Basic Radio Propagation Theory Flashcards
Radio waves travel at the…..?
How fast does it travel?
At the speed of light at approx 300 000 000m/s (÷ 1852m) OR
300, 000km/s (÷ by 1.852km)
=
162 000 NM/s
Define a cycle.
A cycle is a complete series of values of a periodic process.
What is a hertz?
Hertz is a unit of measure to indicate how many cycles passed within a frequency per second.
Define frequency.
A frequency is the number of cycles (oscillations) occurring in a second in a radio wave, expressed in hertz.
So time taken to complete one cycle is determined by the frequency.
Define a wavelength
The physical distance travelled by a radio wave during one cycle of transmission/complete cycle of variation.
Define amplitude
The maximum deflection in an oscillation or wave. It can be + (upper peak of the oscillation) or - (peak to peak)
What is the relationship between a wavelength and frequency?
Wavelength is inversely proportional to frequency
f = 1/lamda
Lamda = speed of light/f
Define phase
The fraction of one wavelength from 0˚ to 360˚
If more than one cycle starts and ends at the same point, this is called?
In phase.
I.e. In phase wavelength rise and pass through the zero datum line together; phase harmony.
If second cycle starts and ends at different places its called?
Out of phase
Anti phase is?
Where there is a 180˚ phase shift, and where reference phase is leading.
When a carrier wave is modulated there are…..
Sidebands on either side of the carrier wave:
- upper sideband
- lower sideband
In HF, whether HF volumes or HF radio, we ignore one of the sidebands; use single sideband.
Radio signals maybe classified by 3 symbols in accordance to ITU radio reg Vol 1. What do the 3 symbols indicate?
E.g. A1A
- first symbol: type of modulation of the main carrier
- second symbol: type (nature) of signal modulating the main carrier
- third symbol: type of info to be transmitted
Pulse characteristics:
What is pulse length?
Pulse width is the length of the pulse given as a unit of time.
They are usually transmitted in series of groups aka pulse strings or pulse trains.
What is pulse power?
Is the power contained within a pulse.
What is continuous power?
Is the power consumed over time, including periods of non transmission.
Carrier/Modulation:
Define carrier waves and list the 4 main types of radio wave acting as a carrier or transporter.
4 main types of radio waves:
- Pulse - simplest form of wave
- Amplitude (AM)
- Frequency (FM)
- Phase (PM)
When a radio wave is modulated (encoded with info), the radio wave is altered and is therefore carrying info on it’s wave. This altered wave is called a carrier wave.
The receiver must remove the carrier wave to reveal the attached info.
Define keying.
Interrupting the carrier wave to break it into dots and dashes. This is a form of pulse wave and is used as morse code.
Define modulation.
Is the changing of a radio wave to encode information onto it. A radio wave itself is of limited use. To attach info to a radio wave we need to alter it either through altering its pulse, amplitude, freq or its phase. This process of impressing and transporting info by radio wave is called modulation.
Define pulse modulation
A modulation form used in radar by transmitting short pulses followed by larger interruptions.
Define amplitude modulation (AM)
The info is impressed onto the carrier wave by altering the amplitude (noise) of the carrier.
Altering the amplitude makes this method sensitive to ‘outside’ noise distortions e.g. Dropped pen or thunder.
Define frequency modulation (FM)
The info impressed onto the carrier wave by altering the frequency of the carrier.
As it is the frequency (cycles that are being altered) on this carrier wave, it does not pick up on noise distortions.
Define Phase modulation (PM)
A modulation form used in GPS where the phase of the carrier wave is reversed.
It is a complex modulation technique and requires a complex demodulator.
Define antenna
When current flows through a wire, an electromagnetic field is generated around it.
Therefore a transducer converts a line of AC into a free electromagnetic wave. So the stronger the current the stronger the electromagnetic field; current proportional to the electromagnetic field.
The input and output signals are the same with the exception that the output is less powerful than the input.
What is a monopole antenna?
Aka whip aerial.
Is a single conducting pole and is the simplest type of antenna. Mounted vertically it is omni directional with a cone of confusion (silence) directly above it.
