Propagation (6a1, 6a2, 6a3, 6a4, 6a5, 6b1 & 6b2)
Introduction to Propagation
When radio waves leave the transmitting aerial they travel in straight lines, unless they are diffracted (bent) or reflected.
Sometimes radio waves get diffracted as they pass over a hill, just as sounds spread out as they pass through an open door.
The radio waves spread out from the transmitting antenna. The further away from the transmitting antenna the greater the area they have to cover, and so are less strong.
Propagation at v.h.f. and u.h.f.
On the v.h.f. and u.h.f. bands hills cause shadows, so
the signals may weaker or not even heard the other side of the hill.
The waves also get weaker when they try to penetrate a building. Glass windows are more transparent to radio waves than bricks and tiles.
The range that can be achieved at v.h.f. and u.h.f. is dependent on antenna height and a clear path and transmitter power.
- Higher antennas are preferable to higher power, as high antennas improve both transmit and receive performance.
- Outside antennas perform better than indoor antennas.
- If losses are high, then a clear path is much more effective in getting a good signal than a 10 or even 100 times increase in power.
- For example amateur satellites can be contacted with very low power at great distances.
Typical Ranges:-
· A small hand-held transceiver - a few km.
· A mast mounted antenna – tens of km.
At v.h.f. and u.h.f. the range that a signal can be heard decreases as frequency increases. So a signal on 432 MHz will not go as far as a 50MHz signal with the same transmitter power and antenna gain.
In general v.h.f. and u.h.f. waves have a range not much beyond “line of sight”. Some refraction takes place at the horizon, so the signals do go a little beyond “line of sight”. Hills and buildings cause path loss.
Propagation at h.f.
Above the earth at heights of between 70km and 400km are a series of layers called the ionosphere.
These layers of “conductive gases” can reflect h.f. signals back to earth.
It is this reflection of h.f. signals that allows worldwide propagation depending on how well the conductive layers reflect the signals back to earth.
The amount of reflection varies with frequency, time of day and season. This is why some bands may be “open” for distant contacts and others “closed”.
During the Day – Higher Frequencies (up to 30MHz or even higher) are bent back to Earth and the Higher Frequency Bands are ‘Open’ to a wide area.
During the Night – Only Lower Frequencies are bent back to Earth. Higher Frequencies escape out to space and the Higher Frequency Bands are ‘Closed’ to a wide area.
Even higher frequencies are not reflected back to earth and they escape out into space, which is why v.h.f. and u.h.f. cannot normally rely on this type of propagation.