1Propagation Mechanisms

2 Reflection Propagating wave impinges on an object which is

large compared to wavelength E.g., the surface of the Earth, buildings, walls, etc. Diffraction Radio path between transmitter and receiver

obstructed by surface with sharp irregular edges Waves bend around the obstacle, even when LOS

does not exist Scattering Objects smaller than the wavelength of the

propagating wave E.g., foliage, street signs, lamp posts

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5First, what do we mean by propagation in free space?

Free space is a region where these is nothing - the vacuum of outer space is a fair approximation for most purposes. There are no obstacles to get in the way, no gases to absorb energy, nothing to scatter the radio waves. Unless you are into space communications, free space is not something you are likely to encounter, but it is important to understand what happens to a radio wave when there is nothing to disturb it.

In free space, a radio wave launched from a point in any given direction will propagate outwards from that point at the speed of light. The energy, carried by photons, will travel in a straight line, as there is nothing to prevent them doing so. They will do this forever. Actually, this is not quite true, photons do eventually decay but as the half life of a photon is of the order of 6.5 Billion years, we don't need to worry about it here. For all practical purposes, a radio wave when launched carries on in a straight line forever traveling at the speed of light.

6As the energy in a radio wave goes on propagating forever

without loss, why do people talk about "free space loss" ?

Here are Some useful equations: Free Space Loss = 32.5 + 20log(d) + 20log(f) dB,

Where D is the distance in km and f is the frequency in MHz

Free Space Loss = 92.5 + 20log(d) + 20log(f) dB, Where D is the distance in km and f is the frequency in GHz

and for the Americans: Free Space Loss = 36.6 + 20log(d) + 20log(f) dB,

where D is the distance in miles and f is the frequency in MHz

7The Regions of the Ionosphere

In a region extending from a height of about 50 km to over 500 km, some of the molecules of the atmosphere are ionised by radiation from the Sun to produce an ionised gas. This region is called the ionosphere, figure 1.1.

During the day there may be four regions present called the D, E, F1 and F2 regions. Their approximate height ranges are: D region 50 to 90 km; E region 90 to 140 km; F1 region 140 to 210 km; F2 region over 210 km.

8Figure 1.1 Day and night structure of the ionosphere

9During the daytime, sporadic E is sometimes observed in the E region, and at certain times during the solar cycle the F1 region may not be distinct from the F2 region but merge to form an F region. At night the D, E and F1 regions become very much depleted of free electrons, leaving only the F2 region available for communications; however it is not uncommon for sporadic E to occur at night.

Only the E, F1, sporadic E when present, and F2 regions refract HF waves. The D region is important though, because while it does not refract HF radio waves, it does absorb or attenuate them. The F2 region is the most important region for high frequency radio propagation as: it is present 24 hours of the day; its high altitude allows the longest communication paths; it usually refracts the highest frequencies in the HF range.

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As signals spread out from a radiating source, the energy is spread out over a larger surface area. As this occurs, the strength of that signal gets weaker. Free space loss (FSL), measured in dB, specifies how much the signal has weakened over a given distance.

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Radio waves travel in a straight line, unless something refracts or reflects them. But the energy of radio waves is not pencil thin. They spread out the farther they get from the radiating source like ripples from a rock thrown into a pond.

The area that the signal spreads out into is called the Fresnel zone (pronounced fra-nell). If there is an obstacle in the Fresnel zone, part of the radio signal will be diffracted or bent away from the straight-line path. The practical effect is that on a point-to-point radio link, this refraction will reduce the amount of RF energy reaching the receive antenna.

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Receive Signal Level Receive signal level is the actual received signal level

(usually measured in negative dBm) presented to the antenna port of a radio receiver from a remote transmitter.

Receiver Sensitivity Receiver sensitivity is the weakest RF signal level

(usually measured in negative dBm) that a radio needs receive in order to demodulate and decode a packet of data without errors.

Antenna Gain Antenna gain is the ratio of how much an antenna boosts

the RF signal over a specified low-gain radiator. Antennas achieve gain simply by focusing RF energy.

If this gain is compared with an isotropic (no gain) radiator, it is measured in dBi. If the gain is measured against a standard dipole antenna, it is measured in dBd. Note that gain applies to both transmit and receive signals.

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Transmit Power The transmit power is the RF power coming out

of the antenna port of a transmitter. It is measured in dBm, Watts or milliWatts and does not include the signal loss of the coax cable or the gain of the antenna.

Effective Isotropic Radiated Power Effective isotropic radiated power (EIRP) is the

actual RF power as measured in the main lobe (or focal point) of an antenna. It is equal to the sum of the transmit power into the antenna (in dBm) added to the dBi gain of the antenna. Since it is a power level, the result is measured in dBm.

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Figure 3 shows how +24 dBm of power (250 mW) can be boosted to +48 dBm or 64 Watts of radiated power.