Radio Theory (EQMT) Flashcards
(41 cards)
What are the 5 components of a wave?
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.
What are modulation and demodulation?
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.
What are the 3 types of modulation?
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.
What frequencies are most civil aviation transmissions conducted on?
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.
What is Bandwidth?
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.
What is the Doppler Effect? How can we calculate its magnitude?
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.
What can the Doppler Effect be used for in aviation signalling? How does this work?
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.
What are the 3 types of wave propagation, how much energy do they use, and what are their approximate ranges?
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.
What types of antenna are there? 4 answers.
1/2 lambda dipoles, 1/4 lambda monopoles, loops and slots.
What shape does the transmission area of a 1/2 lambda dipole antenna create.
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.
How does ground interference cause problems for antenna?
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.
How can ground interference be used beneficially?
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.
What is one potential danger of the ILS caused by radio interference?
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.
How are 1/4 lambda monopoles different from 1/2 lambda dipoles?
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.
What three components/steps should one expect to find in a transmitter?
Firstly, a modulator. Secondly, an amplifier. Thirdly, the transmitter itself.
What five components/steps should one expect to find in a receiver?
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.
How are old, 25KHz channels distinguished form new, 8.33KHz channels?
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.
How did some radio systems used to overcome the issue of dead zones which signal could not reach?
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.
What is cross coupling?
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.
What is the ELT on board a plane?
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.
Where does the HF transmitting equipment live in a plane?
In the tailsection, due to the increased length needed for a HF aerial, due to its increased wavelength.
What is the range of SATCOM, what frequencies does it operate on, and how much energy does it use?
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.
What frequencies is SELCAL transmitted over?
Either VHF or HF, depending on the plane’s capability.
What is ACARS or CPDLC, and what are they used for?
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.
