RF Basics

Two way radio as a subject covers quite a spread of frequencies. For the most part, with respect to land mobile and amateur radio, we are looking at everything from around 1MHz up to 900MHz. I have little to no experience of working with frequencies above 500MHz as this is pretty uncommon in the UK, so I won't be covering it here.
The frequencies used for two way radio have different advantages and disadvantages, with some performing better than others in certain circumstances.
Here we attempt to explore the frequencies in use, their advantages and their disadvantages.

Radio waves exist naturally and are emitted from sources such as the Sun and lightning, but can be artificially generated by the oscillation of charge carriers (electrons) in bulk material (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 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 directly, but inversely related to wavelength, with 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 the gasses in Earths atmosphere. Because the Earth is spherical, the Earth itself limits how far a radio signal at VHF / UHF frequencies can travel in a straight line, wheras frequencies below 30MHz routinely defy this limitation.

As radio waves leave an antenna, part of the wave travels along the ground and is known as Ground Wave. The rest of the wave travels upward and outward from the antenna, and this is known as Skywave.