Radio Wave Propagation

Question 1

What is diffraction and what causes it?

More or less diffraction at lower frequencies?

Answer 1

Spreading of the wave in to a circular waveform caused by the wave travelling through a hole or over a sharp object

More

Question 2

What is attenuation?

Can it happen in a vacuum?

Answer 2

loss of signal strength of a wave

no because there is nothing to give up energy to

Question 3

Ionosphere divided into … layers labelled as …

Answer 3

three

D, E & F

Question 4

What is ionospheric attenuation?

increases with?

Answer 4

Loss of energy to electrically charged atoms at the edge of space

reducing frequency

Question 5

Formula for max range of VHF/UHF space wave in NM?

Answer 5

(1.23 x square of H₁) + (1.23 x square of H₂)

H₁ = height of transmitter in feet AMSL

H₂ = height of receiver in feet AMSL

Question 6

Radar reflection most likely when the wavelength is what size?

Answer 6

Same size or smaller than target

Question 7

What radio waves refract the most?

Answer 7

Lower frequencies/longer wavelengths

Question 8

The gap between the surface wave and first returning sky wave where no signal will be received is called …

Answer 8

dead space

Question 9

What layer of ionosphere is only present by day and is … to earth?

Answer 9

D

Closest

Question 10

VHF and UHF have what kind of propagation?

Answer 10

Space wave (line of sight)

Question 11

What is atmospheric attenuation?

Increases with increase in … and becomes significant above …

Answer 11

electrons in particles of dust and water absorb some of the energy in the wave

frequency

1GHz

Question 12

What is refraction and what causes it?

Answer 12

A change in direction or bending of the radio wave as the medium the wave is travelling through changes density

Question 13

The amount of refraction of a radio wave depends on?

Answer 13

The change of speed of the wave, it’s wavelength and the angle at which it hits the new medium

Question 14

Combination of space waves and surface waves is called?

Answer 14

ground wave

Question 15

What is surface attenuation?

Increases with …

Fastest speeds over …

Answer 15

Loss of energy and slowing of the wave as it loses energy to the earth’s surface it is flowing over

frequency

water

Question 16

In what layer is the density of the ionosphere the greatest?

Is this closer or further from earth?

Answer 16

F - split into F1 and F2 by day and combine to become one at night

Further

Question 17

What is sporadic E?

Answer 17

Unusually long reception range of VHF waves caused by more heavily ionised E layer due to increased solar activity

Question 18

What are sky waves?

Answer 18

Waves refracted in the ionosphere causing them to return to earth over large distances

Question 19

Distance from transmitter to first returning sky wave is called … and will increase/decrease with increase in frequency?

Answer 19

minimum skip distance

decrease (because higher frequency waves do not refract as much, giving shorter skip distance)

Question 20

Reflection sometimes occurs when it otherwise wouldnt when?

Answer 20

Waves hit a surface at a very shallow angle

Question 21

List the three things that influence reflection

Answer 21

wavelength

density of materials

angle of incidence

Question 22

Why is HF better than lower frequencies for sky waves?

Answer 22

Although lower frequency waves refract the most, they are also the ones that suffer most ionospheric attenuation

Question 23

What are surface waves?

What limits the distance of them?

What frequencies travel furthest via surface wave and why?

Answer 23

Waves following the curvature of the earth caused by diffraction of the wave as it passes over hills and buildings etc

limited by surface attenuation

low frequencies travel furthest because less affected by surface attenuation

Question 24

What is atmospheric ducting?

what frequencies?

also known as?

Answer 24

unusually long range propagation of VHF, UHF, SHF and EHF caused by atmospheric conditions creating an atmospheric duct, such as an inversion and rapid decrease in humidity with height. Extends line of sight range by approx 15%.

Also referred to as super refraction

Question 25

What is ionospheric ducting?

what frequency?

distance?

Answer 25

In the VLF band the wavelength is so low that sky waves not normally expected but the point is reached where the waves are no longer refracted but are reflected from ionosphere. Barely any attenuation because the waves barely enter the ionosphere and gap between earth and ionosphere acts as a waveguide

4000nm distances

Question 26

What causes static?

what is noise?

Answer 26

natural phenomenon caused by electrical discharge in convective clouds

insignificant above VHF

man made interference caused by unshielded electrical equipment when ac current and sparking produce electromagnetic fields

affects VHF and above