|
|||||||||||||
|
|
|
||||||||||||
|
|
|||||||||||||
|
Syllabus Sections:- VHF and Above 5C3 37 Recall that contacts at VHF and above can be made by reflecting signals off the lunar surface and that this is known as Earth-Moon-Earth (EME) propagation. As frequencies above HF principally 145MHz and above can normally penetrate the ionosphere and thus can reach satellites and space craft they can also reach the moon, the non man made satellite of the earth. Such bouncing of signal off the moon is known as EME propagation (Earth-Moon-Earth). 5C3 37 continued Understand that as the moon is a poor reflector of radio frequency signals and is a long way from earth, EME contacts generally need high power and high gain antennas accurately pointed at the moon, and very sensitive, low noise receivers or the use of special low signal strength modes to overcome the path loss. The surface of the moon is a poor reflector of radio frequency signals and taking into account the curve surface of the moon the signals will inevitably spread out not only on their trip to the moon but also on their way back.Thus contacts using EME will need high power with high gain antennas, they will need to be directed accurately at the moon and kept aligned with the moon over a period of time. There is therefore a great loss in power from the signals due to these factors and the huge distance the signals are travelling. Low noise receivers often linked with the special low signal strength data modes are used negate / overcome the path loss. Low noise receivers are specially built using expensive components and needing specialist knowledge to assemble the units. 5C3 37 continued Recall that it is possible to make contacts on the VHF bands by reflecting signals off the ionised gases created during an Aurora and that this occurs at high Northerly and Southerly latitudes and is known as Auroral propagation. Whilst normally VHF band signals pass through the Ionosphere, however when the gasses forming the Ionosphere are highly ionised it creates an effect known as an Aurora which occurs at high Northern and Southerly Latitudes. Such ionisation reflects the radio signal and such propagation is known as Auroral propagation. The reflection is not perfect and thus fading occurs see more below. 5C3 37 continued Recall that auroral ionised curtains form vertically in the ionosphere and that movement of there curtains cause rapid flutter on the signals. The
ionisation of the ionosphere's gasses form "ionised curtains"
which under the right circumstance can be seen with the naked
eye, in the pitch blackness of the night sky, and the curtains
move like your window curtains when blown by the wind. The
movement of the curtains changes the reflective nature of the
gases and causes, as well as fading, extremes of fluttering of
the radio signals and also sudden disappearance of signals
altogether. Auroral propagation is very difficult to predict and sometimes the curtains can only be seen by the use of a camera on a tripod and taking a long exposure photograph, about 30 seconds with a wide aperture of say f1.7. Because of the ever changing nature of the "curtains of auroral ionised gases" which move, any propagation from bouncing signals of the curtains will result in flutter at the receiver.Flutter is a rapid change in amplitude, phase, or frequency but most often is in the form of rapid QSB or fading which changes the amplitude of the signal.
--------------------------------------------------------------------- Other features 5D1 37 Recall the Galactic Noise is random noise originating outside the earth's atmosphere. Galactic noise or "Cosmic noise" is not actually sound you can hear directly, but is radio frequency noise coming from outside of the Earth's atmosphere. Radio receivers can pick up the radio waves and convert then into the audio spectrum so that you can hear the sound.5D2 37 Recall that factors affecting a link budget: transmitter power, feeder losses, antenna gains and path loss. So what is a "Link Budget" ?A "Link Budget" is a way of quantifying the link performance from Transmitter to Receiver taking into account all gains and losses of the power from the transmitter by mediums through which the radio propagation is occurring i.e. Antennas, Free Space, cables, wave guides, fibre optic etc as appropriate. 5D2 37
continued Recall that path loss
includes spreading loss and obstruction losses. Propagation
of a radio signal is not confined to a solid tube linking
the space between the transmitter's antenna and the
receiver's antenna. From the moment the signal leaves the
Transmitter's antenna it is spreading out and can be thought
of as being "a cone like shape" of radio waves and as the
cone's open area becomes larger the amount of signal reduces
at any one point in the cone area and this is know as
"Spreading Loss". You can liken this to a torch beam which
spreads out and thus the beam becomes weaker as the distance
increases. Also obstructions such as buildings, solid or merely structures with holes in the sides, also trees, hills and mountains cause the signal strength to reduce and these are call "Obstruction Losses".
--------------------------------------------------------------------------------
|
|||||||||||||
|
|
|||||||||||||
|
|
|||||||||||||
