Low density also means it takes a lot of UV to return
a signal to Earth — especially at the higher frequencies
where less bending occurs. In fact, if the frequency is too
high, the signal is eventually lost to outer space as shown
in Figure 6.
Assuming the signal is returned to
Earth, the distance at which it reaches the
ground again is called the skip distance, and
one such up-and-back trip is called a hop.
Typical hops involving the F layer span from
1,500 to 2,500 miles on the higher HF ham
bands from 14 to 28 MHz.
Note that the signal can also be
reflected by ground or water, then make
another hop. That’s how contacts span the
thousands of miles between North America
and other continents.
Even though each hop attenuates the
signal by about 20 dB (a factor of 100),
modern antennas and transceivers are good
enough for “solid copy” — even between
stations at opposite sides of the planet
when conditions are right.
With all this activity and inter-related effects, is it
possible to predict ionospheric propagation as Hepburn
14 July 2016
FIGURE 8. VOACAP Online prediction chart for circuit
reliability between Jefferson City, MO and Rio de Janeiro
with the same stations and solar activity as in Figure 7. The
heavy black line represents the MUF — Maximum Usable
Frequency — at which communications are possible 90
percent of the time.
FIGURE 7. VOACAP Online coverage map from Jefferson City, MO at 8 PM in
April on the 14 MHz amateur band for stations using half-wave dipole antennas
at a height of 10 meters and CW (Morse code). Coverage should be excellent
across the Americas, west Africa, and much of the eastern Pacific with current
sunspot activity values.