The Radio waves leave the antenna and travel skyward and they will reach the first layer, the D-Layer at low frequencies this layer seems to want to dampen the lower frequencies and these wave cannot reach the other higher layers. The radio waves collide with the free ions in doing so use up some of the energy of the radio wave.
As the radio frequency is increased the wave length become shorter and fewer collisions will take place and the radio wave will continue through this layer and on to the next layers. As the density is lower the higher we go the less ions and less collisions and these layers affect the radio wave in a different manner. It now tends to bend some of the radio wave back toward the earth. The angle that the radio wave is entering the layers is going to affect the angle that the radio wave is "reflected" back to the earth.
During the daylight hours the D-Layer will absorbs most signals that reach the ionosphere. As frequency increases the energy that is absorbs becomes less and signals are passed on through to the other layers. The E-Layer will reflect some of these radio waves and the signal will be reflected at a greater distance from the transmitter.
If the radio signal is increased further in frequency the E-Layer then becomes less of a absorber and the signal then passes on through to the F-Layers and there they are reflected back to earth and those higher frequencies are received at greater distances from the transmitter.
As we increase the frequency of the radio wave, the F-Layers will have less effect on the radio wave and those waves will pass through the FLayers and continue on into space. As in the sketch above we find that the radio wave using the E-Layer will travel about 2000 Km (or about 1300 miles) and the F-Layer to about 4000 Km (or about 2500 miles) The Radio waves also leave the antenna and travel skyward and they will reach the first layer, the D-Layer at low frequencies this layer seem to want to dampen the lower frequencies and thus these wave cannot reach the other higher layers. The radio waves collide with the free ions in doing so use up some of the energy of the radio wave.
As the radio frequency is increased the wave length become shorter and fewer collisions will take place and the radio wave will continue through this layer and on to the next layers. As the density is lower the higher we go the less ions and thus less collisions and these layers affect the radio wave in a different manner. It now tends to bend some of the radio wave back toward the earth. The angle that the radio wave is entering the layers is going to affect the angle that the radio wave is "reflected" back to the earth. These various layers will change as the sun moves overhead and will cause changes in the distance we can communicate. There is no guarantee that the radio waves will behave in the same manner on each day. In order to be sure that we can communicate with some reliability we can consult various document that will predict with some accuracy what will happen to the radio waves at a certain frequency and time of day.
One way we can check on these predictions is to tune to Radio Station WWV which is the product of National Bureau of Standards in Bolder, CO., and WWVH in Hawaii. These stations will transmit the solar terrestrial indices which is the indication of activities of the solar conditions. This station transmits on 2.5, 5.0, 10.0, 15.0,20.0 and 25.0 MHz. They also provide a 'time tick" to accurately set the ships clocks.
The propagation of the radio waves on the Maritime frequencies will vary depending on time of day, season, sunspot activity, etc. In general the higher frequencies are best for mid day communications over a long range, about 800 miles. Lower HF bands are the best choice for nighttime communications over the same range. Medium frequency bands are best for mid day communications over short ranges, 50 miles or so.
The best way to find the 'right' frequency is to listen to the station you want to engage in communications and if you can hear them you should be able to communicate
The antenna serves a dual purpose. It acts as a radiator of RF energy in transmit and efficient collector of weak RF energy in receive mode. To do this efficiently the size or length ofthe antenna is related to the frequency. The physical length is cut or tuned to a specific frequency in the band you will be using. The Federal Communications Commission require that all antennas on Maritime ships be in the Vertical plane. On the VHF frequencies the wave length is about two meters long and we will use a onehalf wave for the actual length of the antenna. This is about 15 or so inches for the VHF channels.
On the HF radio waves the wave length is from 160 meters to 10 meters. It would a real problem to have a vertical antenna 160 meters in length (about 450 feet high). So we use a specific length that we can put on the wheel house and still not have it hit the bridges we pass under. To this shortened vertical antenna we will have to fool the transmitter and coil up some of the length and put it in a box. This box we call an "Antenna Coupler" and this will let us to electrically make the antenna longer or shorter according to the frequency band we want to operate. Most radios have a place in the program to "tune" the transmitter, not really tuning the transmitter but adjusting the Antenna Coupler and thus making the antenna electrically longer or shorter.
