Section 6 - Propagation

6.5 Range

Range is dependant on many factors.

1. Station power output; Obviously station power output is one factor, but not the most significant. Radio waves obey what is known as the "inverse square law". This means that every time the distance from the antenna is doubled, the signal strength is reduced by a factor of four. So, in order to increase range it would be necessary to significantly increase transmitter power.
2. Environment; As detailed previously, buildings and terrain significantly attenuate radio signals reducing the range of our transmissions. This is especially true at VHF and UHF where range will largely be "line of sight". Communication does not extend much beyond the horizon.
3. Antenna choice. If we focus our radio signals in one direction then our erp is significantly increased boosting the range of our transmissions. The antenna gain also works on receive allowing us to hear more distant signals too.
4. Antenna location; The higher our antenna is, the less the path between our antenna and that of the station we are in contact with will be blocked by obstacles. Furthermore, the horizon is more distant to a high antenna than a low one. This is especially important at VHF and UHF frequencies.
5. Frequency in use; The path loss is significantly dependant upon frequency. Radio waves of lower frequencies are attenuated less by obstacles, and depending on conditions, some of the lower frequency signals may travel beyond the horizon as will be shown next.

6.6-6.7 The Ionosphere

The ionosphere is the name of a set of layers of gas in the Earth's atmosphere between 70 and 400km above the surface. These layers become charged (ionised) by the radiation from the sun. This ionisation allows some of them to reflect radio waves back down to the Earth's surface. Because the level of ionisation is dependant on solar radiation, different conditions will be encountered at different times of the day, and also during the different seasons.

Not all radio waves will be reflected by the ionosphere. Only those below a certain frequency are affected, those above that frequency will pass straight through and go off into space. The maximum frequency which will bounce back of the ionosphere at a given time is known as the maximum useable frequency or MUF. This varies as the amount of ionisation varies, but it can be forecasted, and predictions are published in amateur radio magazines.

The effect of the waves bouncing off the ionosphere and returning to Earth allows a radio signal to travel well beyond the horizon. Sometimes it may be reflected back up again by the Earth and the cycle repeated, or it may bounce many times within the ionosphere. Single bounces of over 4000km are possible. This phenomenon is known as skip. As can be seen worldwide communication is possible because of this effect.

Ionospheric propagation is only common on the HF bands. It is rare for the MUF to extend to the VHF bands or beyond. There are however other propagation phenomenon that will allow range to be extended at even these frequencies. We shall discuss them further in the next levels of licence class.