uhf and microwave propagation [read-only]

UHF AND MICROWAVE PROPAGATION

Overview

� This chapter deals the present knowledge and practice in terrestrial propagation of UHF and Microwave

� So this presentation will mainly focus on � So this presentation will mainly focus on following topics� Means of Propagation of Radio Waves

� Estimation of Path Loss during propagation

� Evaluation of Station Performance

� Ideas about making best use of a mode of propagation

History

� Experimentations with UHF and Microwave frequencies have relatively recent history.

� Before 1941 knowledge as well as work in this field was pretty much crippled.was pretty much crippled.

� There was not even clear definition of Ultra High Frequency

� This field took hike after ban on UHF frequencies was lifted post World War II. This opened a new frontier for enthusiastic scientists and amateurs.

History(cont...)

� With the following years distance records were broken one after another along with the discovery of new UHF and Microwave frequency bands for communication.

� Thus the continuous work and dedication in this field has also yielded new concepts in propagation of radio frequencies such as Trophospheric ducting, Aurora, Trasnequatorial spread-F, etc.

Line of Sight

� Like other form of electromagnetic radiation, radio waves also travels in straight line in free space during propagation.

� Visual line(d1) of sight is the distance between � Visual line(d1) of sight is the distance between transmitting antenna to the horizon.

d1=√(12.7*h)h is elevation of the observer

Line of Sight(cont..)

� Due to atmospheric refraction, a correction factor is required for this distance which is given as

dr=√(12.7*hs*k)

dr is distance of radio horizon(in km)dr is distance of radio horizon(in km)

hs is elevation of station above sea level(in meter)

k is effective radius factor

Free Space and Atmospheric Losses

� Radio frequency undergo attenuation when it propagates through vacuum or Earth’s atmosphere.

� Uhf and Microwave communication is further limited by attenuation due to atmospheric gases, hail, snow, by attenuation due to atmospheric gases, hail, snow, rain, etc.

� Path-loss due to free space is given as the ratio of transmitter power to receiver power

( )2

4300rP

fdtP π=

Free Space and Atmospheric Losses(cont…)

� Path-loss expressed in dB as

Lfs=32.45+20 log(d)+20 log(f)

Lfs is path-loss in dB

d is distance in kmd is distance in km

f is frequency of radio wave

Free Space and Atmospheric Losses(cont…)

� Apart from free space losses, attenuation by different other natural factors should be considered such as� Losses from atmospheric gasesLosses from atmospheric gases

� Rain Attenuation

� Snow and Hail

� Fog and cloud

Total System Performance

� Total system performance is the combined evaluation of transmitting and receiving sation.

� Total system performance(Ps) is given asPs=Perp-PnPerp is transmitter station performance(in dBW)Perp is transmitter station performance(in dBW)Pn is receiver system noise power(in dBW)

� Signal to noise ratio for the system is given asS=Ps-PlPI is total path loss(dB)

� Transmitter Effective Radiated Power(Perp) is given by equation

Total System Performance(cont…)

Perp=Pt + Gta - LtfGta is transmitter antenna gain(in dBi)Pt is transmitter power(in dBW) Ltf is transmitter feed-line loss(in dBW)

Likewise, receiver system noise power(P ) is given as� Likewise, receiver system noise power(Pn) is given asPn=10 log(kBTs) + Grak is Boltzmann’s constantB is BandwidthGra is antenna gain(in dB)Ts is receiver system noise temperature

Modes of Propagation

1. Troposheric Refraction

2. Troposheric Scattering

3. Ionospheric Modes

4. Reflection4. Reflection

5. Knife-edge Diffracation

6. Absorption

Troposheric Refraction

� Lower 10km of the atmosphere is known as troposphere.

� Tropospheric refraction is most commonly and widely used for propagation of radio signal over long distance.long distance.

� This propagation mode is based on the fact that radio waves like other electromagnetic waves are also refracted as it passes through the troposphere.

� Troposphere being a non-homogeneous medium supports refraction.

Troposheric Refraction(cont…)

� INDEX OF REFRACTION� Index of refraction(N) of air can be calculated as

P is atmospheric pressure, millibars(mb)2

510*73.36.77

Te

TpN +=

P is atmospheric pressure, millibars(mb)

e is partial pressure of water vapour(mb)T is temperature, kelvins


� Super-refraction or Ducting� It is an interesting phenomenon that tends to happen during periods of stable, anticyclonic weather.

� This duct almost acts like a wave-guide, so once the radio wave enters the ducting region it is propagated to the other wave enters the ducting region it is propagated to the other end with often much less attenuation that a direct signal would be between the same points.

