High Frequency Radio Telephony

HF (High Frequency) telephony in aviation refers to the use of HF radio frequencies for voice communication between aircraft and ground stations over long distances. HF telephony enables reliable communication in remote areas or over oceans where other forms of communication, such as VHF (Very High Frequency), may not be effective. HF Radio communication uses sky waves for its propagation.

Maximum Usable Frequency (MUF) for HF Transmission

We are now aware that, for the same angle of transmission, the sky waves return from a greater height with increase in frequency. Beyond a certain frequency, the sky waves would not return to earth, but would escape from the atmosphere. Maximum usable frequency (MUF) is the name given to the highest usable frequency below HF bands. Frequencies higher than MUF will form escape rays and would not serve our purpose.

Transmission characteristics of Maximum Usable Frequency (MUF)

Maximum Usable Frequency (MUF) travels the shortest path through the ionosphere. Therefore, at MUF we can expect the minimum ionospheric attenuation and static interference. High ionisation levels at noon time results in higher MUF. Although transmission at MUF appears to be the best option, we should cater for fluctuations in the ionosphere.

Transmission characteristics of Optimum Working Frequency (OWF)

Optimum working frequency is 0.85 times the Maximum Usable Frequency (MUF). Optimum Working Frequency (OWF) ensures Maximum Usable Frequency (MUF) is not exceeded due to ionospheric fluctuations and avoid the transmission becoming escape rays. That is the reason HF transmission frequencies are halved at night due to low ionisation levels. If you look at the aerodrome information, there are two frequencies given for Day and Night HF Transmission. The Night frequency is approximately half the day time frequency.

Fading of Radio Waves

Increasing and decreasing signal quality in received signal is called Fading. This is also called as waxing and waning of signals. Fading is caused due to weak signal strength. Higher distances, low power, weather and terrain reduce signal strength.

MultiPath Fading of Radio Waves

When the same radio wave signal is received both as a sky and surface wave in the receiver they tend to interfere with each other causing MultiPath Fading. When sky waves travel to the ionosphere and return back to the earth its phase changes from the originally transmitted signal. These out of phase signals cause interference. In other words, interference is a result of out of phase reception of surface and sky waves of the same signal. One of the ways to overcome Multipath fading is by using different frequencies for transmission and reception. The other way is to create space diversity, by using two or more antenna for reception.