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

  • WaveguideAntenna = 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.

Layers of the ionosphere

 

Transmission paths

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

Ground waves { Surface waves - Waves that hug the ground as a result of diffraction.
Space waves - Waves that travel roughly in a straight line, though which may be reflected off the Earth's surface.
    Sky waves - Waves from a terrestrial transmitter that are totally internally reflected back to Earth from the ionosphere.

Wave transmission paths

  • 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.

BAsic antenna design
  • 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.

    radio horizon Space wave

Sky Waves (Ionospheric Waves)

Ionosphere reflectionSkywaves 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.)

sky waveNow 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.

Effect of the various ionospheric layers on different frequencies.

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.

atmospheric radiation window (Earthguide at Scripps Institute of Technology)

AM versus FM radio

AM FM
Noise Subject to elcectromagnetic interference and the effects of this are difficult to deal with. Much less susceptible to noise and gives far better audio fidelity.
Bandwidth LW & MW stations are each allotted a 9kHz channel bandwidth; each SW station is allotted 5kHz. Limited information can be transmitted. Much greater bandwidth (~200kHz) means more information can be transmitted, and so stereo is possible.
Wavelength effects  

Subject to

  • diffraction by smaller objects ~ a few metres in size
  • reflection, causing multipath interference.

but these are due to the wavelengths rather than the fact that the waves are frequency modulated.

 

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.

 

Radio Wave Propagation Mechanisms

Part
of
EM
spec
trum
Radio
Band
Approx
Freq &
Wave-
length
Comments
Transmission
Path

B
L
O
C
K

R
A
D
I
O

W
A
V
E
S

ELF
Extremely
Low
Frequency

100 Hz

5000 km

Communications to mines and to submarines down to several hundred metres.
(λ ~ size of Europe).

S
U
R
F
A
C
E

W
A
V
E
S

S
P
A
C
E

W
A
V
E
S

skywave at night icon

S
K
Y
W
A
V
E
S

O
N
L
Y

A
T

N
I
G
H
T

R
A
D
I
O

B
A
R
R
I
E
R

ULF
Ultra Low
Frequency

1 kHz

500 km

Communications through ground; comms to mines.
(λ ~ size of England)

VLF
Very Low
Frequency

10 kHz

50 km

Marine & military. Messages to submarines down to 20 metres.
(λ ~ size of Greater London)

LF
Low
Frequency

100 kHz

5 km (LW)

AM (Amplitude Modulation) radio; aeronautics navigation. Countrywide communications, so no car radio retuning needed on long trips. Interference between surface & sky waves produces fading. (λ ~ length of an hour's walk) . LW means Long Wave.

MF
Medium Frequency

1 MHz

500 m (MW)

AM radio (most heavily used band for commercial broadcasting). Local & continent-wide transmissions. Naval use.
(λ ~ size of a hill); MW means Medium Wave.

HF
High Frequency

10 MHz

50 m (SW)

AM radio. Radar. "Radio hams" (i.e. amateur radio operators). A very crowded band, used for commercial and military medium and long range (sometimes right round the Earth!) communication. Relatively low audio fidelity. The term "High Frequency" is misleading because HF radio waves are actually at a lower frequency than most radio and radar frequencies in use.
(λ ~ size of an olympic swimming pool). SW means Short Wave.

S
U
R
F
A
C
E

W
A
V
E
S

S
K
Y
W
A
V
E
S

VHF
Very High
Frequency

100 MHz

5 m

Stereo FM (Frequency Modulation) radio stations, e.g. BBC Radio 1. TV. Two-way radios. Radar. Communication with low earth orbit satellites (LEOs). Space wave is main propagation mechanism. Range limited to, say, 120 km, so car radios need retuning on long trips. (λ ~ size of a car).

S
P
A
C
E

W
A
V
E
S

R
A
D
I
O

W
I
N
D
O
W

M
I
C
R
O
W
A
V
E
S
UHF
Ultra High
Frequency

1 GHz

50 cm

DAB (Digital Audio Broadcasting). TV. Mobile phones. Police, military, two-way radios. Radar. Wireless protocols, e.g. WiFi, Bluetooth, DECT and GPS. Satellite communications (limited at present). Microwave ovens.
(λ ~ size of a car wheel)

SHF
Super High
Frequency

10 GHz

5 cm

Satellite communications. Radar.
(λ ~ size of a potato)
EHF
Extremely
High
Frequency

100 GHz

5 mm

Satellite communications. Radar.
Extremely prone to atmospheric attenuation.
(λ ~ size of a pea). This is sometimes called the "millimetre band".

I
N
F
R
A
R
E
D

W
A
V
E
S

Far
Infrared

2THz

150μm

Not used. Research into future use for fast-switching.
(λ ~ size of a pin point)

F    B
A    A
R    R
      R
I     I
R    E
      R

Mid
Infrared

10 THz

30μm

Remote controllers for TVs, DVD players and audio equipment. (Some PDAs)
(λ ~ size of a human cell)

P    I
A   R
R     
T   W
I     I
A    N
L    D
     O
     W

NIR
Near
Infrared

100 THz

3μm

Remote controllers for TVs, DVD players & audio equipment. (Some PDAs)
(λ ~ size of a bacterium)