Increasing radio coverage range
Radio communications at frequencies above 30 Mhz normally travel between transmitter and receiver, in what is known as ‘line of sight’.
In other words they don’t bounce off the atmospheric layers, or the ground, as they can do below 30 Mhz.
You may have noticed that if you are listening to an old radio receiver that has Medium Wave (MW) & Long Wave (LW) wavebands, that you can hear ‘foreign’ radio stations at night.
This is because despite being called medium and long wave, both frequencies are below 30 Mhz. This means that the radio waves between transmitter and receiver can ‘bounce’, which increases range.
So why don’t we use frequencies below 30 Mhz to achieve long range?
Well for a start the antenna length would be too long.
This is because the the lower the frequency, the longer the antenna needs to be, to be ‘resonant’.
It would therefore be impractical for handheld or vehicle communications systems, to use low frequencies.
Why do ‘Line of Sight’ communications only go a short distance?
The simple answer is that the earth is round, and there are objects such as buildings and hill in the way.
If two people were talking on handheld ‘walkie talkies’ and started walking away from each other, gradually the curvature of the earth would come between them, and stop communication.
Buildings and other objects can also either reduce or prevent radio signals from getting between the transmitter and a receiver.
Therefore to overcome the reduction or complete blocking of the radio signal, antennas are often put on high masts, or high buildings.
This increase in height helps overcome the curvature of the earth, and also objects in the line of sight, between the transmitter and the receiver.
This principle is used effectively by space satellites.
Space Satellites receive a radio signal from a Satellite ground station, and rebroadcast the transmission back down to earth over a large coverage area, known as the satellites ‘footprint’.