CHAPTER 4. Electromagnetic Spectrum

Transcription

1 ELEC4504 Avionics Systems 9 CHAPTER 4. Electromagnetic Spectrum 4.1. Electromagnetic (EM) Waves In free space (or the atmosphere) the electric field is perpendicular to the magnetic field and both are perpendicular to the direction of propagation. The speed of propagation is the same as the speed of light (about 3 x 10 8 m/s) c The wavelength λ is related to frequency f by the equation λ = --- f e.g. if f = 300 MHz then λ = 1 meter The polarity of an EM wave is defined as the orientation.of the electrical field vector. Typical polarities are: - vertical - horizontal - circular (left or right). In circular polarization, the electric and magnetic field vectors rotate in a corkscrew fashion as the wave propagates 4.2. Classification of the spectrum (somewhat arbitrary) Table 1: Band f λ VLF (Very Low Frequency) 30kHz-300kHz 10km - 1km LF(low Frequency) 300kHz-3MHz 1km - 100m HF(High Frequency 3MHz-30MHz 100m - 10m VHF(Very High Frequency) 30MHz-300MHz 100m - 10m UHF(Ultra High Frequency) 300MHz - 3GHz) 1m - 10cm SHF(Super High Frequency) 3GHz - 30GHz 10cm - 1cm Question: Why are so many parts of the spectrum used for avionics purposes? Answer: - propagation properties - bandwidth usage 4.3. Propagation: Ionosphere - upper levels of the atmosphere - atoms ionized by bombardment of solar particles (solar wind) - height and electron density very variable (see Figure 1) -depends on time of day and solar activity (sunspot cycle)

2 10 ELEC4504 Avionics Systems Effect of the Ionosphere on EM waves - rotates polarity (Faraday Effect) - increased electron density decreases speed of propagation - leads to refraction - amount of refraction depends on - electron density - frequency, f, of wave (higher frequency, less refraction) 2 f index of refraction n = p f 2 where f p depends on electron density Note: Since the ionosphere is, as a whole, neutral the electron density is the same as the ion density, but, being much lighter, the electrons have a much greater effect on the EM wave Where there is a gradient in the electron density, refraction and reflection of the waves take place (see Figures)

3 ELEC4504 Avionics Systems 11 Thus, for frequencies below 30 MHz it is possible to transmit signals over long distances

4 12 ELEC4504 Avionics Systems 4.4. Bandwidth The second factor in determining spectrum usage is bandwidth - to transmit information on an electromagnetic wave, it is usually necessary to modulate a carrier signal - carrier: a single frequency tone

5 ELEC4504 Avionics Systems 13 - modulate: alter carrier signal in a manner which codes the information to be transmitted There are three basic types of modulation: AM (amplitude modulation), FM (Frequency Modulation) and PM (Phase Modulation). These differ only on the parameter of the carrier which is varied (modulated) to transmit in AM (Amplitude Modulation) - the amplitude of the carrier is varied in accordance with the information i.e. s = A( t) ) cos( 2π f C t) where At) ( ) is determined by the information being transmitted For a sinusoidal modulation of frequency the equation of an AM signal is At () = 1 + Acos( 2π f m t) t T Figure 1: AM modulated signal in the time domaim. In this case the period of the carrier is t (therefore the frequency f c = 1/t) The period of the modulating signal is T (therefore the modulating frequency is f m = 1/T) and the spectrum is f m f m f c - Thus the bandwidth required for this signal is 2 f m - Problems with AM

6 14 ELEC4504 Avionics Systems - not very efficient - susceptible to interference FM (Frequency Modulation) In frequency modulation, the amplitude of the carrier is maintained at a constant level while the the frequency of the carrier is varied by an amount proportional to the amplitude of the modulating signal. i.e. s = Acos{[f c + pm(t)]t} where: -A is the (constant) amplitude of the carrier signal. -m(t) is the waveform of the information to be transmitted - p is a proportionality constant relating the frequency shift to the modulating signal amplitude For sinusoidal modulation the spectrum of an FM signal is approximately: 2.5f DMAX 2.5f DMAX f c Where f DMAX is the maximum frequency deviation The advantages of FM are: - high quality speech transmission - relatively immune to interference PM (Phase modulation) Because frequency is simply the rate of change of phase, phase modulation is similar to frequency modulation. The amplitude of the carrier is maintained at a constant level and the phase of the carrier is varied by an amount proportional to the amplitude of the modulating signal. i.e. S = cos [f c t + A(t)]

7 ELEC4504 Avionics Systems 15 The spectrum for phase modulation is: 2f m 2f m f c PM is used mainly for data transmission. As can be seen in these examples, the bandwidth required is approximately proportional to the modulating frequency. The modulating frequency is determined by the required information rate (data rate) Thus high data rate > high frequency > high bandwidth Spectrum Management Because spectrum is an international commodity it is controlled by international agreement. The ITU (International Telecommunications Union) is the United Nations organization which coordinates the allocation of frequencies to the activities which use them (e.g. r5adionavigation, satellite communications, radar systems). ITU organizes World Radiocommunication Conferences (WRC) every two to three years to modify the frequency allotments as required (or requested) The low end of spectrum is very congested and so to get more bandwidth it is necessary to use higher frequencies.

