Bands & Propagation

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 amount to the oscillation of charge carriers in bulk material. An example would be the oscillating of the electrons (charge carriers) in an antenna (bulk material). 

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 can become ionised from the Suns radiation, causing radio waves at certain frequencies to be reflected or absorbed. 

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 directly, but inversely related to wavelength, with frequency being measured in Hertz and wavelength being measured in metres.

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

Convert Frequency to Wavelength

Convert Wavelength to Frequency

Although normally used with "HF" radios, and mentioned in the context of conversations surrounding the topic of HF, this falls below 3MHz which actually makes it a MF band.

Propagation is limited to local contacts during the day, but DX is possible at night, especially around sunrise and sunset 

Unlike the higher HF bands, long distance propagation is often better around sunspot minimum when solar activity, and therefore noise level, is lower. 

maximum power on 160 metres is limited to 32 watts at frequencies above 1850kHz where amateurs are secondary. 

Some text

60m

Less affected by D-Layer absorption than 80 m.
Is an ideal candidate for near vertical incidence skywave (NVIS), the most commonly used technique capable of providing seamless local-to-medium distance HF communications. Information about the critical frequency (foF2) of the ionosphere at any one time is highly important for setting up and maintaining reliable NVIS radio links. 

Some text

Some text

Some text

Some text

Some text

Some text

Some text

Some text

Some text

Some text

Some text

https://www.voacap.com/hf/

The ionosphere is the ionized part of the upper atmosphere of Earth
from about 48 km to 965 km above sea level

• 
  

• ionized by solar radiation

• Ionization is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons. The resulting electrically charged atom or molecule is called an ion.

• 
  

• The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth,[18] stretching from a height of about 50 km (30 mi) to more than 1,000 km (600 mi). It exists primarily due to ultraviolet radiation from the Sun. 

• Ultraviolet (UV), X-ray and shorter wavelengths of solar radiation are ionizing, since photons at these frequencies contain sufficient energy to dislodge an electron from a neutral gas atom or molecule upon absorption. In this process the light electron obtains a high velocity so that the temperature of the created electronic gas is much higher (of the order of thousand K) than the one of ions and neutrals. The reverse process to ionization is recombination, in which a free electron is "captured" by a positive ion. Recombination occurs spontaneously, and causes the emission of a photon carrying away the energy produced upon recombination. As gas density increases at lower altitudes, the recombination process prevails, since the gas molecules and ions are closer together. The balance between these two processes determines the quantity of ionization present. 

• Ultraviolet (UV), X-ray and shorter wavelengths of solar radiation are ionizing, since photons at these frequencies contain sufficient energy to dislodge an electron from a neutral gas atom or molecule upon absorption. In this process the light electron obtains a high velocity so that the temperature of the created electronic gas is much higher (of the order of thousand K) than the one of ions and neutrals. The reverse process to ionization is recombination, in which a free electron is "captured" by a positive ion. Recombination occurs spontaneously, and causes the emission of a photon carrying away the energy produced upon recombination. As gas density increases at lower altitudes, the recombination process prevails, since the gas molecules and ions are closer together. The balance between these two processes determines the quantity of ionization present. 

• The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur.[1] From the planetary surface of the Earth, the average height of the troposphere is 18 km (11 mi; 59,000 ft) in the tropics; 17 km (11 mi; 56,000 ft) in the middle latitudes; and 6 km (3.7 mi; 20,000 ft) in the high latitudes of the polar regions in winter; thus the average height of the troposphere is 13 km (8.1 mi; 43,000 ft). 

• The lowest part of the Earth's atmosphere, the troposphere, extends from the surface to about 10 km (6 mi). Above that is the stratosphere, followed by the mesosphere. In the stratosphere incoming solar radiation creates the ozone layer. At heights of above 80 km (50 mi), in the thermosphere, the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The number of these free electrons is sufficient to affect radio propagation. This portion of the atmosphere is partially ionized and contains a plasma which is referred to as the ionosphere. 

• The tropopause is the atmospheric boundary layer between the troposphere and the stratosphere, and is located by measuring the changes in temperature relative to increased altitude in the troposphere and in the stratosphere. In the troposphere, the temperature of the air decreases at high altitude, however, in the stratosphere the air temperature initially is constant, and then increases with altitude. The increase of air temperature at stratospheric altitudes results from the ozone layer's absorption and retention of the ultraviolet (UV) radiation that Earth receives from the Sun.

D Layer 48 - 90km
E Layer 90 - 150km  (includes Es Layer)
F Layer 150km - 500km