6a.1 Recall the basic structure of the ionosphere: D, E and F layers and their order.
Understand that ionisation is caused mainly by ultraviolet rays from the sun.
6a.2 Recall that the level of ionisation changes with the time of day,
the time of year, and according to the 11-year sunspot cycle.
Understand that the sunspot number is an indicator of solar activity
and that more sunspots give better HF propagation as a result of increased ionisation.
Radio
waves reach a receiving stations either by ground waves or by
reflection from the ionosphere. The Ionosphere consists of several
layers that have become ionised (charged) mainly by ultraviolet rays
from the sun.
The three layers of the Ionosphere that mostly affect radio waves are called the D, E and F layers.
The level of ionisation of each of these layers varies according to:
The time of day
The time of year
The 11- year sunspot cycle
Counting the number of sunspots gives an indication of HF propagation.
The higher the number of sunspots, the better HF prorogation.
6a.3 Recall that reflection from the F layer is the main mode of HF propagation.
Most HF prorogation takes place by reflection from the F layer. During
the day this consists of two layers but at night these combine to form
one layer
6a.4 Understand the meaning of ground wave, sky wave, skip distance and skip zone (deadzone).
The D layer is good at absorbing LF signals. The D layer is present
during the day and disappears at night. This is why on 160 and 80
metres there is little to be heard during the day because the signals
transmitted are absorbed and not reflected. At night the D layer
disappears and signals can be reflected from the E and F layer.
On the diagram, an HF signal (above 80m) passes through the D and E
layers and is then reflected back to earth. The angle of incidence is
equal to the angle of reflection, so if the take off angle from the
antenna is lowered the signal will travel further. To maintain UK to UK
contacts a high angle of take off is required.
The ground wave is the signal following the surface of the Earth and is
restricted in how far it can travel. High power medium wave
broadcasting stations can transmit between 50 to 100km by ground wave.
The sky wave is one that is reflected from the ionosphere.
The skip distance is the distance, measured on the ground, between station A and Station B
The skip zone or deadzone is the zone between the end of the
groundwaveA and the start of the groundwave B. In this area stations A
and B would not be received because they too close to pick up sky waves
and too far to pick up ground waves.
6a.5 Recall that high atmospheric pressure can cause ducting in the
troposphere, which increases the range of VHF and UHF signals.
Recall that the range of VHF signals can occasionally be significantly
increased by reflection from highly ionised areas in the E layer
(Sporadic E).
6a.6 Recall that VHF and UHF signals normally pass through the
ionosphere, and at these frequencies propagation is within the
troposphere situated below the ionosphere.
It
is not normal for VHF and UHF signals to be reflected from the
ionosphere. They usually pass through and out into space. This is why they
are used for satellite communication. Most long distance contacts on
VHF/UHF are made by transmission
through the troposphere under certain conditions.
The
troposphere is the lowest layer of the atmosphere where the weather
occurs. This is about 12Km wide. When the pressure in this area rises
radio waves at VHF and UHF can be ducted over considerable distance
either:
By reflection along a duct between the Earth's surface and an upper layer of warm air
By reflection within an upper layer of warm air
Another way in which VHF
signals can travel further than normal is by reflection from the E
layer when it is strongly ionised. This is called Sporadic E. In the UK
this occurs mainly in summer when there are areas of high pressure..
6a.7 Recall that snow, ice and heavy rain can attenuate signals at UHF and above.
Attenuation of signals at UHF
As the wavelength gets shorter snow,
ice and heavy rain can absorb (attenuate) UHF signals. So if these conditions
occur between two stations the signal strength may drop
6a.8 Recall and manipulate the formula v=f×λ. Calculate frequency or wavelength given the other parameter.
For calculations, the velocity of radio waves will be given. Relationship between frequency and wavelength
As amateurs we tend to use the terms frequency and wavelength interchangeably. For example the 20m band or 14MHz band.
The equation for converting one to the other is:
V=f×λ
Where V is the velocity of radio waves in metres per second, f is the frequency in Hertz and λ (Lambda) is the wavelength in metres per second.
The speed of radio waves = 300,000,000 m/s 0r 3x108 m/s
So V=f×λ and f=V/λ
and λ = V/f
NB In the exam you will be expected to calculate frequency or wavelength. The speed of radio waves will be provided.
