ValenceElectrons wrote: ↑Tue Mar 21, 2023 3:19 pm
I've seen reference to "the ground plane" above and elsewhere. With respect to my project, is that the radio module PCB itself?
Yes.
Radio waves pass energy back and forth between a magnetic field (the electrical equivalent of momentum) and an electric field (the electrical equivalent of potential energy). A radio wave passing through a block of conductive material creates the electric field by pushing electrons to one face of the block.
The wave can't do that if the block of conductive material is connected to an electrical power source though.. any electrons the wave pushes around are replaced by the power source. The only way the wave can cope with that is to align its own 0V state (which it has to pass through twice every cycle) to the surface of the block.
Mechanically, it's equivalent to a wave moving along a rope that suddenly hits a point where the rope is tied off. The wave's only option is to put one of its zero-displacement nodes at that point. And since the wave can't move the point, all the energy in the wave has to reflect from that point and go back the other way.
Functionally, that means a block of conductive material held at a fixed voltage is a mirror for radio waves.
That's useful because antennas detect radio waves by collecting energy from different positions in the wave. The maximum possible difference between two points on a sine wave lives between 0 degrees (sin=1) and 180 degrees (sin=-1), and for a wave moving through space those points will be half the wavelength apart.
The antenna that takes advantage of that has two pieces of wire, each 1/4 wavelength, with a resistor connected between them. The passing radio waves push electrons from the far tip of one wire to the far tip of the other (half a wavelength away). Those moving electrons are an electrical current that generates voltage across the resistor in the middle.
If you put a single quarter-wave wire on a ground plane held at a fixed voltage, the wave 'sees' a reflection of the real quarter-wave wire in the mirror surface of the ground plane. In practice, the parts of the wave that would go to the tip of the reflected wire reflect from the ground plane and go to the top of the real wire. The math works out so the single quarter-wave antenna over a ground plane is equivalent to a dipole antenna in free space.
That's especially handy because the surface of the Earth can be considered a plane with a fixed voltage (fun fact: the resistance between any two points on the planet is about 300 Ohms). Most radio towers are quarter-wave conductors pretending to be dipoles.
To get the advantage of a ground plane, you need the conductive surface to extend far enough to capture useful reflections. Extending the PCB ground plane with a piece of aluminum connected to GND can give you a larger reflective surface.
ValenceElectrons wrote: ↑Tue Mar 21, 2023 3:19 pm
And if so, should the helical antenna (assuming I stick with it for now!) be mounted parallel off the end, or upright and perpendicular to, the module's plane?
It doesn't matter.. the reflections from the ground plane will make incoming waves travel to the appropriate point for a dipole regardless of the real wire's orientation. The math just gets more complicated when the wire isn't straight and perpendicular to the plane.
However..
The wave that comes out of a perpendicular-to-the-plane virtual dipole is different from the wave that comes out of a wire with a different shape or orientation.
The math to express that precisely is exceptionally ugly, so I'm going to hand-wave a high-level argument by resorting to a couple of well-supported physical principles: superposition and reversability.
Superposition means that every atom in an antenna is doing its own thing, and has no knowledge-of/interaction-with any other atom in the antenna. We can break an antenna down into as many tiny-little slices as we want, and treat each of them as an isotropic antenna (radiating equally in all directions). The behavior of the antenna as a whole is exactly the sum of the effects of all those slices stacked up (superimposed) on each other in space and time.
Reversability is a fact of the way the universe works: if electrical energy excites an electron in a way that sends out a radio wave, the same radio wave coming in will produce the same electrical excitation. Basically, if we reverse time we can swap cause and effect. And if we don't actually want to reverse time, an incoming wave with the same properties as a time-reversed wave will produce the same effect as a 'real' time-reversed wave.
(That last bit is known as 'symmetry': physics being unaffected by changes like moving the experiment to another place, pointing it a different direction, doing it at a different time, etc)
The upshot of all that is: the ideal thing to receive a radio wave emitted by some slice of an antenna is exactly that same slice of the same antenna. Failing that, we want the equivalent slice of an identical antenna located somewhere else.
We can get the original antenna back by superimposing all the slices, but in the process we lose some symmetry: the sum of the slices in space depends on where the slices are relative to each other. That means, to get the best performance, the receiving antenna needs to be the same shape as the transmitting antenna, and needs to be in the same orientation.
To demonstrate the loss of symmetry in orientation, imagine a vertical dipole: the waves it emits push electrons up and down in any conductive material they pass through, and for any point on the wave, the best place to capture the most energy is half a wavelength above or below it.
If that wave passes through a horizontal dipole, almost nothing happens. The wave pushes electrons across the thickness of the wire, but doesn't to anything to create voltage between points half a wavelength to the left and right of each other. For ideal dipoles, the reception would be zero.
The same is true for antannas of any other shape and orientation. For a helical antenna, the wave that generates the radio signal is coiled along the wire. The ideal thing to receive that wave is another helix with the same pitch, diameter, and twist, whose slices are the same distance and orientation from the ground plane as the transmitting antenna.