes for the ring were made in the enclosure which I decided to use. Next, the pipe was bent to form a ring and the two ends were passed through the holes in the enclosure, as the photo on the right shows. For the wire loops I used a 4-core single-strand cable of the type commonly used by installers of burglar alarm systems. This cable was fed repeatedly into and out of the ring at the open ends until it became difficult to add another turn. I managed to get 11 turns into the ring, which resulted in 44 turns of single wire. I would have preferred more turns, but 44 should be a reasonable starting point. Finally, the individual wires at the ends of the cable were soldered together in such a way that only the two ends of the resulting coil remained. During the soldering process each solder joint was covered with heat shrink tubing to prevent short circuits between joints.
Having come this far, the next step was to tune the antenna to the frequency in the VLF band which I hope to monitor. The NWC station at Harold E. Holt, North West Cape, Exmouth, Australia seems a good candidate, but this needs to be confirmed. Since this station transmits at 19.8 kHz I decided to tune my antenna to have maximum response at this frequency. First, the inductance of the coil had to be obtained, which I found to be 6.33 mH. Next, the required value of a parallel tuning capacitor to achieve resonance at 19.8 kHz was obtained from the formula
This gave a value of 10 nF.
The antenna was excited at frequencies from 10 kHz to 30 kHz using a GW Instek AFG-2005 function generator and a GW Instek GDS-1052-U oscilloscope to measure the voltage across the antenna terminals. The results are shown in the graph below. With the choice of five 2.2 nF capacitors making up the total parallel capacitance, the antenna has a peak response at 20.5 kHz. The response above the -3 dB level extends from about 12 kHz all the way to 30 kHz, so in theory it should be possible to monitor stations at frequencies other than 19.8 kHz. Of course there will be tuning in the receiver, but more about this when I get around to the receiver design.