� Ducting occurs when angle with which radio wave enters the duct is equivalent to or greater than the curvature of.

� Alternately if rate of change of index of refraction of air with altitude(dN/dm) is greater than 0.157 ducting occurs.


� During this phenomenon radio waves entering duct are refracted enough to bring them back to earth surface.

� During super-refraction radio horizon theoretically extends to infinity.

Figure:-Ducting phenomenon


� Temperature Inversion� Temperature inversion frequently occur in trposphere.� The are so called because, in contrast to usual lapse of temperature with increase in altitude, temperature rises with altitude.with altitude.

� These inversion aid the uhf and microwave propagation as this inversion sharpens the vertical boundary of troposphere. Thus the amount of refraction becomes so severe that signals extend a great distance as though caught in a duct.

� Lowest frequency which is super-refracted is determined by the depth of inversion.


� Tropospheric inversion can be classified on the basis of how they are created, such as1. Radiation Inversion2. Subsidence Inversion3. Advective Inverison3. Advective Inverison4. Evaporation Inversion

Tropospheric Scatter

� This mode of propagation is particularly used for the long distance propagation of radio frequency(practically upto 300km).

� This mode of propagation uses the tropospheric� This mode of propagation uses the troposphericscatter phenomenon, where radio waves at particular frequencies are randomly scattered as they pass through the upper layers of the troposphere.

� This mode is very useful when the transmitting station spread beyond horizon.

Tropospheric Scatter(cont…)

� If antennas are pointed along the great circle, their beam will intersect a common volume of air which lie almost at the centre of the path.

� Part of the signal common to both station will � Part of the signal common to both station will undergo tropospheric scatter aiding the radio communiction.

� But frequency is the limiting case for this communication, only the frequency up to 2GHz can be transmitted through tropospheric scatter.


� Scattering Loss� Scattering loss in this propagation can be assessed as

Lc is aperture-to-medium coupling loss(dB)f is frequency(MHz)øs is scattering angleLs is scattering angle(dB)

css Llog(f) 101021L +++= φ


� Naturally occurring factors which contribute to the scattering are� Rain Scatter

� Snow, Hail, and Particle Scatter� Snow, Hail, and Particle Scatter

� Lighting Scatter

Ionospheric Modes

� The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km to more than 1000 km.

� Ionospheric mode of propagation relies on refraction of radio waves in the ionosphere.

� F2-layer is the most important ionospheric layer for HF propagation, though F1, E, and D-layers also play some role.

Ionospheric Modes(cont…)

� Highest usable frequency for F-layer is about 70MHz, while for Sporadic-E maximum of 220MHz of frequency has been use.

� Phenomenon like Aurora, meteor scatter, and � Phenomenon like Aurora, meteor scatter, and transequatorial spread-F have been observed at 432MHz.


� Aurora� Scattering from Aurora borealis, has been widely used for many years.

� By pointing the directional antenna towards the centre of aurora, very long towards the centre of aurora, very long distance transmission on radio frequency is carried out.

� Contacts over 2000km have been made of 144MHz and over 1900km on 432MHz.

� Aurora is closely related to solar activity.


� During massive solar storms, high energy stream of electron and proton spew out of sun. Earth’s atmosphere trap these ions in northern and southern magnetic polar region.

� Thus the trapping of ions increase the density of the E-� Thus the trapping of ions increase the density of the E-layer dramatically, which is sufficient to scatter the radio frequency.

� This phenomenon is restricted to E-layer only.


� Transequatorial Spread-F� First discovered in 1947, transequatorial spread-f makes possible contacts up to 8000 km across equator in UHF and VHF range, at least up to 432MHz.

� Station engaged in this mode of propagation must be roughly equidistance from the equator.

� Physical mechanism of TE Spread are not well-known but this phenomena like other ionospheric mode is linked with the solar activity.


� Meteor Scatter� Meteor scattering relies on reflecting radio waves off the intensely ionized columns of air generated by meteors.

� While this mode is very short duration, often only from � While this mode is very short duration, often only from a fraction of second to couple of seconds per event.

� This mode allows remote stations to communicate to a station that may be hundreds of miles up to over 1,000 miles (1,600 km) away, without the expense required for a satellite link.

� Meteor scatter is generally useful on VHF frequencies between 30 and 250MHz .

Reflection

� Rigid objects like buildings, towers, and airplane are large enough can act as the reflector for UHF and microwaves.

� Contacts in the path ranging from 300 to 700 km is reported from this mode.reported from this mode.

� In this mode, two station point their antenna towards the common reflector. Reflecting objects must be in line-of-sight to both the stations.

