Since electromagnetic radiation is a wave phenomenon, it has certain characteristics associated with waves, such as “polarization”, “phase”, and “wave interference”.
The oscillations of an EM wave occur back and forth across the direction of the wave’s propagation. This means that the wave has a certain “polarity”. If the wave’s oscillations are up and down, the wave is said to be “vertically polarized”; if they are left and right, the wave is said to be “horizontally polarized”. Of course, the wave could also be polarized at any angle between those two extremes.
The concepts of “phase” and “wave interference” are a bit trickier to explain. Imaging a tank of water with some sort of vibrating element to generate waves stuck into it. If another vibrating element is stuck into the water operating at the same vibration rate and same intensity, it will generate waves of the same frequency and height (or “amplitude”), but the peaks and valleys of the waves generated by the second vibrating element will not necessarily coincide with those of the first. In other words, they won’t have the same “phase”.
The phase of the two sets of waves could be matched up, with the peaks and valleys of both coinciding; or they could be completely out of phase, with the peaks of one coinciding with the valleys of the other and the reverse, a condition known as “antiphase”; or they could have a phase difference anywhere between those two extremes.
The really interesting thing is that the two sets of waves add to each other. If the two sets of waves are exactly in phase, they add up into a single set of waves of twice the amplitude of one of the sets of waves. If they are exactly antiphase, they cancel out, and the water in the tank is smooth. If they are between those two extremes, the additive effect is intermediate. This phenomenon is known as “wave interference”.
Radio waves also have phase and so radio waves of the same wavelength can interfere. A single transmission may go from point A to point B by various paths. For example, one path may be a straight line, while another may be a long path due to a reflection or “bounce” off a mountain. Such “multipath effects” caused the “ghosting” seen in the days of analog TV transmissions, with a faint video image appearing slightly offset from the main image. They can also cause “phase delays” that seem to alter the direction of the beam by interference. Controlled interference effects can be used to deliberately shift the direction of a radio beam, a scheme known as “electronic steering”, discussed later.
Radio waves generally propagate over a line-of-sight, weakening with distance, as anybody who’s driven from town to town in a car with a radio realizes, with the music fading out as one town is left behind and becoming stronger as another town is approached.
At night, radio waves can bounce off the ionospheric layer in the upper atmosphere, allowing them to propagate over the horizon, if in a somewhat unpredictable fashion. Such “ionospheric bounce” or “ionospheric backscatter” tends to work better at lower frequencies, below the VHF (30 MHz / 10 meter) range. Low frequency radio waves emitted low to ground, known as “ground waves”, will also tend to curve over the horizon, due to a wave phenomenon known as “diffraction”.
Ionospheric bounce can allow signals to be picked up over very long distances under special circumstances. Anybody who’s ever played around with a broadcast radio receiver late at night knows remote stations can be picked up, sometimes from very far away. A condition known as “ducting” can also arise where the signal bounces down from a high atmospheric layer and then bounces back up again at a lower atmospheric layer, with the two layers forming a tunnel or “duct” that allows the signal to propagate for very long distances.
Other atmospheric effects can interfere with radio signals. Higher frequencies can be blocked by heavy rainstorms or snowstorms, and lightning can throw “noise” into radio transmissions. Particle flows from eruptions on the Sun, known as “solar flares”, can cause massive disruptions of radio communications, and in the worst cases can even disrupt electrical power distribution grids. There is also a variable background of radio noise from human sources that can cause unwanted interference.
Radio waves vary in their interactions with solid matter. Radio waves, like light, can be absorbed or reflected by matter. Metals and water, for example, tend to reflect radio waves, while soils tend to absorb them. Also as with light, radio reflections can be “specular”, as if bounced off a mirror, or “diffuse”, as if bounced off a rough and uneven surface.
If radio waves can penetrate a material, the penetration is greater at longer wavelengths. Radio waves can penetrate buildings well enough, and very long wavelengths can even penetrate a good depth through the sea or into the ground. Very short wavelengths are strongly attenuated and have limited penetration.