CHAPTER 4. Electromagnetic Spectrum

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 108 m/s) The wavelengthλ is related to frequency f by the equation

e.g. if f = 300 MHz thenλ = 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)

Question:

Why are so many parts of the spectrum used for avionics purposes? Answer:

- propagation properties - bandwidth usage

4.3. Propagation:

4.3.1 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)

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

4.3.1.1 Effect of the Ionosphere on EM waves

- increased electron density decreases speed of propagation - leads to refraction

- amount of refraction depends on - electron density

- frequency, , of wave (higher frequency, less refraction)

index of refraction

where 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)

f

n

1

f

f

---–

=

f

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

- 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

4.4.1 AM (Amplitude Modulation)

- the amplitude of the carrier is varied in accordance with the information i.e.

where is determined by the information being transmitted For a sinusoidal modulation of frequency the equation of an AM signal is

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

- Thus the bandwidth required for this signal is 2 f_m

- Problems with AM

s

=

A t )

( )

cos

(

2

π f

t

)

A t )

( )

A t

( )

=

1

+

A

cos

(

2

π f

t

)

- not very efficient

- susceptible to interference

4.4.2 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:

Where f_DMAX is the maximum frequency deviation The advantages of FM are:

- high quality speech transmission - relatively immune to interference

4.4.3 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_ct + A(t)]

f_c

2.5fDMAX

The spectrum for phase modulation is:

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

4.4.4 Spectrum Management

Because spectrum is an international commodity it is controlled by international agreement. The ITU (Inter-national 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 fre-quency 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 frequen-cies.

f_c 2f_m 2f_m

4.4.5 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 500 - 1600 kHz HF Communications: Oceanic or Polar

communica-tions

3 - 30 MHz (vari-ous bands)

VHF Navigation ILS (Instrument Landing Sys-tem)

108 - 112 MHz

VOR (VHF Omnirange) 108 - 118 MHz Communication VHF Comm (continental air

traf-fic control)

118 - 136 MHz

UHF Navigation DME (Distance Measuring

Equipment

960 - 1215 MHz

TACAN (Tactical Air Naviga-tion)

960 - 1215 MHz

GPS (Global Positioning Sys-tem)

1575.42 MHz

Communication UHF Comm (Military aircraft) 225 - 399 MHz Radar Air Traffic Control Radar 1030 and 1090

MHz

SHF Navigation MLS 5.031 - 5.1907

GHz Communication Satellite communication

Radar Airborne weather radar 5 - 10 GHz

Radar Altimeter 4.2 - 4.4 GHz Above

30GHz

Radar Millimeter radar for synthetic vision (landing)

35GHz

Passive Infrared imaging for synthetic vision

4.5. :Antennas: General

4.5.1 Purpose:

The purpose of an antenna is to provide the link between the electromagnetic wave and either a transmitter or a receiver

4.5.2 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.

Figure 2: Isotropic Antenna Pattern

b) Directivity: If an antenna is not omnidirectional it is more sensitive (or radiates more power) in some direc-Antenna

tions than in others. Directivity is a measure of this.

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.

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.

Antenna

Antenna

4.5.2.1

Examples of aircraft antennas:

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.

Figure 5:

Basic configuration of half wave dipole

Figure 6:

Half wave dipole used for VOR/ILS (CL-601 Challenger)

To Receiver or Transmitter

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.

Figure 7: Basic configuration of quarter wave monopole

Figure 8:

Quarter wave monopoles used for VHF Comm and DME Aircraft Skin Reflection of antenna λ/4 VHF Comm DME

c) Long wire (LF, HF) - not practical on turbojet aircraft

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)

4.5.2.2 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. LONG WIRE ANTENNAS

4.5.2.3 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)

4.5.2.4 Methods for determining antenna placement:

4.5.2.4.1 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 struc-tures

4.5.2.4.2 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 materi-als 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

4.5.2.5 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)