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:

• 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
  

MF

The ground wave propagates at all times, day and night. The day time service area of a MF station is determined by the characteristics of the ground wave's mode of propagation. This is affected by the carrier wave frequency and the effective conductivity of the ground. On the sea the conductivity is higher than on earth. The distance significantly depends on the power of the transmitting station. All registered ship stations should have enough power to reach shore stations within 150 nautical miles (the minimum requirement for Sea area A2).

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.

HF

HF communications are always transmitted via the ionosphere as skywave propagation. This provides good long range communications. The state of the ionosphere is constantly changing, primarily due to the daily cycle of day and night. At night, the skywave returns to earth at a greater distance because it is refracted from a higher layer in the ionosphere.

Ideal communication is using OTF (Optimal Traffic Frequency). But the ship's radio operator is limited to those frequencies designated for maritime traffic. The official list of those frequencies are published in ITU books, but also in many other publications, like ALRS vol. 1-6. Those frequencies are arranged into so called bands. There are so called 4MHz, 6MHz, 8MHz, 12MHz, 16MHz, 18/19MHz, 22MHz and 25/26MHz bands. All of those bands contain frequencies for voice communications as well as DSC and other (telex, telegraphy...) frequencies. For priority communications designated frequencies are only up to the 16 MHz band as it is enough to cover roughly half of the world.

So, the operator should use the frequency band nearest to the OTF. Exact frequency depends on the communication priority (priority/routine) and type of communication (DSC, voice, telex...). The list of priority frequencies (see appendix) for non-SOLAS vessels is strongly recommended to be clearly visible nearby the radio station. For the routine ship to ship communications the working frequency should be taken from the list (see appendix).

Those frequencies are available also in the publications published by ALRS, ITU or others. Note: these frequencies are simplex. For ship to shore (duplex) communications, the working frequencies are already inside the channels. The channel number is from 3 (YXX) to 4 (YYXX) figures where Y or YY means the Mhz band, while XX means consecutive number within the Mhz band. For example: CH 603 means third channel within 6 MHz band, while CH 1210 means tenth channel within 12 MHz band.

HOW TO CHOOSE THE FREQUENCY

When the operator knows the distance to the other ship or shore station, he must calculate the OTF and then choose the nearest frequency band and then find the appropriate frequency. While the average operator on board ship does not have all data necessary to calculate OTF, the OTF is very often estimable by experience. The following simulation could be useful for quick calculation. Note: the results do not represent an exact situation, but are rounded figures based on experience.