1. Electromagnetic Wave Theory II
Lecture 8

2. Ground Wave Propagation
Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz
Example
AM radio

3. Disadvantages Ground Wave Propagation
.Requires relatively high transmission power
.They are limited to very low, low and medium frequencies which require large antennas
.Losses on the ground vary considerably with surface material

4. Advantages
Given enough power they can be used to communicate between any two points in the world
They are relatively unaffected by changing atmospheric conditions

5. Space wave propagation
This includes radiated energy that travels in the lower few miles of the earth’s atmosphere. They include both direct and ground reflected waves.
Direct waves travel in essentially a straight line between the transmitting and receiving antennas. The most common name is line of sight propagation.
The field intensity at the receiving antenna depends on the distance between the two antennas and whether the direct and ground reflected waves are in phase.

6. Line-of-Sight Propagation

7. Line-of-Sight Propagation
Transmitting and receiving antennas must be within line of sight
Satellite communication – signal above 30 MHz not reflected by ionosphere
Ground communication – antennas within effective line of site due to refraction
Refraction – bending of microwaves by the atmosphere
Velocity of electromagnetic wave is a function of the density of the medium
When wave changes medium, speed changes
Wave bends at the boundary between mediums

8. Line-of-Sight Equations
Optical line of sight
Effective, or radio, line of sight
d = distance between antenna and horizon (km)
h = antenna height (m)
K = adjustment factor to account for refraction, rule of thumb K = 4/3

9. Line-of-Sight Equations
Maximum distance between two antennas for LOS propagation:
h1 = height of antenna one
h2 = height of antenna two

10. LOS Wireless Transmission Impairments
Attenuation and attenuation distortion
Free space loss
Noise
Atmospheric absorption
Multipath
Refraction
Thermal noise

11. Sky Wave Propagation

12. Sky Wave Propagation
Signal reflected from ionized layer of atmosphere back down to earth
Signal can travel a number of hops, back and forth between ionosphere and earth’s surface
Reflection effect caused by refraction
Examples
Amateur radio
CB radio

13. Sky Wave Propagation
For many years, numerous organisations have been employing the High Frequency (HF) spectrum to communicate over long distances. It was recognised in the late 30's that these communication systems were subject to marked variations in performance, and it was hypothesised that most of these variations were directly related to changes in the ionosphere.

14. Sky Wave Propagation
Considerable effort was made to investigate ionospheric parameters and determine their effect on radio waves and the associated reliability of HF circuits. World-wide noise measurement records were started and steps were taken to record observed variations in signal amplitudes over various HF paths.

15. The results of this research established that ionised regions ranging from approximately 70 to 1000 km above the earth's surface provide the medium of transmission for electromagnetic energy in the HF spectrum (2 to 30 MHz) and that most variations in HF system performance are directly related to changes in these ionised regions. The ionisation is produced in a complex manner by the photoionization of the earth's high altitude atmosphere by solar radiation.

16. Sky Wave Propagation
Within the ionosphere, the recombination of the ions and electrons proceeds slowly enough (due to low gas densities) so that some free electrons persist even throughout the night. In practice, the ionosphere has a lower limit of 50 to 70 km and no distinct upper limit, although 1000 km is somewhat arbitrarily set as the upper limit for most application purposes.

17. Sky Wave Propagation
The vertical structure of the ionosphere is changing continuously. It varies from day to night, with the seasons of the year, and with latitude. Furthermore, it is sensitive to enhanced periods of short-wavelength solar radiation accompanying solar activity. In spite of all this, the essential features of the ionosphere are usually identifiable, except during periods of unusually intense geomagnetic disturbances.

18. PREDICTABLE IONOSPHERIC PARAMETERS
The presence of free electrons in the ionosphere produces the reflecting regions important to High Frequency (HF) radio-wave propagation. In the principal regions, between the approximate heights of 75 km and 500 km, the electrons are produced by the ionising effect of ultraviolet light and soft x-rays from the sun. for convenience in studies of radio-wave propagation, the ionosphere is divided into three regions defined according to height and ion distribution: the D,E, and F regions.

