#6: Radio Propagation & Antennas Flashcards
What are ‘L’, ‘C’, and ‘X’ bands? What are their rough frequencies?
Satellite bands; range from about 1 - 300GHz.
How to electromagnetic waves ‘move’? Explain with a diagram.
They oscillate (in a sinusoidal manner, assuming it is a sine wave), with a vertical electric component and a perpendicular magnetic component.
The size (i.e. diameter/length) of an antenna is related to the _____ of the signal it is receiving/transmitting.
wavelength
What’s a ‘dipole’ antenna? Draw a diagram.
Two conductors, each of length related to the signal wavelength. One conductor is positively charged (positive ions build up at the end), and the other negatively.
What’s the ‘far-field’ pattern of an antenna, and how is it expressed?
The region far away from the antenna. It gives an indiction of the gain in all directions. This can be used to make a polar plot of the antennas overall gain/direction pattern.
What’s the equation of the field intensity of an antenna with respect to its length ‘L’?
What is the equation to convert from dB’s to gain?
dB = 10 log G
What is the difference between multi-mode and single-mode fibre? Draw a diagram.
What is the ‘refractive index’? What is its equation?
How much light bends when passing into a different medium
n = c / v
c = speed of light in a vacuum
v = speed of light in the given medium
what is Total Internal Reflection?
a perfect angle where light is completely reflected
fibre users a core and cladding with different refractive indexs so that the light can bounch and totally reflect.
What does the term ‘numerical aperture’ mean within the context of fibre optics? What is its equation?
the maximum angle of incidence (e.g. approach angle into fibre optic core) beyond which the light would refract into the cladding instead of propopogating through the core.
What is a ‘phased array’ within the context of antennas?
An array of antennas that are electronically steered to point in different directions without moving the antennas.
How do phased arrays change direction/radiation patterns without physically moving the antennas? Draw a diagram.
They change the phasing between supply currents to the antennas. A slight delay between different currents creates interferance between waves and changes the radiation pattern.
What is the formula for the transmission distance of an antenna at height ‘h’ and a receiver on the ground?
d(tx) (km) = 4sqrt(h (tx) )
Explain the doppler effect, and mention its equation.
When a source or receiver is moving, the apparent frequency changes (e.g. if a source is moving away from a stationary receiver, it looks like the radio waves are further apart and thus the frequency is lower).
How is a Yagi antenna different to a dipole? Use a diagram.
