2.4 Radio Transmission Flashcards
What is transmission path?
The way in which radio can be transmitted from a transmitter to a receiver.
At high frequency ground waves and sky waves are the main transmission paths.
When is line of site transmission used?
At VHF and above line of sight transmission is used, which involves the receiver and transmitter being able to be seen by each other.
At HF there are two methods of transmission. What are they?
GROUND WAVES, which involves waves that travel near to the surface of the earth.
SKY WAVES, which are refracted back to earth by the ionosphere.
What are surface waves?
Involve very long wavelengths, of generally 1km.
Reach the receiver by diffracting around the earth’s surface, and also by objects when the objects height does not exceed the wavelength.
Surface waves can reach places were shorter wavelengths cannot.
Surface waves can be transmitted all over the country, and do not have to be re-tuned when the receiver is moved to different places.
What are space waves?
Travel in line of sight or refract off the earth’s surface to the receiving aerial.
The different densities of the atmosphere means that the waves travel at a slightly higher speed in the upper than in the lower parts of the atmosphere causing them to refract.
Sky waves: In this mode of transmission a signal is sent out towards the ionosphere. Here it is reflected back to Earth’s surface but this depends on a few things…?
The frequency of the radio waves
The power of the transmitter
The level of ionisation at the time
What happens when sky waves have been reflected back again?
It leaves an area of land between the initial signal and its current position.
In this gap no signal can be received, and this is called a skip zone. This can happen multiple times.
The waves are not actually reflected, they are refracted by the ionosphere.
The wave could exist if it had enough energy the ionosphere, but generally the signal always is ‘sent’ back to earth again.
What is meant by work function?
Minimum energy to remove an electron.
What is a ground wave, name the two types of ground wave?
Surface waves- transmitted with a very long wavelength and reach the receiver by diffracting around objects that do not exceed the wavelength of the signal.
Space waves- travels in the line of sight or they refract off the earths surface to the receiver ariel.
What is amplitude modulation?
Process of transferring information by varying the amplitude of a wave.
This is how binary digits are encoded.
What is frequency modulation?
Process of transferring information by varying the frequency of a wave.
This is how binary digits are encoded.
What is time division multiplexing?
A technique used to send pulse code modulated or digital signals from a number of devices along the same path at what seems to be the same time.
The signal from each device is split into packets of bits which are then sent to the receiver from the transmitter.
When the signals reach their destination they are reassembled in a sequence by a de-multiplier, they are then passed to the appropriate device for decoding.
What are sky waves?
Occur with short wavelength waves.
The signals is refracted by the ionosphere.
Gives several ‘skip zones’ (regions where no signal can be received).
What is the modulating characteristics of AM?
In AM, a radio wave known as the “carrier” or “carrier wave” is modulated in amplitude by the signal that is to be transmitted.
The frequency and phase remain the same.
What is the modulating characteristics of FM?
In FM, a radio wave known as the “carrier” or “carrier wave” is modulated in frequency by the signal that is to be transmitted.
The amplitude and phase remain the same.
Pros and cons of AM?
AM has poorer sound quality compared with FM, but is cheaper and can be transmitted over long distances.
It has a lower bandwidth so it can have more stations available in any frequency range.
Pros and cons of FM?
FM is less prone to interference than AM.
However, FM signals are impacted by physical barriers.
FM has better sound quality due to higher bandwidth.
How is the attenuation of surface waves reduced?
Surface waves are weakened (attenuated) by the earth’s electric field, therefore they are polarised vertically by using a vertical dipole aerial.
This then reduces the amount of contact the radio wave has with the earth’s electric field.
Explain why radio waves of frequency 600 kHz cannot be used for an Earth–Moon radio link.
Because waves will not leave earth
This frequency is reflected by the ionosphere
Describe how some radio waves can travel beyond the horizon by following the earth’s curvature.
State the name of the radio wave with a wavelength suitable for these waves.
Diffraction
Long wave
Descrive how sky waves travel beyond the horizon and state the name of the radio wave with a wavelength suitable for these sky waves.
Refraction by ionosphere
Short wave
What are sidebands?
In electronic signal transmission, a sideband is the portion of a modulated carrier wave that is either above or below the basic ( baseband ) signal. The portion above the baseband signal is the upper sideband ; the portion below is the lower sideband .
What are bandwidths?
The range of the frequencies in a transmitted signal.
What is a carrier wave?
A high-frequency electromagnetic wave modulated in amplitude or frequency to convey a signal.
