AOR AR8000 Operating Manual page 114

You will note that large areas of the Earth's surface lays between the point of
transmission and reflection, in this area there will be little or no reception. For
this reason "F" layer propagation is often referred to a "SKIP" and the reflected
signal as "SKY WAVE".
Generally speaking only frequencies below 30MHz are reflected by the
ionosphere. Higher frequencies pass straight through even the "F" layers and
will continue outward into space for ever.
Choice of frequency
Depending upon the time of day and desired skip distance, different
frequencies will be selected by Radio Amateurs and commercial users such
as Oceanic Air Traffic.
For instance the "MUF" (Maximum Usable Frequency) is often stated for a
path between two locations. Choosing a frequency above the "MUF" will not
produce results as transmissions will pass straight into space.
Many propagation predictions and statistics are published and usually
available from most country's National Amateur Radio and Shortwave
Listeners representatives.
Various publications are produced giving transmission and contact details for
World wide reception. These titles include:
World Radio TV Handbook (WRTH), BPI Communications, 1515 Broadway,
New York 10036, NY USA.
Passport To World Band Radio, IBS North America, Box 300, Penn's Park PA
18943, USA.
Listings for utility services are also widely published and available.