Constructive and Destructive Interference, Directivity
This is an excellent time to point you towards antenna arrays!
So, maybe you remember the double-slit experiment from school. In case you never did that, or need a refresher:
Imagine having e.g. a laser, that is, something emitting a plane wavefront (from the left in the below picture). You shine that light on a plane with two small slits:
from wikipedia:
You've got two very small slits, emitting (half-) circular wavefronts that have the same phase (in the plane of the slits); when you look from a distance, you'll see that these wavefronts overlay constructively, whenever the run length difference of both waves is 0 or any other multiple of a wavelength. They interfere destructively, i.e. they cancel out, whenever the run length difference is an odd multiply of half the wavelength. These things line up in directions from the centre between the two slits, so that we can talk of directivity!
As you can imagine, adding more slots in the same spacing just makes the 0-difference direction "stronger" and all suppressed directions even more suppressed.
Light's just slightly higher-frequent radio
So, light is just electromagnetic waves, just as 10m radio waves. The latter are just slightly easier to handle – we don't need to build something that makes a plane wave front and then have some opaque material with slits in there. We can just put antennas with circular emission pattern at the point of the slits!
The whole idea of "we add delayed versions of the same RF wave up" is what makes multi-element antennas (and any aperture antenna) work: The elements of the Yagi are just sized and spaced that way, that they re-emit energy just in the right moment to lead to constructive interference of the radio wave in the main direction of the Yagi antenna. By the way, what works in TX works exactly the same in RX.
Due to the geometry of the antenna system defining that main direction this way, Yagis are fixed-direction (unless you rotate them). That's fine for many applications, but I think you need something with which you can find a good direction to receive from.
Enter: Antenna Arrays
Now, what would you have to change in the above double-slit experiment to change the direction(s) in which the main maximum occurs? You'd simply change the distance between the slots. Or, you could find a "magic" device that you install in one of these slots that delays the phase of the wave coming from that slot by some adjustable fraction of the full wave cycle, and with that you could steer the beam.
While that's nontrivial for photonics, it's relatively easy for the relatively narrowband HF signals: We call such a device a phase shifter:
v
+----------------------> | Antenna 1
Transmitter amplifier ---> splitter
| v
+-----> Phase shifter--> | Antenna 2
^
|
Direction control
Tadah, by adjusting the phase shift, you adjust in which direction your transmitted radio waves constructively add up; that's your antenna system's main direction! And you get a gain in that direction. If you want better directivity, repeat the above scheme; from wikipedia:
Remember, what works in TX works in RX, too! Adjust the phase of the received wave of one antenna, add the electric signals up, and get a signal where the reception is good from one direction, and worse from the others.
How to Shift your Phases (All your Phase are belong to us)
For most of the previous century, using (mechanical!) analog phase shifters was state of the art for beam forming, and it's been extensively used, especially in radio sensing applications; and since that's something that military people like to do (for example, to have knowledge of where someone's transmitter is standing, or to build a radar with which you can look very far in a very specific direction), cost was …… less of an issue.
That changed around the turn of the millennium, when Software-Defined Radio (SDR) became a viable thing. Idea is simple: Take your RF signal, digitize it (like a sound card converts electrical signals from a microphone to digital numbers), and do the phase shifting and adding up in software. Done! Computers are cheap, and fast enough.
So, what you'd need is
- a set of omnidirectional antennas in the polarization of your choice
- a place to set them up in a line, in typically quarter- or half-wavelength distances
- a SDR receiver for each of them
- a way to "null" the phases of these receivers (otherwise they'll just be random)
- a computer (seems like you have one!)
- and a tiny bit of software to add up these streams
For 1., antennas: The easiest and – perpendicular to its conductors – omnidirectional antenna is the dipole. And in fact, many laaaaarge antenna arrays are simply made of dipoles, or similarly low-gain antennas. See EISCAT, for example, which I know hams that were involved with:
For 3. Receivers: For 30 MHz, basically all RTL-SDR dongles will do their job. Depending on where, in which quality and in which quantities you buy them, 6 to USD 40 a piece.
(from the OSMOcom wiki linked above:)
For 4., you'll just need something to calibrate your receivers. A transmitter in a known direction would work – you can calculate the phase shifts that the antennas should have with easy trigonometrics, and then just adjust their phases to have that, in doubt, manually. It'd be good idea to feed all receivers from the same reference oscillator source, so that you don't have continuously readjust (because the phases will drift away pretty quickly if they run from different reference oscillators), and that'll involve a bit of soldering (unsolder the oscillator from all but one dongle, add an amplifier and clock distributor to that one dongle, solder in cable to where the oscillator(s) used to be).
For 6., software: a tiny bit of GNU Radio, and pipe the result (e.g. via network) to the SDR receiver software of your choice (GQRX, SDR#, HDSDR, LinRad, I don't know a tenth of the candidates) to do the demodulation.
