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8/10/2019 Krisna DSK Udayana 06 Propagation Mechanism
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Propagating wave impinges on an object which is
large compared to wavelength
.g., e sur ace o e ar , u ngs, wa s, e c.
Diffraction
Radio path between transmitter and receiver
obstructed by surface with sharp irregular edges
Waves bend around the obstacle, even when LOS
Scattering
Objects smaller than the wavelength of the
propaga ng waveE.g., foliage, street signs, lamp posts
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Free space is a region where these is nothing - thevacuum of outer space is a fair approximation for most
ur oses. There are no obstacles to et in the wa no
gases to absorb energy, nothing to scatter the radiowaves. Unless you are into space communications, freespace is not something you are likely to encounter, but
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. ,
photons, will travel in a straight line, as there isnothing to prevent them doing so. They will do thisforever. Actually, this is not quite true, photons do
even ua y ecay u as e a e o a p o on s o eorder of 6.5 Billion years, we don't need to worry aboutit here. For all practical purposes, a radio wave whenlaunched carries on in a straight line forever travelinga e spee o g .
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Here are Some useful e uations: Free Space Loss = 32.5 + 20log(d) + 20log(f) dB,
Where D is the distance in km and fis the frequency inMHz
ree pace oss . og og ,Where D is the distance in km and fis the frequency in
GHz Free Space Loss = 36.6 + 20log(d) + 20log(f) dB,
where D is the distance in miles and fis the frequencyin MHz
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In a region extending from a height of about 50km to over 500 km, some of the molecules of
the Sun to produce an ionised gas. This regionis called the ionosphere, figure 1.1.
present called the D, E, F1 and F2 regions. Their
approximate height ranges are: reg on o m;
E region 90 to 140 km; F1 region 140 to 210 km;
reg on over m.
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8Figure 1.1 Day and night structure of the ionosphere
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During the daytime, sporadic E is sometimes observed in
the E re ion, and at certain times durin the solar c cle theF1 region may not be distinct from the F2 region but merge
to form an F region. At night the D, E and F1 regions
,
the F2 region available for communications; however it isnot 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
it is present 24 hours of the day;
its high altitude allows the longest communication paths;
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usua y re rac s e g es requenc es n e range.
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As si nals s read out from a radiatin source the ener isspread out over a larger surface area. As this occurs, the
strength of that signal gets weaker. Free space loss (FSL),
,
weakened over a given distance.
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Radio waves travel in a strai ht line, unlesssomething refracts or reflects them. But the energy
of radio waves is not pencil thin. They spread out
e ar er ey ge rom e ra a ng source e
ripples from a rock thrown into a pond.
The area that the signal spreads out into is called
the Fresnel zone ronounced fra-nell . If there isan 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 apoint-to-point radio link, this refraction will reduce
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antenna.
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Receive Signal Level Receive signal level is the actual received signal level
antenna port of a radio receiver from a remotetransmitter.
Receiver Sensitivit
Receiver sensitivity is the weakest RF signal level(usually measured in negative dBm) that a radio needsreceive in order to demodulate and decode a packet of
ata w t out errors. Antenna Gain
Antenna gain is the ratio of how much an antenna- .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 ain is measured
against a standard dipole antenna, it is measured indBd. Note that gain applies to both transmit and receivesignals.
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Transmit Power
of the antenna port of a transmitter. It ismeasured in dBm, Watts or milliWatts and does
not include the signal loss of the coax cable orthe gain of the antenna.
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 thesum of the transmit power into the antenna (in
.Since it is a power level, the result is measuredin dBm.
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Figure 3 shows how +24 dBm of power (250 mW) can be boosted
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.