Chapter 2 - Propagation

Question 1

What happens to the layers within the Ionosphere at night?

Answer 1

D Layer - Disappears
E Layer - Remains weakly ionised at night
F Layer - F1/F2 combine at night to form a single layer.

Question 2

How does the ground wave propagate and how does the sky wave propagate? What is a typical skip distance in km?

Answer 2

Ground Wave Propagation:
Ground waves travel along the Earth’s surface, bending to follow its curvature. This enables long-distance communication, with ranges up to thousands of kilometers at lower frequencies.

Sky Wave Propagation:
Sky waves refracts off the ionosphere and return to Earth. This allows for long-distance communication, with typical skip distances ranging from 300 to 4000 kilometers.

Typical Skip Distance:
300 to 4000 kilometers.

Question 3

List and describe the abnormal and regular variations in the ionosphere. In each case, which layers are affected and what happens to the ranges / skip distance of the sky wave?

Answer 3

Regular and thus predictable changes in the ionosphere:

1. Diurnal Changes: The height and density of all ionospheric layers vary with the time of day, increasing around noon, thus extending range and skip distance due to increased daytime ionization.
2. Seasonal Changes: All ionospheric layers experience higher ionization in summer due to more intense sunlight, resulting in an increased range and skip distance.
3. Sunspot Cycle: The 11-year cycle of solar activity mainly affects the F layer by increasing ionization, extending range and skip distance during periods of higher sunspot activity.

Abnormal and hence irregular and unpredictable variations in the ionosphere:

1. Sporadic ‘E’: Localized clouds of ionization in the E layer, caused by meteors or electrical discharges, can unpredictably enhance or disrupt HF communication, affecting range and skip distance.
2. Dellinger Fade Outs: Intense UV radiation from solar flares increases ionization in the D layer, causing signal absorption and resulting in short-term communication blackouts and reduced range.
3. Magnetic Storms: Solar flares emit charged particles that cause rapid ionization changes in all layers, leading to fading and reduced range and skip distance for several days.
4. Atmospheric Disturbance: Events like volcanic eruptions or nuclear explosions alter the ionospheric equilibrium in all layers, causing unpredictable changes in range and skip distance.

Question 4

What is NVIS and what can it be used for?

Answer 4

Near Vertical Incidence Skywave - it provides a means of communicating in mountainous or other types of terrain that block line of sight communications.

Question 5

What part of the atmosphere will the space wave propagate through?

Answer 5

Troposphere

Question 6

Two ships have antennas at 32ft. What is the range of VHF comms? What happens if the height is increased to 50ft?

Answer 6

Range(nm) = √2 x Tx Height + √2 x Rx Height
Range(nm) = √2 x 32 + √2 x 32 = 16nm

Range(nm) = √2 x 50 + √2 x 50 = 20nm

Question 7

What atmospheric condition causes extended ranges at UHF. What causes a reduction in range?

Answer 7

Super Refraction
Sub refraction

Question 8

What is the wavelength of 300Mhz? What is the frequency of a wave that is 2m long?

Answer 8

λ = c / f

λ = 3 x10^8 / 300 x10^6 = 1m

f = c / λ

f = 3 x10^8 / 2 = 150 Mhz