Propagation

Question 1

Mathematical definition of Decibels (dB)?

Answer 1

Power ratio (dB) = 10log(P2/P1)

Question 2

Antenna types.

Answer 2

Omani-directional
Directional
- Planar (mechanical)
- Phased (passive beam steering)
- AESA (Active Electronically Steered Arrays)

Question 3

Derive an equation for Antenna Gain

Answer 3

Area covered by an antenna beam at distance R with antenna dimensions a, b, is:
R(wavelength)/a * R(wavelength)/b
Therefore antenna gain…
G = 4 pi R^2 / ((R^2 (wavelength)^2) / ab)
= 4 pi a b / (wavelength)^2
= 4 pi Ae / (wavelength)^2
Where Ae = G (wavelength)^2 / 4 pi

Question 4

Advantages/Disadvantages of Low/High frequencies

Answer 4

Low: Long wavelength, large equipment, less info capacity, less affected by atmosphere.
High: Short wavelength, small equipment, more info capacity, more technically challenging.

Question 5

Propagation modes?

Answer 5

Ducting
Surface
Sky waves
Direct/space

Question 6

What happens to waves as they travel the earths surface?

Answer 6

They are slowed down within the earth and coupling between the above and below ground components results in bending of the wave front towards the surface.

Question 7

What happens to waves near the coast?

Answer 7

A surface wave refracts as it crosses from land to sea or vice-versa.

Question 8

How do waves reflect off the ionosphere?

Answer 8

Waves below the plasma resonant frequency cause electrons to oscillate which then re-emit the wave, effectively reflecting it.

Question 9

What is the skip distance?

Answer 9

The dead zone between their highest range surface wave, the the lowest range sky wave.

Question 10

Derive a formula to describe transmitted energy for an isotopic antenna.

Answer 10

Ep = Et / 4 pi R^2
Antennas have gain so...
Ep = Et Gt / 4 pi R^2
Recovering antenna has effective area Aer
Er = Ep Aer
Er = Et Gt Aer / 4 pi R^2