What is a dipole?
An antenna which has two wires that equal 1/2 of a wavelength. So each wire is 1/4 of the transmitting wavelength.
Length of the conductor must carefully match the wavelength in order to receive it.
What happens when a second conductor is placed within the electromagnetic field of another wire which is being AC fed?
The electromagnetic field of the AC fed conductor will generate (induce) voltage (AC) into the second conductor which exactly reproduces the transmission. That is, the signal is strengthened.
How does an electromagnetic wave propagate from the source?
At the speed of light and with electric(E) and magnetic (H) fields
What are the relationship(s) between the E and H fields?
• E fields are parallel to the wire while H fields are perpendicular
to the wire.
• E and H fields are perpendicular to each other but are in phase.
• Together these oscillations are perpendicular in propagation
direction.
Define polarisation
Polarisation describes the way an electromagnetic field is propagating from it’s source.
Name the 3 polarisation types
- Linear: Plane of oscillation is fixed in space
Plane of oscillation is rotating:
- Circular
- Elliptical
How is a type of polarisation determined?
By the orientation of the plane of oscillation of the E field.
A linear polarisation can be polarised horizontally and vertically. Explain why this is.
Linear polarisation is an electromagnetic wave propagating in one direction. Since the polarisation depends on the E field, and E fields are parallel to the wire (transmitter), if a conductor is horizontally placed then the polarisation is horizontal. If the transmitter is vertically placed then the polarisation is vertical.
The receiver must be placed in the same direction as the conductor to receive the transmission.
Types of antenna:
List the common kinds of directional antennas and an example of it’s use.
1) Loop antenna, used in old ADF receivers
2) Parabolic antenna, used in old Wx radars
3) Slotted array, used in more modern Wx radars
4) Helical antenna, used in GPS transmitters
Wave propagation:
What is the Ionosphere and where can it be found?
Is the ionised component of the Earth’s upper atmosphere from 60 to 200km above the surface.
State the layers (or regions) in the ionosphere
There are 3 layers in the ionosphere named D, E and F.
The depth of these layers varies with time. D layer disappears at night and the layer extends upwards comprising of only E and F layers.
What are sky waves?
The electromagnetic waves which are refracted from the E and F layers of the ionosphere are called sky waves
How many radio wave types are there and how are they categorised.
There are three types of radio waves and are categorised by the how the wave propagates e.g.
- Surface waves propagate travelling along the curvature of the earth
- Sky waves propagate by refracting from the E and F layers of the
Ionosphere. - Space waves propagate following line of sight; directly from transmitter to receiver.
Which frequencies do Space waves use?
VUSE
What is the Doppler effect?
Describes the apparent frequency change of a electromagnetic wave due to the relative motion between the transmitter and receiver.
If the transmitter and receiver are converging then the frequency will increase and the wavelength will decrease.
If the transmitter and receiver are moving away from each other the frequency will decrease and the wavelength will increase.
What is skip distance?
The distance between the transmitter and the point on the surface of the earth where the first sky wave arrives.
What is the skip (dead) zone?
The distance between the limit of a surface wave and the sky wave.
Skip distance is defined as the ground distance between MF/HF transmitter and the returning position of the first sky wave.
Describe fading.
When a signal loses power and ‘fades’ out
What is the critical angle?
The highest angle in which a transmission of a sky wave is emitted into space and is refracted. Go lower than this critical angle and the transmissions will be absorbed by the layers in the Ionosphere.
Why are radio waves in the VHF band and above are limited?
Because they are absorbed by the ionosphere and do not have a surface wave.
VHF band frequencies follow line of sight.
What is reflection?
This is when a radio wave, when hitting an object, part of if not all of the wave bounces off in a different direction.
This is why you get multiple pictures layered on top of each other on a tv screen.
What is refraction?
When a radio wave alters it’s direction due to the substance and velocity at which the wave hits an object.
The change in direction can be so abrupt it can sometimes be classified as a reflection.
What is diffraction?
The tendency of a radio wave to propagate in a curve i.e. Ability to propagate around the curve of the earth.
