Radio Basics and Propagation

The radio spectrum is divided into named frequency bands. Each band has different practical propagation characteristics, so aviation chooses the band to suit the route, service and required range.

Ground, sky and space waves

A VHF tower may be easy to hear at cruise level yet disappear behind a ridge during low-level flight. That is not normally a transmitter failure. It is the expected geometry of a space-wave system.

HF is different. A signal can travel upward to the ionosphere and return to Earth a great distance away. This makes it useful where VHF stations cannot provide continuous coverage, especially over oceanic or remote routes.

HF range depends on the ionosphere, a region of the upper atmosphere whose gases are ionised by solar radiation. Its condition changes with sunlight, season, location and solar activity.

Ionospheric layers

Critical frequency is the highest frequency returned vertically by an ionospheric layer. MUF, maximum usable frequency, is the highest frequency likely to support a particular path at a particular time. LUF, lowest usable frequency, is the lowest frequency that can be used successfully once absorption and noise are considered. The optimum working frequency is a practical choice between LUF and MUF, selected for reliable communication rather than merely possible communication.

Why HF frequencies change with time

During daylight, stronger ionisation generally supports a higher usable HF frequency family. At night, the D layer disappears, absorption reduces, and the usable family normally shifts lower. Oceanic operators therefore plan day and night frequencies rather than assuming one HF channel will work throughout the flight.

Skip distance and skip zone

Skip distance is the distance from the transmitter to the point where the first refracted sky wave returns to Earth. The area between the end of useful ground-wave coverage and that first sky-wave return is the skip zone, also called dead space. An aircraft in that zone may not hear the station on that selected frequency even though aircraft nearer to, and farther from, the station can hear it.

Fading

Fading is unwanted variation in received signal strength. It can result from multiple paths arriving together, such as ground and sky waves, changing ionospheric path length, and changes in polarisation. Two waves can add together or partly cancel each other, so speech may rise and fall without either station changing power.

Automatic volume control, also called AVC or AGC, reduces the effect of large signal-strength changes. Diversity reception uses more than one aerial, receiver or path so that a fade affecting one signal does not necessarily remove all usable reception.

Radio range is not a fixed number printed on a transmitter. It is the result of geometry, equipment, terrain, atmosphere and the radio environment at that moment.

Line-of-sight range

A useful first estimate for VHF line-of-sight range is:

At 10,000 ft, the estimate is 1.25 × √10,000. The square root of 10,000 is 100, so the approximate range is 125 NM. This is an estimate, not a guarantee. It assumes an unobstructed path and does not account for the height of the ground aerial.

Ground shadow and terrain screening

Hills, buildings, terrain folds and the curvature of the Earth can block VHF and UHF space waves. The region behind an obstruction is a ground shadow. It is especially relevant to aircraft on the ground and at low level, where the aerial cannot see the transmitting station. Climbing often restores contact because the direct path clears the terrain.

Real range limiters

Frequency interference is therefore a range limitation even when the wanted station is technically within line-of-sight range. A strong interfering transmission can make a weaker operational call unusable.

Good radio communication depends on the complete system: microphone, headset, receiver controls, transmitter tuning, aerials and the crew’s method of use.

Microphones, headphones and controls

Use press-to-talk deliberately. Check the intended frequency before transmitting, press before speaking, pause briefly for the transmitter to key, speak clearly, then release promptly. Holding the switch while not speaking blocks the channel.

Simplex, duplex and planned frequencies

SELCAL, intercommunication and navigation aids

SELCAL uses a four-letter tone code. It is checked before departure. On long HF sectors, it lets the crew silence the loudspeaker until the ground station calls by sending the assigned tone pair. The flight-deck interphone connects crew positions, the cabin interphone connects cabin and flight-deck crew, and the passenger-address system carries announcements to passengers.

Advantages and disadvantages of R/T