While I’m very familiar with handling audio-frequency signals in electronics and have a basic understanding of how radio circuits work, there are huge gaps in my knowledge around radio waves, their propagation and reception, and rather a key part of radio system design: aerials.
My planned aerial design for VLF/ELF reception was three air-cored coils positioned in orthogonal directions, like this :
(see also Hardware Issues)
Unlike, say FM aerials, these small loop antennas will pick up the magnetic component of the electromagnetic signal (as do the coils in typical AM radios). I assumed that the signal received on the N-S axis would be different from E-W and Up-Down. This seemed like the way to capture directional information, assuming the Up-Down direction would also be useful, ie. overall collecting as much information as possible.
Small loop antennas are much less than a wavelength in size, and are mainly (but not always) used as receiving antennas at lower frequencies…
…Surprisingly, the radiation and receiving pattern of a small loop is quite opposite that of a large loop (whose circumference is close to one wavelength). Since the loop is much smaller than a wavelength, the current at any one moment is nearly constant round the circumference. By symmetry it can be seen that the voltages induced along the flat sides of the loop will cancel each other when a signal arrives along the loop axis. Therefore, there is a null in that direction. Instead, the radiation pattern peaks in directions lying in the plane of the loop, because signals received from sources in that plane do not quite cancel owing to the phase difference between the arrival of the wave at the near side and far side of the loop. Increasing that phase difference by increasing the size of the loop has a large impact in increasing the radiation resistance and the resulting antenna efficiency.
So my planned N-S coil would actually have nulls in those directions, similarly for E-W, and the Up-Down coil would in effect be omnidirectional NSEW. D’oh!
There is a lot to electromagnetic radiation! (eg. near vs. far field reception is something I need to read up on). It’s weird stuff.
Reading around the topic a little more, the null positions are the key to radio direction finding (RDF). A coil will have two nulls, 180° apart, which RDF gets around by adding a sense antenna, which may be a simple vertical whip aerial. This will be omnidirectional and (if I understand correctly) when summed at the right levels with the coil signal will effectively remove one of the nulls.
Luckily, hardware-wise I’m still at the planning/experimentation stage, so haven’t wasted too much time winding coils. Looks like I’ll have to change the design, provisionally having N-S and E-W coils plus a sense antenna, a la RDF. There are lots of designs for all kinds of aerials and receivers at VLF.it (I’ve just started assembling a list of reference links, that site is at the top of the list).