� Appreciable loss is experienced in this mode of propagation for higher frequency(above 10GHz)

Reflection(cont…)

� Radar EqutionThe radar equation may be applied to communications path completed by reflection

22

2

2r

)4(4P

rtt

R

SG

R

GP

πλ

π+=

Pt transmitted power(in Watt)

Pr received power(in Watt)

Gt transmitter antenna gain

Gr receiver antenna gain

Rt range from transmitter to reflector(in m)

Rr range from receiver to reflector(in m)

wavelength(in m)

S is radar cross section of reflector(in m)

222r

)4(4P

rt RR ππ+=

λ

Reflection(cont…)

� In terms of dB

Lr is total loss of reflection path(in dB)

153log1022

2

−

=

rtr

RR

SL

λ

Lr is total loss of reflection path(in dB)

Knife-Edge Diffraction

� Radio signal can also be diffracted over the peaks of well-defined ridges and mountains

� Though high losses accompany diffraction(even from 20dB to 50dB), path is completed otherwise not possible.possible.

� Signal level decrease rapidly as the angle of diffraction increases.

� In order to minimize the loss in transmission, diffracting ridge should have sharp profile i.e. free form trees and other absorbents.

� Losses also decrease as the elevation of the knife-edge reduces


� Attenuation over knife-edge diffraction in terms of dB is

8.38log201

−

=

d

fhLk

Lk is knife edge diffraction loss(in dB)h is elevation of knife edge relative to the two stations(in m)f is frequency(in MHz)d1is distance from the near station to knife edge(in km)

Fresnel Zone

� A Fresnel zone is one of a theoretically infinite number of a concentric ellipsoids of revolution which define volumes in the radiation pattern of a usually circular aperture.

� Fresnel zones result from diffraction by the circular aperture.

Fresnel Zone(cont…)

� Normally radio waves will travel in a straight line from the transmitter to the receiver.

� But if there are obstacles near the path, the radio waves reflecting off of those objects may arrive out of phase with the signals that travel directly and of phase with the signals that travel directly and reduce the power of the received signal (Interferences), this effect is a reason of “Fading”.

� On the other hand, the reflection can enhance the power of the received signal if the reflection and the direct signals arrive in phase.


� Calculation of Fresnel Zone

Fn is Nth Fresnel zone outer boundary from direct path(in m)

fd

ddNFn

)(31.17

21=

N is any positive number

f frequency(MHz)d1 is distance from one station to some point in

path(in m)d2 is distance from second station to same point in

path(in m)d is total path distance(in m)


� It is very useful in determining how close to a line-of-sight path a potential diffracting object may lie without causing unacceptable loss.

� It is found that greatest Fresnel-zone losses occur when diffracting object lie within first 0.6 Fresnel Zone.diffracting object lie within first 0.6 Fresnel Zone.

� Even the small objects, close to either of the station can act as diffractor and adversely affect the signal.

� Level of attenuation due to an object within Fresnel zone is also affected by the shape and size of the object.

Absorption

� Low-frequency radio waves travel easily through brick and stone and VLF even penetrates sea-water.

� At microwave or higher frequencies, absorption by molecular resonance in the atmosphere (mostly molecular resonance in the atmosphere (mostly water, H2O and oxygen, O2) is a major factor in radio propagation.

� Though attenuation due to absorption increase with frequency, precise attenuation is difficult to calculate.

Summary

Band Frequency Wavelength Propagation via

VLF Very Low Frequency 3–30 kHz 100–10 km Guided between the earth and the ionosphere.

LF Low Frequency 30–300 kHz 10–1 kmGuided between the earth and the D layer of the ionosphere. Surface waves.

MF Medium Frequency 300–3000 kHz 1000–100 mSurface waves. E, F layer ionospheric refraction at night, when D layer absorption weakens.

HFHigh Frequency (Short Wave)

3–30 MHz 100–10 mE layer ionospheric refraction. F1, F2 layer ionospheric refraction.

VHF Very High Frequency 30–300 MHz 10–1 m

Infrequent E ionospheric refraction. Extremely rare F1, F2 layer ionospheric refraction during high sunspot activity up to 80 MHz. Generally direct wave. Sometimes tropospheric ducting.

UHF Ultra High Frequency 300–3000 MHz 100–10 cm Direct wave. Sometimes tropospheric ducting.

SHF Super High Frequency 3–30 GHz 10–1 cm Direct wave.

EHFExtremely High Frequency

30–300 GHz 10–1 mm Direct wave limited by absorption.

uhf and microwave propagation [read-only]

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