8 16 ELEC4504 Avionics Systems Aeronautical Usage of EM Spectrum Table 2: Band Usage System Frequencies VLF Navigation Omega (discontinued) 10 khz LF Navigation LORAN C 1 MHz Non-Directional Beacon HF Communications: Oceanic or Polar communications khz 3-30 MHz (various bands) VHF Navigation ILS (Instrument Landing System) VOR (VHF Omnirange) MHz MHz Communication VHF Comm (continental air traffic control) MHz UHF Navigation DME (Distance Measuring Equipment TACAN (Tactical Air Navigation) GPS (Global Positioning System) MHz MHz MHz Communication UHF Comm (Military aircraft) MHz Radar Air Traffic Control Radar 1030 and 1090 MHz SHF Navigation MLS GHz Communication Satellite communication Radar Airborne weather radar 5-10 GHz Radar Altimeter GHz Above 30GHz Radar Millimeter radar for synthetic vision (landing) 35GHz Passive Infrared imaging for synthetic vision

9 ELEC4504 Avionics Systems :Antennas: General Purpose: The purpose of an antenna is to provide the link between the electromagnetic wave and either a transmitter or a receiver Definitions a) Antenna Pattern: A means of describing the directional sensitivity of an antenna e.g. An omnidirectional (or isotropic) antenna has a perfectly spherical pattern - it is equally sensitive in all directions. This is illustrated by the following figure. Note: Antennas are reciprocal devices. This means that they have the same characteristics (including antenna pattern) whether they are transmitting or receiving. The main difference is that for a receiving antenna the pattern indicates the directional sensitivity whiel for a transmitting antenna it indicates the directional power ouput distribution. Antenna Figure 2: Isotropic Antenna Pattern b) Directivity: If an antenna is not omnidirectional it is more sensitive (or radiates more power) in some direc-

10 18 ELEC4504 Avionics Systems tions than in others. Directivity is a measure of this. Antenna Figure 3: Antenna Pattern for a Directional Antenna c) Gain: If an antenna is directional its sensitivity compared to that of anomnidirectional antenna is its gain. Antenna Gain Figure 4: Illustration of the Gain of a Directional Antenna In general the closer the antenna is in size to the wavelength with which it is intended to interact, the more efficient it is. d) Polarity All antennas are designed to transmit or receive EM waves of a given polarity. Thus there are vertically, horizontally and circularly polarized antennas.

11 ELEC4504 Avionics Systems Examples of aircraft antennas: a) Half-wave dipole (TV antenna) The polarity of this antenna is determined by the direction of the two elements which make it up. Thus the VOR/ILS antenna in Figure 6 is horizontally polarized. λ/2 To Receiver or Transmitter Figure 5: Basic configuration of half wave dipole Figure 6: Half wave dipole used for VOR/ILS (CL-601 Challenger)

12 20 ELEC4504 Avionics Systems b) Quarter wave monopole (or stub, or whip) An electrically conducting surface reflects EM waves as a mirror reflects light. This effect is used to make a half-wave dipole with a single element as shown in Figure 7. Because the single element is half the length of the half wave dipole it is ususally called a 1/4 wave monopole. When the reflecting surface is horizontal, (the normal case) the polarity is vertical. λ/4 Aircraft Skin Reflection of antenna Figure 7: Basic configuration of quarter wave monopole VHF Comm DME Figure 8: Quarter wave monopoles used for VHF Comm and DME

13 ELEC4504 Avionics Systems 21 c) Long wire (LF, HF) - not practical on turbojet aircraft LONG WIRE ANTENNAS Figure 9: Long Wire Antenna on a Dakota Aircraft d) Loop (sensitive to magnetic field) e) Horn (microwave, usually radar) may include a parabolic reflector) Antennas: Aircraft Installation Usually the antenna pattern should be as close to omnidirectional as possible since the relative direction from the aircraft to the ground station is a function of the aircraft attitude and can be almost any direction. exception: weather radar, satellite communications Note: these require attitude stabilization It is also preferable to have low drag Antennas on the bottom of aircraft may require protection from debris thrown up by the undercarriage.

14 22 ELEC4504 Avionics Systems Siting of antennas -Watch for shadowing from wings/ horizontal stabilizer -Interference from one system to another -VLF antennas may require skin current mapping -as close as possible to the avionics bay of the equipment it is serving - to reduce cable losses and - to reduce the number of connectors (through bulkheads) Methods for determining antenna placement: Analytical (Mathematical) modelling -OK for Low frequencies (a/c parts modeled as rods) and for microwave and above (a/c skin modeled as a series of planes) -VHF/UHF very difficult since the wavelengths are comparable to the size of the aircraft structures Scale modelling This method requires a large anechoic chamber. This is a room whose walls are lined with EM wave absorbing material to reduce the effect of reflections on the measurements A scale model of the aircraft is used. It is necessary to construct the model aircraft from materials whose properties are scaled in the same proportion as the mode. Sometimes it is difficult to find materials whose properties scale correctly e.g. for a 1/10 model, the skin should be made of a material whose electrical resistivity is 1/ 10th that of aluminum Problems with antennas - Poor bonding between antenna and aircraft skin. This is especially true for 1/4 wave monopoles since they depend on the skin to work properly - Cabling losses and faults cause a reduction in the signal level at the receiver (for receiving antennas) or the radiated power (transmitting antennas) - Connectors introduce losses into the signal path.