RF Basics

As a subject, two-way radio communications utilise a broad range of frequencies. For the purposes of land mobile and amateur radio, we are generally concerned with frequencies between approximately 1 MHz and 900 MHz. Frequencies above 500 MHz are not covered in detail here, as they are relatively uncommon in UK land mobile applications and fall outside my area of practical experience.

Each frequency range offers distinct advantages and disadvantages, making some bands more suitable than others for particular operating environments and applications.

The following sections examine the frequency bands commonly used for two-way radio communications and discuss their respective characteristics, strengths, and limitations.

Radio waves occur naturally and are emitted from sources such as the Sun and lightning during storms, but can be artificially generated by the oscillation of charge carriers (electrons) within a conductor such as an antenna.

A transmitter generates an alternating electric current which is fed to an antenna. This causes electrons to oscillate in the antenna, and this generates magnetic and electric fields which radiate from the antenna as radio waves.

During the reception of radio waves, the electrons in the antenna are caused to oscillate because of coupling with the oscillating electric and magnetic fields of the radio waves. The oscillation of the electrons in the antenna causes an oscillating current, which is then fed to the receiver. 

Radio waves travel almost completely unimpeded through our atmosphere. the exception to this is where layers of gas at high altitude become ionised from the Suns radiation, causing radio waves to be reflected back towards the surface at some frequencies, or absorbed as heat by the atmosphere at other frequencies.

Electromagnetic radiation covers a span of frequencies way beyond what we need to worry about for two way radio. Here we will concentrate only on the radio frequencies from MF (Medium Frequency) to UHF (Ultra High Frequency).

Frequency is a measure of how often something occurs in some time period. In the case of radio waves, we are measuring how many times some point of a wave passes a point per second, and this is expressed in Hertz (Hz). 

Frequency is inversely related to wavelength, with wavelength increasing as frequency decreases, and vice versa.

Frequency being measured in Hertz and wavelength being measured in metres.

Radio waves generally travel in straight lines and are attenuated (reduced in strength) by objects in their path, including soil, rock, buildings, and gases in Earth's atmosphere. Because the Earth is spherical, and because signals at VHF (30–300 MHz) and UHF (300–3000 MHz) are strongly attenuated by obstacles and do not normally follow the curvature of the Earth, the Earths horizon limits how far these signals can travel in a straight line.

Frequencies below 30 MHz routinely overcome this limitation because they are less affected by atmospheric attenuation and, at some frequencies, can travel via ground-wave transmission.

As radio waves leave an antenna, part of the energy travels along the Earth's surface and is known as a ground wave. The remainder travels upward and outward from the antenna and is known as a sky wave.