We will be concerned with the frequencies in the MF (medium frequency) bands, especially from 2Mhz to 3 MHz and in the HF (high frequency) bands, 3 MHz to 30 MHz.
The broadcast AM band is in the Low MF band, that is, from 550 Khz to 1500 Khz. And the broadcast FM band is in the VHF band: 88.1 MHz to 107.9 MHz band. Airplanes are in the VHF band around 118.7 MHz AM. Our VHF FM Bridge to bridge radios are in the area of 156.00 MHz FM. EPIRB's are in the UHF band and are on 406 MHz. How about our RADAR units that are on 3 Ghz (giga hertz) or in the SHF band. The INMARSAT Satellite's are in the 1.5 Ghz and 4.5 Ghz bands, SHF. So in using our GMDSS system we are in several different frequency bands.
In these band we experience different Radio Propagations, or that these radio waves act different at different frequencies. So go far and some just got to the horizon and stopped. We call this difference: radio propagation. Once the radio wave leave the antenna this radio wave travels out in space and is radiated in a circular pattern at right angles to the antenna wire. Here we see a vertical antenna, like the ones that we see on the bridge. As this Radio wave travels out and upward into the sky there are some layers of ionized particles, some thick and some thin. The radio wave tends to bounce off these layers of particles and return to earth. There are several layers at different thickness and at different altitudes. We have given these layers names.
The first layer above the earth and at about 90 to 100 miles is called the D-Layer the next is the E-Layer and is about 100 to 130 miles. Above the E-Layer is the F-Layer and at the zenith of the day this layer splits into two layers called the F1-Layer (the lower one) and the F2-Layer. This F1 and F2 layers are about 200 to 300 miles in altitude. These layers vary in altitude during the various time of the day and night. Some go a way at night.
These layers are formed by the solar radiation (mainly by ultra-violet light) and is so intense that when it strikes gas molecules they ionize the gas molecules and set a positive ion free. The free electrons actually affect radio waves. This action takes place in the Ionosphere which is from about 70 to about 500 miles from the surface of the earth. And as the title indicates this area contain these gas molecule and releases the electrons by this action and is thus called IONosphere.
The D-Layer is the lowest from 90 to 100 miles above the earth. It is present during the day when the sun is sending this radiation at its maximum. Because the density ofthe ionosphere is high at this altitude the ions and electrons will recombine quickly and after sunset the solar radiation is blocked by the earth the electrons levels fall quickly and thus this D-layer all but disappears. Just above this O-Iayer is the E-Layer and its about 100 to 150 miles above the earth. At this altitude the ions and electrons recombine quickly and act somewhat like the D-Layer and after sunset this layer also disappears.
The next layers are the F-Layers and these are the ones we use for our long distance communications. As the day starts this layer starts to split into two layers the two layers are at their maximum at the zenith of the sun. At night the two layers, F1 and F2 recombine and become the F layer. In the winter times this F layer and its children F1 and F2 are lower in the ionosphere and will effect the radio waves.
