Radio Theory (EQMT)

Question 1

What are the 5 components of a wave?

Answer 1

Period: The time of one full cycle of the wave.
Frequency: the number of full cycles per second (1/s=1Hz)
Amplitude: the maximum positive or negative deviation from the location of the wave at 0 degrees. Units vary depending on type of wave.
Phase: the proportion of one full cycle of the wave which any given wave has currently completed.
Wavelength: the relationship between the propagation speed and the period. The distance between locations where phase = 0 degrees on the same wave.

Question 2

What are modulation and demodulation?

Answer 2

Modulation is the process of attaching a signal to a carrier wave, such that it can be transmitted at a different frequency. Demodulation is the process of reading the message signal from the carrier signal and reconstructing the original transmission from it.

Question 3

What are the 3 types of modulation?

Answer 3

Frequency modulation, amplitude modulation and phase modulation. In each case, the stated property of the carrier signal is fluctuated in order to encode the message signal.

Question 4

What frequencies are most civil aviation transmissions conducted on?

Answer 4

The majority of civil aviation transmissions are conducted in the VHF (very high frequency) or UHF (ultra-high frequency) ranges. VFH falls between 30 and 300 MHz, although aviation transmissions don’t start until 108MHz. UHF goes from 300MHz to 3 GHz. Aviation VHF communications radio, the most common kind, words on 118-137MHz.

Question 5

What is Bandwidth?

Answer 5

Bandwidth is the amount of space on either side of the carrier signal’s frequency which is occupied by the message frequency. It is important that this space is not overlapped by any other signals, as this could cause interference which makes the message unintelligible.

Question 6

What is the Doppler Effect? How can we calculate its magnitude?

Answer 6

The Doppler Effect is the apparent red- or blue-shifitng of signals, caused by the motion of the signal-sending body relative to the signal-receiving body. Its magnitude is the cosine of the angle between the line of the transmitter’s motion and the shortest line connecting the transmitter to the receiver.

Question 7

What can the Doppler Effect be used for in aviation signalling? How does this work?

Answer 7

It can be used in D-VORs (Doppler-VHF Omni-Directional Radar). The way it works is that a series of Doppler Radars measure the Doppler shift magnitude of things they detect, and check whether those things are in motion relative to the radar device. If they aren’t the system then automatically filters them from the radar operator’s screen, reducing clutter and only showing pertinent information.

Question 8

What are the 3 types of wave propagation, how much energy do they use, and what are their approximate ranges?

Answer 8

Ground Wave propagation is the highest-energy type of wave propagation, and has an approximate range of 600NM.
Sky Wave propagation is medium-energy, and has a range of up to 10,000NM, though this is highly variable according to atmospheric conditions.
Line-of-Sight propagation is the lowest-energy type, and has a range of 300NM. It is powerful enough to retain coherence through the Ionosphere, though, and is therefore used for talking to satellites.

Question 9

What types of antenna are there? 4 answers.

Answer 9

1/2 lambda dipoles, 1/4 lambda monopoles, loops and slots.

Question 10

What shape does the transmission area of a 1/2 lambda dipole antenna create.

Answer 10

Basically like a doughnut, if the antenna itself were sticking through the hole in the middle. The cone with no signal at the top/bottom is called the zone of silence.

Question 11

How does ground interference cause problems for antenna?

Answer 11

Some of the transmissions hit the ground and reflect back, causing interference. This creates a wobbly looking dome of signal around the antenna, with the wobbles being called feathers.

Question 12

How can ground interference be used beneficially?

Answer 12

If half lambda dipole antennae are arranged horizontally, there is so much ground interference that the “feathers” created become so pronounced that the positive regions of interference reach outwards much further, and the regions of negative interference reach, effectively, to zero. This creates “lobes” of signal extending from the transmitted at various angles, the lowest of which is 3 degrees. This is useful for sending the ILS (instrument landing system) signal, as planes should approach at 3 degrees.

Question 13

What is one potential danger of the ILS caused by radio interference?

Answer 13

The lobes created by a horizontal half lambda dipole don’t just stop at the 3 degree one we want. There is a further one at 9 degrees, and if a plane hits this one by accident instead of the 3 degree one, it confuses the hell out of the autopilot and causes the plane to go nose up very quickly, possibly causing a stall.

Question 14

How are 1/4 lambda monopoles different from 1/2 lambda dipoles?

Answer 14

They all need a built in ground, which is usually mounted on the lower part of their housing, in order to reflect the signal. Also, their cone of silence is much wider, and their doughnut of signal stretches further.

Question 15

What three components/steps should one expect to find in a transmitter?

Answer 15

Firstly, a modulator. Secondly, an amplifier. Thirdly, the transmitter itself.

Question 16

What five components/steps should one expect to find in a receiver?

Answer 16

Firstly, a radio frequency amplifier. Secondly, an automatic gain compensator, to make sure the volumes of transmissions are the same. Thirdly, an intermediate amplifier and a demodulator, Fourthly, a squelch removal circuit. Lastly, another amplifier and the speaker.

Question 17

How are old, 25KHz channels distinguished form new, 8.33KHz channels?

Answer 17

25 KHz channels get the whole numbers, e.g. 188.000, whereas new ones add 0.005 if they would be the same, i.e. 118.005. The next two channels are then called 118.010 and 118.015, to avoid the need to do maths.

Question 18

How did some radio systems used to overcome the issue of dead zones which signal could not reach?

Answer 18

By putting multiple masts up, and then spacing their transmissions out within the 25KHz channel by 8.33KHz increments. Obviously, this is becoming less possible as channel spacing all moves to 8.33KHz.

Question 19

What is cross coupling?

