In the High Frequency range and partly in the MF range (actually from about 1 MHz to 30 MHz), sky-wave propagation is most affected by ionospheric reflection, and as the density of atmospheric ionization fluctuates, so must the frequency, a higher frequency corresponding to higher density of ionization. Variations in propagation length occur according to such factors as time of day, year, sunspots as well as other ionospheric conditions.
During the day, the reflection of the wave back from the ionosphere occurs lower (layer E), so more waves are necessary for longer propagation and therefore, a higher frequency must be used. At night, the wave reflections are from the higher layer F2, so a lower frequency generally suffices.
The data about the height of ionosphere is provided daily by several observatories. The most known European observatory is the National Observatory of Athens in Greece, which publishes data for the whole world (www.iono.noa.gr). Here an operator can find the current status of layer height ( h ) and density (f0).
The angle of reflection (alpha) depends on both the layer height and distance (d) across land or sea of the point of reflection and may be calculated by the formula:
alpha = arc tg ((d/2)/h)
Where only one reflection occurs during propagation, this is called 'single hop' propagation. The more hops, the greater the distance of propagation, but the lesser the quality of transmission and the more energy required. Whenever possible single hop is used.
To reach a certain station (ship, RCC, etc.) with a single hop the communication may be done only within the limits from LUF (Lowest Usable Frequency) and MUF (Maximum Usable Frequency). The optimum communication is called OTF (Optimum Traffic Frequency), which is around 85% of MUF. As the radio operator may not choose just any frequency, but only those designated for maritime traffic, the operator should use the nearest available frequency. MUF and OTF are calculated by the formula:
MUF=f0/cos (alpha)
OTF=MUF * 0.85
LUF depends upon transmitter power, sensitivity and tuning of the receiver
The distance the signal travels before reaching the earth again is called the skip distance. It will also be found that there is often an area between the limit of the ground wave coverage and the reflection zone (where the skywave returns to earth) where no signal can be received. This area is called the dead zone or skip zone. To communicate within the skip zone, the frequency must be lowered.
In general:
During the night a signal is propagated as a sky wave via the ionosphere as well as a ground wave, so the distance of possible communication is greater. The reflection from the ionosphere causes what is called a skip distance, but within MF propagation, this distance is covered by the ground wave so there is no skip zone.
Normally the ship is using DSC to establish the connection with other stations. For priority communications (distress, urgent, safety) the DSC frequency 2187.5 kHz should be used. But for the ship's routine calls (all non-priority) the simplex frequency 2177 kHz is used for ship to ship calls, while for ship to shore call the duplex frequency 2189.5 kHz (ship transmit)/2177 kHz (ship receive) must be used.
When the connection is established, the radio operator should move to working frequency. For priority communications (distress, urgent, safety) by radiotelephony frequency 2182 kHz should be used. For routine communications, the operator should consider the frequency list for a particular MF region (see appendix). Ship to ship communications are possible on frequencies 2045 kHz or 2048 kHz in region 1, while for regions 2 and 3, the frequencies are 2635 kHz or 2638 kHz. For ship to shore communications, the shore station provides the ship working frequencies.
During the day, the reflection of the wave back from the ionosphere occurs lower (layer E), so more waves are necessary for longer propagation and therefore, a higher frequency must be used. At night, the wave reflections are from the higher layer F2, so a lower frequency generally suffices.
The data about the height of ionosphere is provided daily by several observatories. The most known European observatory is the National Observatory of Athens in Greece, which publishes data for the whole world (www.iono.noa.gr). Here an operator can find the current status of layer height ( h ) and density (f0).
The angle of reflection (alpha) depends on both the layer height and distance (d) across land or sea of the point of reflection and may be calculated by the formula:
alpha = arc tg ((d/2)/h)
Where only one reflection occurs during propagation, this is called 'single hop' propagation. The more hops, the greater the distance of propagation, but the lesser the quality of transmission and the more energy required. Whenever possible single hop is used.
To reach a certain station (ship, RCC, etc.) with a single hop the communication may be done only within the limits from LUF (Lowest Usable Frequency) and MUF (Maximum Usable Frequency). The optimum communication is called OTF (Optimum Traffic Frequency), which is around 85% of MUF. As the radio operator may not choose just any frequency, but only those designated for maritime traffic, the operator should use the nearest available frequency. MUF and OTF are calculated by the formula:
MUF=f0/cos (alpha)
OTF=MUF * 0.85
LUF depends upon transmitter power, sensitivity and tuning of the receiver
The distance the signal travels before reaching the earth again is called the skip distance. It will also be found that there is often an area between the limit of the ground wave coverage and the reflection zone (where the skywave returns to earth) where no signal can be received. This area is called the dead zone or skip zone. To communicate within the skip zone, the frequency must be lowered.
In general:
- day MUF is higher than night MUF
- summer MUF is greater than winter MUF
- if sun activity arises, MUF and OTF increases, skip distance decreases
- if sun activity decreases, MUF and OTF decrease, skip distance increases
During the night a signal is propagated as a sky wave via the ionosphere as well as a ground wave, so the distance of possible communication is greater. The reflection from the ionosphere causes what is called a skip distance, but within MF propagation, this distance is covered by the ground wave so there is no skip zone.
Normally the ship is using DSC to establish the connection with other stations. For priority communications (distress, urgent, safety) the DSC frequency 2187.5 kHz should be used. But for the ship's routine calls (all non-priority) the simplex frequency 2177 kHz is used for ship to ship calls, while for ship to shore call the duplex frequency 2189.5 kHz (ship transmit)/2177 kHz (ship receive) must be used.
When the connection is established, the radio operator should move to working frequency. For priority communications (distress, urgent, safety) by radiotelephony frequency 2182 kHz should be used. For routine communications, the operator should consider the frequency list for a particular MF region (see appendix). Ship to ship communications are possible on frequencies 2045 kHz or 2048 kHz in region 1, while for regions 2 and 3, the frequencies are 2635 kHz or 2638 kHz. For ship to shore communications, the shore station provides the ship working frequencies.
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