hf notes
hi there
i will copy and paste my notes about the HF.
bear in mind thatnormally when you fly in those areas where VHF is not available due to limted coverage, the ATC will direct you to contact the next secor on the HF.
HF radio communications are used in those areas where VHF radio communications are not available due to their limited signal coverage: as matter of fact HF radio signals, as opposed to VHF radio signals which propagate via direct ground waves, use as preferred path of propagation sky-waves reflected back by the ionosphere.
The signal coverage and thus the distance at which the signal can be received depends upon the reflectivity of the ionosphere, upon the impact angle and upon the frequency.
The ionosphere is a layer stretching from about 50km above the earth surface up to 500km, where due to the action of the ultra-violet radiations from the sun the negatively charged electrons become separated from their atoms which acquire a positive charge: when a radio signal enters the ionosphere it experiences refraction which is the path alteration occurring as a radio signal travels through mediums of different density and molecular properties - resulting into its path being deflected from its straight line, bouncing back towards the earth.
The magnitude of the refraction and the distance at which the signal can be received, depend upon the reflectivity of the ionosphere and upon its thickness affected by the amount of ultra violet solar radiations available and thus by to the time of the day - during the day time when solar radiations are abundant up to four different layers referred to as D, E, F1 and F2 in ascending order of height can be counted while at nite as the solar radiations attenuate the number of layers is reduced to two - D and F -; by the time of the year determining the distance of the earth from the sun and by other cosmic factors such as the sunspots or solar flares.
The amount of energy reflected back to earth is also determined by the impact angle defined by the vertical and the propagation path: the angle producing the first sky wave is referred to as the critical angle and its distance from the transmitter represents the skip distance.
An angle steeper than the critical one will result into a large amount of energy travelling through the ionosphere suffering too much attenuation while a shallower angle will result into an increase in the skip distance and thus into an increase in the so called dead zone where the is no radio coverage.
The critical angle is determined amongst other factors by the frequency, the higher the frequency the larger the critical angle and the greater is the skip distance; in addition the higher the frequency the greater is the degree of ionization required to reflect the signal back to earth.
The ideal frequency will then be a frequency referred to as MUF or Maximum Useable Frequency, that will keep the skip distance just short of the receiver: a higher frequency than the MUF will result into an excessive critical angle and thus into an excessive skip distance putting the receiver within the dead zone, while a lower frequency will cause the signal to suffer excessive attenuation.
The MUF is also affected by the time of the day, as matter of fact at nite when the E layer is at its weakest and the signal has to travel all the way up to the F layer to find a similar degree of reflectivity, in order to obtain the same skip distance as during the day a lower frequency must be used.
An altered or modulated signal such a radio signal is comprised of a carrier wave – the original signal - and of two side bands referred to as the upper side band or USB and the lower side band or LSB. The transmitter to limit the amount of power required to broadcast the signal and thus increasing its range will attenuate the carrier wave together with one side band, focusing all the power available to transmit the remaining side band. For aeronautical HF communications normally only the USB is used.
also watch out for those pilot eaters out there ready to launch at you, as soon as you as a sort of obvious question!!!