Answer 19

The process of wiring together a number of radio communication systems, such that a number of incoming and outgoing frequencies can be “tied together”, effectively turning them into one big region with one big channel band. This might be done, for example, at night, when traffic reduces to the point that one controller could work 3 desks at once. If they are cross-coupled, that is literally possible.

Question 20

What is the ELT on board a plane?

Answer 20

The emergency locator transmitter, this is the third VHF radio transmitter, usually located towards the back of the plane, near the dorsal. It is not only a normal VHF transmitter, but it also transmits the emergency signal if needed. It is this one, as opposed to the other two, because this one’s location makes it much more likely to survive in a crash.

Question 21

Where does the HF transmitting equipment live in a plane?

Answer 21

In the tailsection, due to the increased length needed for a HF aerial, due to its increased wavelength.

Question 22

What is the range of SATCOM, what frequencies does it operate on, and how much energy does it use?

Answer 22

The range is worldwide, besides some polar regions, and it runs on a frequency of 1525-1559MHz receiving, 1626-1660 sending, and only uses 20 watts to send.

Question 23

What frequencies is SELCAL transmitted over?

Answer 23

Either VHF or HF, depending on the plane’s capability.

Question 24

What is ACARS or CPDLC, and what are they used for?

Answer 24

They are Aircraft Communications Addressing and Reporting System, or Controller-Pilor DataLink Communication. They are both text message connections between ground and plane on the HF and VHF frequencies, intended to cut down on voice comms and also save time re-boroadcasting routine messages.

Question 25

What are QDM, QDR, QTE and QUJ?

Answer 25

References pilots may ask for about their angles to beacons.
QDM is the angle between the direction to the beacon, and true north.
QDR is the inverse: the angle between magnetic north and the direction of the beacon.
QUJ and QTE are the same, but for true north rather than magnetic north.

Question 26

What is the technical way to say “directions to things by reference to drawing straight lines”?

Answer 26

Azimuthal directions.

Question 27

What are VDFs and UDFs, and why are they useful?

Answer 27

VHF Direction Finders and UHF Direction Finders. They work out which direction an aircraft is in by looking which direction their VHF or UHF comms signals came from. They are useful because it does not require planes to carry any more equipment, You just look for the signals they were already sending for other reasons anyway.

Question 28

How is direction finding normally done (not Doppler)?

Answer 28

Using 3 aerials, called “loop and sense” aerials. The way they work is by having 1 aerial per axis. One remains constant, and the other two are switched on and off gradually and alternately, following out of phase sine patterns. The signals are then combined with the static one, to distinguish positive from negative, and with each other, to distinguish coming from going. and then the direction in which the signal is strongest is the one the plane is in.

Question 29

How accurate is position-finding using VORs and DMEs?

Answer 29

VORs are quite inaccurate, so the area of uncertainty can be many kilometers. DMEs are very accurate, and can be within tens of meters. 1 VOR and 1 DME together is accurate enough for area control, but not accurate enough for aerodrome control.

Question 30

What is area navigation / performance based navigation?

Answer 30

Navigating by triangulating your position, rather than by flying directly at navigational beacons.

Question 31

What are RNAV and RNP, and what’s the difference.

Answer 31

RNAV (radio navigation) is the old form of instrument navigation, wherein planes must fly directly at navigation beacons in order to know where they are going. Must by 95% accurate, minimum.
RNP is Required Navigation Performance, and is the modern sort of instrument navigation. Here, computers are working out where they reckon the plane is and what it’s doing, and tell the pilot if this is wrong. Still needs to be 95% accurate.

Question 32

What is a radio-magnetic indicator, and where might you find one?

Answer 32

It is basically a compass overlaid with dials which show where different radio sources are. You would fine one in the cockpit of a plane.

Question 33

What are the main limitations of NDBs (non-directional beacons)?

Answer 33

They work via ground-wave propagation, so they are quite prone to interference. Furthermore, they are prone to being effected by coastal air diffraction, which can make them appear up to 60 degrees wrong.

Question 34

What frequencies and bandwidth do VORs operate on?

Answer 34

They have a bandwidth of 50KHz, and operate between 108 and 118MHz. Between 108 and 112MHz, it shares the space with localiser frequencies, though, and therefore only operates on the even numbered first decimal numbers, i.e. 108.00, 108.05, 108.20, 108.25, etc.

Question 35

What is the demodulated frequency and modulation mode of the reference signal in VOR and D-VOR?

Answer 35

Both are 30Hz, demodulated. D-VOR uses an AM reference signal, because the rotating signal must naturally be FM (because it’s the Doppler effect). Regular VOR has an FM reference signal.

Question 36

What are the approximate inaccuracies of VOR and D-VOR?

Answer 36

4 degrees and 1 degree respectively.

Question 37

What frequencies do DMEs broadcast on, and how are they selected?

Answer 37

DMEs broadcast between 962 and 1213MHz, but they are selected by tuning a corresponding VOR. If no VOR exists, a “ghost” VOR will be created whose frequency tunes the DME.

Question 38

How far from the runway are the outer and middle markers?

Answer 38

4NM and 0.5NM, respectively.

Question 39

What are the demodulated frequencies used by the localiser and glide path signals, and how are they arranged?

Answer 39

90Hz and 150Hz, with 90Hz on the top and left, as you approach the runway.

Question 40

Which localiser antenna shows you the path to take, and which one correct for side lobes and off-course regions?

Answer 40

The course antenna gives the path, the clearance antenna sends the other signal.

Question 41

At what ranges and angles do the clearance and course antennae transmit?

Answer 41

The course antenna transmits 25NM is a 20 degree cone, center on the runway centre. The clearance antenna transmits 17NM, in a 70 degree cone centered on the runway centre.