Propagation - Shortwave Bands - AOR AR8000 Operating Manual

This provides a screen for potential incoming interference and passes any
noise down the cable away from the receiver and toward the earth rod.

(23) Propagation - shortwave bands

VHF and UHF transmissions generally only propagate relatively short
distances when compared to short wave signals. For all intensive purposes
they may be considered as line-of-sight plus a bit.
Where as point to point communication between mobile users or when in built
up areas may only be a couple of miles, aircraft at heights of 30,000 feet may
be heard for many miles (50 to 200 with the right conditions).
Occasionally "tropospheric" weather conditions or "sporadic E" layer ionisation
enable VHF-UHF signals to travel many hundreds of kilometres.
Unlike VHF and UHF transmissions which generally propagate only on a
localised basis (to the horizon plus a small amount), shortwave transmissions
may travel for many thousands of kilometres. Depending upon the frequency
in use, time of day, season of the year and sun spot activity, transmissions
may propagate completely around the World.
Radio signals are electromagnetic waves very similar to light beams. As such
they do not readily follow the curvature of the Earth but attempt to travel out
into space.
The ionosphere
Luckily the frequency spectrum of shortwave is often reflected back down to
Earth by the upper layer of the Earth's atmosphere called the ionosphere.
When the reflected signals reach the Earth again they may either be
received or reflected back up into space. If lucky, they will be reflected by the
ionosphere yet again down toward the Earth providing reception into another
and possibly more distant location.
The ionosphere is constructed of many layers of ionised gas. Of particular
interest to shortwave listeners' are the lower "E" and upper "F1" & "F2" layers
although a lower "D" layer exists during day time.
"D" layer
During day time the lower "D" layer forms around 60 to 80 kilometres above
the Earth's surface. This "D" layer tends to absorb low frequencies reducing
the distance covered by medium wave transmissions. In the night time when
the "D" layer dissipates, medium and low frequency transmissions may
propagate over much greater distances.