19. Sky Wave Propagation
Each region is subdivided into layers called the D,E, Es, F1, and F2 layers, also according to height and ion distribution. These are not distinctly separated layers, but rather overlapping regions of ionisation that vary in thickness from a few kilometres to hundreds of kilometres. The number of layers, their heights, and their ionisation (electron) density vary both geographically and with time. At HF, all the regions are important and must be considered in predicting the operational parameters of radio communication circuits.

20. The D region
The D region lies between the approximate limits of 75 and 90 km above the earth's surface.
The electron density is relatively small compared with that of the other regions, but, because of collisions between the molecules of the atmosphere and free electrons excited by the presence of an electromagnetic wave, pronounced energy loss occurs. This energy loss, dissipated in the form of thermal energy of the electrons or thermal (electromagnetic) noise, is termed absorption. Higher in the E and F regions, electron collisions with atmosphere molecules can also affect the condition for reflection that occurs wherever there is a marked bending of the wave. This is explained by the fact that as the wave nears its reflecting level, there is a slowing down or retardation effect, which allows additional time for collisions to occur and thus for absorption to take place. Absorption of this type is called deviative absorption.
Because of the low electron density, the D region does not reflect useful transmissions in the frequency range above 1 MHz. However, D-region absorption is important at all frequencies and, because its ionization is produced by ultraviolet solar radiation, it is primarily a daytime phenomenon
The degree of absorption is expressed by the absorption factor. After sunset in the D region, ionization decreases rapidly and non-deviative absorption becomes negligible 2 to 3 hours later.
Non-deviative D-region absorption is the principal cause of the attenuation of HF sky waves, particularly at the lower frequencies during daylight hours.

21. THE E REGION
The approximate true height range of the regular E layer is well established at 90 to 130 km and it is assumed that the maximum electron density occurs at 110 km and the semi-thickness is 20 km.
For communication, the most important characteristic feature of the E region is the temporal and geographic variation of its critical frequency. In almost all other respects, the features of the E layer are very predictable compared with those of the F2 layer.
A large volume of vertical-incidence ionosonde data has been collected over about three solar cycles, and many features of the E region are therefore well known. The minimum virtual height of the E region and the variation of maximum electron density within this region as a function of time and geographic location are readily obtained from the ionograms.

22. THE F REGION
For HF radio communications, the F region is the most important part of the ionosphere. It is not regular and because of its variability, short time scale estimates of the important F-region characteristics are required if predictions of the operational parameters of HF radio systems are to be meaningful
There are many characteristic features of the F region important to HF radio communications. This layer is actually divided into two separate layers, F1 and F2 layers.
The F1 layer is of importance to communication only during daylight hours or during ionospheric storms; it lies in the height range of about 200 to 250 km and undergoes both seasonal and solar cycle variations, which are more pronounced during the summer and in high sunspot periods.
The F2 layer is located between 250 to 350 km above the earth’s surface. During the night the F1 and F2 layers combine into a single layer

23. Effects of the Ionosphere on the Sky wave
If we consider a wave of frequency , f incident on an ionospheric layer whose maximum density is N then the refractive index of the layer is given by

24. Critical Frequency
If the frequency of a wave transmitted vertically is increased, a point will be reached where the wave will not be refracted sufficiently to curve back to earth and if this frequency is high enough then the wave will penetrate the ionosphere and continue on to outer space. The highest frequency that will be returned to earth when transmitted vertically under given atmospheric conditions is called the critical frequency.

25. Maximum Usable Frequency
There is a best frequency for communication between any two points under specific ionospheric conditions. The highest frequency that is returned to earth at a given distance is called the Maximum Usable Frequency (MUF).

26. Optimum Working Frequency
This is the frequency which provides the most consistent communication and is therefore the best to use. For transmission using the F2 layer it is defined as

27. Lowest Usable Frequency
This is set by the attenuation in the ionosphere. A practical value of this is usually taken as 3 MHz.

28. Satellite Communication
In these systems a communication satellite is placed into synchronous orbit about mi above the earth’s surface. The transmitter sends a signal using a highly directional antenna to the satellite. This signal is reamplified within the satellite and transmitted back to earth. This allows transoceanic links, frequencies range from 1 GHz to 40 GHz. The received signals and the retransmitted signals are usually at different carrier frequencies.

