krisna dsk udayana 06 propagation mechanism

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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krisna dsk udayana 06 propagation mechanism

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