# Propagation Between the emission of radio photons by a transmitting antenna and their capture by a receiving antenna (or reflection by an object) they move from one location to the next with the speed of light. There are many different modes of propagation, and deciding which one should be adopted for the solution of any particular challenge to the radio engineer depends on matching its characteristics to the problems to be solved. Once the mode of propagation has been chosen it is generally straightforward to select the optimum radio band in which to work, and the best receiving and transmitting antennas. Identifying the manner of propagation is thus the first step in radio system design. The following paragraphs review the various processes of radio propagation. Radio quanta, like all photons, have no electric charge and very little mass, so they move in straight lines in all but the very strongest gravitational fields, where their paths will be slightly curved by gravitational attraction. In fact the interaction of gravity with radio photons is such a weak effect that it can be neglected in all except the most rare situations encountered only in radio astronomy. We shall not consider it further here, and with that very slight proviso shall assume that radio quanta travel in straight lines and, of course, at the speed of light. If it is only the far field that is of interest they may be taken as originating from point sources since the transmitting antennas are far away. The simplest of all possible radio propagation environments is free space. We begin by assuming that neither atmosphere nor any solid objects are present to complicate things. When transmitting, we will be concerned, as said, only with the far field, and are thus always very distant from the antenna, which can therefore be approximated as a point source, of negligible dimensions. > [!warning] > This section is under #development # Millimeter-Wave Propagation Radio wave propagation holds the key to understanding receiver design, the transmitter power requirements, antenna requirements, interference levels, and expected distances for wireless communication links. At mmWave frequencies, where the wavelengths are smaller than a centimeter— even smaller than the size of a human fingernail—most objects in the physical environment are very large relative to the wavelength. Lampposts, walls, and people are large relative to the wavelength, and this causes very pronounced propagation phenomena, such as signal blockage (e.g., shadowing) when an obstacle is in the way of the path between the transmitter and receiver. However, reflection and scattering allow wireless links to be made between the transmitter and receiver, even when there are physical obstructions that block the line-of-sight (LOS) paths, as long as steerable antennas are used to “find” objects that bounce or scatter energy. Fortunately, highly directional multiple-element antennas, capable of being electrically steered, can be made in very small form factors and integrated inexpensively. The wavelengths at mmWave frequencies are so small, in fact, that the molecular constituency of air and water play a major role in defining the free space distances achievable across the sub-terahertz spectrum. Radio waves are dramatically attenuated by atmospheric absorption caused by the oxygen molecule at 60 GHz and the water molecule at 180 and 320 GHz. ![[Pasted image 20250107142139.png]] > [!Figure] > _The attenuation (dB/km) in excess of free space propagation due to absorption in air at sea level across the sub-terahertz frequency bands. The far left (unshaded) bubble shows extremely small excess attenuation in air for today’s UHF and microwave consumer wireless networks, and other bubbles show interesting excess attenuation characteristics that are dependent on carrier frequency_ (source: IEEE). > [!warning] > This section is under #development