HF RADIO TRANSMISSION
FREQUENCY, DISTANCE AND TIME OF DAY
428 ENVOY™ TRANSCEIVER REFERENCE MANUAL
Frequency, distance and time of day
The extent to which an HF radio wave is reflected depends on the frequency that is
used. If the frequency is too low, the signal is absorbed by the ionosphere. If the
frequency is too high, the signal passes straight through the ionosphere. Within the HF
band, low frequencies are generally considered to be in the range of 2 to 10 MHz.
High frequencies are above 10 MHz.
A frequency chosen for day-time transmission may not necessarily be suitable for
night-time use. During the day, the layers of the ionosphere are thick. The layers
absorb lower frequencies and reflect higher frequencies. At night, the ionosphere
becomes very thin. The low frequencies that were absorbed during the day are
reflected, and the high frequencies that were reflected during the day pass straight
through.
Summer HF radio communications usually operate on higher frequencies than those
used in winter over the same distance.
Solar activity varies over an 11-year cycle. Higher frequencies need to be used during
periods of peak activity.
It is important to remember that you may need to change the frequency you are using
to achieve the best communication. The general rules of thumb for effective HF radio
communication are:
• the higher the sun, the higher the required frequency
• the further the distance, the higher the required frequency
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