The longer the wavelength, thus the lower the frequency, the better the ability of the radio wave to bend and travel in a curve.
What is absorption?
A part of the frequency is attenuated (diminished) or lost due to matter turning the radio wave energy into another form e.g. Heat energy.
i.e. LF/MF are absorbed in the D layer during the day due to the ionisation levels within the D layer during the day.
What is interference?
Two or more signals crossing each other causing attenuation (if out of phase), or cancellation (if anti phase).
What are the categories of radio waves?
Ground/Surface, sky and space.
What frequency do surface waves use?
VLF to MF
What frequencies do Sky waves use?
MF to VHF
What frequencies do Space waves use?
VHF to EHF
Super: SHF
Extremely: EHF
How do sky waves travel?
Through absorption and refraction.
How do surface waves travel?
Across the surface. The lower the frequency the lower to the ground the wave is.
How do space waves or higher frequency waves travel?
Space waves are limited by line of sight, however when they are projected into space, the wave can return back to earth via a GPS signal.
What is the formula for max theoretical range?
D = 1.23 (√Tx +√Rx)
D - NM
Tx - Height of transmitter (ft)
Rx - Height of receiver (ft)
What is a whip aerial?
Straight conducting wire, like a car aerial.
What is a dipole aerial?
2 conducting wires. Each wire is 1/4 of the wavelength being received so 2 wires means a dipole is receiving 1/2 of the total wavelength from the signal.
What is a loop antenna?
Used for ADF and is an electrical loop that measures different voltages.
AM modulations most sceptical to which kind of intereference?
Static interference i.e. CB
How does PM work and what is it used for?
- phase of carrier wave is reversed
- used by GPS
- requires complex demodulator
NDB and VOR both have COC, why?
Because both use the whip (monopole) aerial which has weak or no signal directly above the monopole aerial.
What do they mean when polarising is referred to?
How an electromagnetic field is propagating.
E field always parallel to the wire, while H field perpendicular to the E field.
What is a loop antenna used for?
ADF receivers
What are parabolic antennas used for?
Older ATC radar and A/C Wx radars
What are slotted planar radars used for?
Used for Wx radars in most airliners now
What are helical antennas used for?
GPS
What affects wave propagation?
Wx Ionosphere Solar activity Antenna - omnidirectional and directional Frequencies
What is reflection?
A signal bounces off (reflects) off an object back in the same direction as the transmission.
What is refraction?
Change in direction of a signal due to the change in speed caused by density.
I.e. Over land more friction so speed of signal is slower compared to water which is smoother and so the speed of the signal is faster.
What is diffraction?
Signals change due to gap in which the wave travels through. As gap alters wave path.
What are the wavelengths for VLF, LF and MF?
V: 100 to 10km (Myriametric)
L: 10 to 1km (kilometric)
M: 1km to 100m (hectometric)
What are the wavelengths for HF, VHF and UHF?
H: 100 - 10m (decametric)
V: 10 to 1m (metric)
U: 1m to 10cm (decimetric)
What are the wavelengths for SHF and EHF?
S: 10 - 1cm (centrimetric)
E: 1cm - 1mm (millimetric)
Which antennas are directional and which are not?
Directional: Loop Parabolic Slotted planar Helical
Omnidirectional:
Sense
Dipole
The advantage of the use of slotted antennas in modern radar technology is to:
Virtually eliminate lateral lobes and as a consequence concentrate more energy in the main beam
In aviation electronic systems the so called Doppler principle may be used in:
VOR, GPS and MTS and the turbulence mode of AWR
Diffraction is?
DIffraction is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings.
Comparing a parabolic reflector with a flat plate antenna of the same size:
The flare plate antenna generates less side lobes than the parabolic reflector
What instruments use an OBI?
VOR
ILS
RNAV
PBN
What Nav aids use an HSI?
VOR
ILS
RNAV
PBN
What Nav aids use an RMI?
VOR
NDB
RNAV
PBN
What Nav aid uses a RBC (fixed and moving) and RMI?
NDB