29. Satellite-Related Terms
Earth Stations – antenna systems on or near earth
Uplink – transmission from an earth station to a satellite
Downlink – transmission from a satellite to an earth station
Transponder – electronics in the satellite that convert uplink signals to downlink signals

30. Ways to Categorize Communications Satellites
Coverage area
Global, regional, national
Service type
Fixed service satellite (FSS)
Broadcast service satellite (BSS)
General usage
Commercial, military, amateur, experimental

31. Classification of Satellite Orbits
Circular or elliptical orbit
Circular with center at earth’s center
Elliptical with one foci at earth’s center
Orbit around earth in different planes
Equatorial orbit above earth’s equator
Polar orbit passes over both poles
Other orbits referred to as inclined orbits
Altitude of satellites
Geostationary orbit (GEO)
Medium earth orbit (MEO)
Low earth orbit (LEO)

32. Geometry Terms
Elevation angle - the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite
Minimum elevation angle
Coverage angle - the measure of the portion of the earth's surface visible to the satellite

33. Minimum Elevation Angle
Reasons affecting minimum elevation angle of earth station’s antenna (>0o)
Buildings, trees, and other terrestrial objects block the line of sight
Atmospheric attenuation is greater at low elevation angles
Electrical noise generated by the earth's heat near its surface adversely affects reception

34. GEO Orbit Advantages of the the GEO orbit
No problem with frequency changes
Tracking of the satellite is simplified
High coverage area
Disadvantages of the GEO orbit
Weak signal after traveling over 35,000 km
Polar regions are poorly served
Signal sending delay is substantial

35. LEO Satellite Characteristics
Circular/slightly elliptical orbit under 2000 km
Orbit period ranges from 1.5 to 2 hours
Diameter of coverage is about 8000 km
Round-trip signal propagation delay less than 20 ms
Maximum satellite visible time up to 20 min

36. LEO Categories Little LEOs Big LEOs Frequencies below 1 GHz
5MHz of bandwidth
Data rates up to 10 kbps
Aimed at paging, tracking, and low-rate messaging
Big LEOs
Frequencies above 1 GHz
Support data rates up to a few megabits per sec
Offer same services as little LEOs in addition to voice and positioning services

37. MEO Satellite Characteristics
Circular orbit at an altitude in the range of 5000 to 12,000 km
Orbit period of 6 hours
Diameter of coverage is 10,000 to 15,000 km
Round trip signal propagation delay less than 50 ms
Maximum satellite visible time is a few hours

38. Frequency Bands Available for Satellite Communications

39. Satellite Link Performance Factors
Distance between earth station antenna and satellite antenna
For downlink, terrestrial distance between earth station antenna and “aim point” of satellite
Displayed as a satellite footprint (Figure 9.6)
Atmospheric attenuation
Affected by oxygen, water, angle of elevation, and higher frequencies

40. Satellite Footprint

41. Satellite Network Configurations

42. Power Budget for SATCOM
The power relation between a transmitted and received power of any space wave is given as follows
where Pr is the received power
Pt is the transmitted power
Gt is the gain of the transmitting antenna
Gr is the gain of the receiving antenna
d is the distance (km) between the antennas
f is the frequency in MHz

43. Examples
1. What is the horizon for a transmitting antenna height 225 feet above ground level? What is the total horizon if the receiver is of height 25 feet above ground level?
2. If the transmitting antenna is 1000ft above ground level and the receiving antenna is 20 ft high what is the radio horizon?

44. 3. Determine the distance to the radio horizon for an antenna 40 ft above sea level
4. Calculate the radio horizon for a 500 ft transmitting antenna and receiving antenna of 20 ft. calculate the required increase in height for the receiving antenna if a 10% increase in radio horizon were required.

45. 5. Calculate the power received at a satellite given the following conditions
Power gain of the transmitting antenna is
The transmitter drives 2 kW of power into the antenna at a carrier frequency of 6.21 MHz
The satellite receiving antenna has a power gain of 30
The transmission path is km
6. Determine the maximum distance between identical antennas equally distant above sea level