6a.1 Recall that radio waves travel in straight lines, unless diffracted or reflected.
When radio waves leave an antenna they travel in straight lines. However they can be bent by:
Diffraction
-Diffraction means the waves bend round an obstacle or spread out as
they go through a small gap and end up in places they would not
normally be expected.
For example when radio waves pass over the sharp edge of a mountain
they can be diffracted away from the straight line in which they are
travelling down to the other side of the mountain.
Reflection -
Radio waves can be reflected by the ionosphere. This consists of a
number of layers of ionised (charged) molecules. See below
6a.2 Recall that radio waves get weaker as they spread out. As a radio wave spreads out from the antenna the signal becomes weaker.If
you have a VHF receiver local stations will be very strong, but,
assuming they are using the same power and antenna, more distant
stations will be weaker
6a.3 Recall that at VHF and UHF hills cause “shadows” and that waves get weaker in penetrating buildings but glass windows are more transparent to radio waves.
Although VHF and UHF signals can be
diffracted from a mountain top it is more likely that diffraction does
not take place. The area behind the mountain will be in an RF shadow
and often no signal can be received because the rocks in the mountain
absorb any signal trying to pass directly through the mountain.
Buildings also absorb RF energy. Walls
absorb more RF. Walls reinforced with metal will absorb even more RF
energy. However, glass absorbs less RF.
6a.4 Recall that the range achieved at VHF/UHF is dependent on antenna height and a clear path and transmitter power. Understand that higher antennas are preferable to higher power as they improve both transmit and receive performance. Recall that outdoor antennas will perform better than indoor antennas.
There are three ways to increase the range of a VHF and UHF signal:
Increase the antenna height
Ensure that the antenna is above other buildings and outside rather than inside.
Increase transmitter power
use a directional antenna
Increasing the height is
better than using more power because it will increase the strength of
signal transmitted and received. Increasing power only improves the
transmitted signal. Indoor antennas will suffer because of absorption of the signal by bricks, rooves and windows.
6a.5 Recall that, at VHF/UHF, range decreases as frequency increases and that in general, VHF/UHF waves have a range not much beyond ’line of sight’.
At VHF and UHF the range decreases as
the frequency increases. So, if you use the same power into a dipole on
each band you would expect to get further on 4m, then 2m and further on
2m than 70cm.
Propagation - Ionosphere basics
6b.1 Recall that the ionosphere comprises layers of conductive gases at heights between 70 and 400km above the earth.
The ionosphere consists of layers of
conductive gases between 70 and 400km. These are important in
reflecting signals back to earth.
.
6b.2 Recall that on HF most communication relies on the waves being reflected by the ionosphere. Recall that HF can provide world-wide propagation depending on how well the ionosphere bends the waves back to the earth. Recall that this varies with frequency, time of day and season. On the HF bands the conductive gases in the ionosphere reflect and refract radio waves allowing world-wide communication The layers of conductive gases are
constantly changing their height and thickness. This depends on the
amount of waves (e.g. light) from the Sun. And so propogation
will depend on the time of day and the season. Different layers reflect or absorb
radio waves at different frequencies in different ways. So propogation
also depends on the frequency used.
The diagram opposite shows the
reflection of radio wave from the top layer of conductive gases. In
this case station A can communicate with Station B over a distance of
about 2500km. The communication distance can be increased if the
the conducting layer is higher, the angle of take off from the antenna
is lower and if conditions allow multiple hops.
