Radio Communications

Multipath propagation

Multipath radio signal propagation occurs on all terrestrial radio links. The signals spread out from the transmitter over a range of angles and will encounter all sorts of object: hills, mountains, buildings reflective surfaces such as the ground, water, etc. The signals will reflect off all of these, and reach the receiver via paths other than the direct line of sight - which is often the main signal that is wanted.

Additionally, other effects such as ionospheric reflection give rise to multipath propagation. These multiple signals can give rise to interference in a variety of ways including distortion of the signal, data loss and multipath fading. (The variety of signal paths arising from the multipath propagation can also be used to advantage, to cleverly increase the capacity of the channels they use.)

Jargon

• Antenna = arial
• Signal = information (i.e. sound, image, or text) transmitted or received via electromagnetic waves in radio, television, radar, telephony, or telegraphy.
• Terrestrial radio/TV = radio/TV signals received through a conventional aerial, as opposed to a satellite dish.
• Transmission medium = any material substance which can propagate waves or energy
• Attenuation = the gradual loss in intensity of a signal through a medium.
• Ground wave = either a surface wave or a space wave.
• RADAR = RAdio Detection And Ranging
• Cable = e.g. twisted pair wires, coaxial cable, optical fibre.
• Waveguide = a structure which guides waves (see diagram). The most common meaning is a hollow conductive metal pipe used to carry microwaves.
• Radio ham = amateur radio operator.
• The ionosphere is an extended region of the upper atmosphere in which neutral air atoms are ionized by cosmic rays and photons from the sun. Here, electrons and ions float freely. The ionosphere is not uniform, and there are layers in which the free electron density is greater than other parts of the ionosphere. The ionosphere is able to refract electromagnetic waves, the amount of refraction depending on the free electron density and the signal frequency.

Transmission paths

Radio waves are classified according to the paths they take from transmitter to receiver. They can be:

• Surface waves. The portion of the wavefront propagating close to the Earth's surface undergoes diffraction at the surface and so follows the curvature of the Earth, allowing radio coverage well beyond the visual horizon. Beyond the horizon, the signal is purely made up from the diffracted surface wave. Part of the process is currents being induced in the surface of the earth, which slows down the wave-front at the surface. The degree of attenuation of the wave is dependent upon factors such as:
  
  
  • The nature of the surface. (A smooth sea surface is best).
    
  • The signal frequency: losses rise with increasing frequency. (Surface wave transmission is impracticable above the MF portion of the radio spectrum).
  • Antenna type and orientation. Vertically polarised signals are subject to much less attenuation than horizontally polarised ones. This is why MW & LW broadcasters use vertical antennae.
    
    
  Surface waves are very stable and are the way we receive medium wave signals during the daytime.

Japanese antenna experts Yagi and Uda discovered that by adding elements of various lengths and spacings in front of and behind a dipole antenna that the performance and effectiveness of the dipole could be greatly enhanced and the pattern of the dipole radio frequency energy could be focused in one direction and that the antenna could be tuned to get various results. The dipole driven element, which receives the power from the transmitter, will be "resonant" when its electrical length is 1/2 of the wavelength of the frequency applied to its feed point. The reflector is the element that is placed at the rear of the driven element. Its length is about 5% longer than the driven element and its resonant frequency is slightly less. The directors are the shortest of the elements and this end of the antenna is aimed at the receiving station. Their length will be about 5% shorter than that of the driven element and their resonant frequencies are slightly greater. The directors are used to provide a directional pattern and gain.

• Space waves
  
  

  Direct (line-of-sight) signal (actually these signals can undergo a certain amount of refraction in the lower atmosphere i.e. troposphere.) Satellites receive and transmit direct signals.	Reflected signal (reflection off objects such as Earth's surface, hills, large buildings). In MF and lower HF bands, the direct wave and reflected wave tend to cancel each other out, bearing in mind there is a 180 degree phase shift on reflection.

  
  

Sky Waves (Ionospheric Waves)

Skywaves are radio waves that have at some stage on their journey to a receiver have been totally internally reflected within the ionosphere back to the Earth's surface.

As a radio signal penetrates deeper and deeper into one of the ionospheric layers, it encounters a gradually increasing free electron (and ion) density within that layer. This corresponds to a decrease in 'optical density' (more specifically a decrease of refractive index), so the signal travels faster. As this wave travels on upwards - at an oblique angle of incidence - through this decreasing refractive index, it gradually refracts more and more away from the normal (following a curved path - unlike in the diagram!). Depending upon the wave frequency, the free electron density and the original angle of incidence, the wave will either eventually undergo total internal reflection and start heading back to Earth, or it will exit the topside of the ionosphere, insufficient refraction having taken place for total internal reflection. Signals with frequencies above about 30 MHz (VHF and beyond) usually penetrate the ionosphere whatever the angle of incidence and are thus not normally returned to the Earth. Thus skywaves operate at frequencies up to about 30 MHz - though they are most useful in the range 1-30 MHz. (At about 60 MHz the amount of refraction becomes negligible.)

Now when the total internally reflected radio signal reaches the Earth's surface (ground or water), the latter reflects it back upwards towards the ionosphere. As a result, like a rock "skipping" across water, the wave may actually "bounce" or "skip" between the earth and ionosphere two or more times (multihop propagation). The ground and ionosphere together act rather like a "waveguide" (see diagram of waveguide) in propagating the skywave. Since at shallow incidence losses remain quite small, signals of only a few watts can sometimes be received many thousands of miles away. With a single "hop," path distances up to 3500 km may be attained. Transatlantic connections are mostly achieved with two or three hops.

For the lower frequencies, signals that are sent vertically upwards will return to the transmitter.

Skywave propagation on the sunlit side of the Earth can be badly disrupted during sudden ionospheric disturbances, which are casued by geomagnetic storms, solar flares, sunspots, etc.

Frequencies below about 10 MHz (wavelengths longer than 30 metres), including broadcasts in the mediumwave and shortwave bands (and to some extent longwave), propagate most efficiently by skywave at night. Frequencies above 10 MHz typically propagate most efficiently during the day.

Because the lower-altitude layers of the ionosphere largely disappear at night, the refractive layer of the ionosphere is much higher above the surface of the Earth at night. This leads to an increase in the "skip" or "hop" distance of the skywave at night.

Because of the losses in both the refractive and reflective processes, the less hops that you can use to get to your destination the better. To get less hops, you need a lower angle of incidence at the ionosphere, making each hop longer.

There is sometimes a dead zone where the surface wave has run out and the ionisation levels and frequency in use will not allow the wave to return to earth. On a band like 10m this may mean that communications over relatively short ranges is virtually impossible. For example it will be rare to hear signals on 10m from stations between 20km away (where the surface wave runs out) and 1500km away where the first ground reflection occurs.

Skip Zone (Quiet Zone): The space between the point where the ground wave is completely dissipated and the point where the sky wave is received.

AM versus FM radio

Bandwidth limitations

Each radio, TV or other communication requires a certain bandwidth, and this takes up a portion of the e-m spectrum. E.g. the maximum number of FM channels that can be fitted into the allotted range of frequencies for FM is 100. Researchers are exploring the possibility of making more use of infrared carrier frequencies.

DAB (Digital Audio Broadcasting)

With DAB, a digital signal is superimposed on a carrier wave.