In 1924, Sir Edward Appleton was one of the first to demonstrate the existence of a reflecting layer at a height of about 100 km (now called the Elayer). This was soon followed by the discovery of another layer at around 250 km (now called the F-layer). This was achieved by broadcasting a continuous signal from one site and receiving the signal at a second site several miles away. By measuring the time difference between the signal received along the ground and the signal reflected from the atmosphere (and knowing the velocity at which the radio wave propagates) it was possible to calculate the height of the atmospheric reflecting layer. Today, the standard technique for detecting the presence of ionised layers (and determining their height above the surface of the earth) is to transmit a very short pulse directed upwards into space and accurately measuring the amplitude and time delay before the arrival back on earth of the reflected pulses. This ionospheric sounding is carried out over a range of frequencies. The ionosphere provides us with a reasonably predictable means of communicating over long
distances using HF radio signals. Much of the short and long distance communications below 30 MHz depend on the bending or refraction of the transmitted wave in the earth’s ionosphere which are regions of ionisation caused by the sun’s ultraviolet radiation and lying about 60 to 200 miles above the earth’s surface. The useful regions of ionisation are the E-layer (at about 70 miles in height for maximum ionisation) and the F-layer (lying at about 175 miles in height at night). During the daylight hours, the F-layer splits into two distinguishable parts: F1 (lying at a height of about 140 miles) and F2 (lying at a height of about 200 miles). After sunset the F1-and F2-layers recombine into
a single F-layer (see Figures 1.8 and 1.10). During daylight, a lower layer of ionisation known as the D-layer exists in proportion to the sun’s height, peaking at local noon and largely dissipating after sunset. This lower layer primarily acts to absorb energy in the low end of the high frequency (HF) band. The F-layer ionisation regions are primarily responsible for long distance communication using sky waves at distances of up to several thousand km (greatly in excess of those distances that can be achieved using VHF direct wave communication, see Figure 1.9). The characteristics of the ionised layers are summarised in Table 1.2 together with their effect on radio waves.