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• Coverage of an area by NVIS techniques, which is normally in the skip zone i.e.
that area which is normally too far away to receive ground-wave signals, but not
yet far enough away to receive sky-waves reflected from the ionosphere.
• NVIS requires no infrastructure such as repeaters or satellites. Two stations
employing NVIS techniques can establish reliable communications without the
support of any third party.
• Signals propagated via the NVIS mode are relatively free from fading.
• Antennas optimised for NVIS can usually be erected near to the ground. Simple
dipoles work very well.
• The path to and from the ionosphere is short and direct, resulting in lower path
losses due to factors such as absorption by the D layer.
• NVIS techniques can dramatically reduce noise and interference, resulting in an
improved signal/noise ratio.
• With its improved signal/noise ratio and low path loss, NVIS works well with low
power thus facilitating spectrum sharing.
The disadvantages of NVIS operation include:
• Due to differences between daytime and night-time propagation, a minimum of
two different frequencies must be used to ensure semi-reliable around-the-clock
communications.
• A need for antennas producing a high angle of radiation, the simplest of which
would be a horizontal or semi horizontal dipole, not a convenient antenna for
smaller vehicles or vessels.
The selection of an optimum frequency for NVIS operation depends upon many variables.
Among these are time of day, time of year, solar activity, type of antenna used,
atmospheric noise, and atmospheric absorption.
NVIS could provide for the shared use of the maritime mobile bands included in
Appendices 17 and 25 of the ITU Radio Regulations for localised fixed and land mobile
service operations. It could also be used for broadcasting and other point to point and
point to multipoint radiocommunication services. Maritime HF communications are
unlikely to be used at ranges less than 300 km because the MF maritime bands will offer
a more reliable service. Ship stations receiving from a coast station located within the
same general area as NVIS communication links being operated on land will therefore be
beyond the range of those NVIS links. A possibility remains that ship stations operating
close to the shore using long distance HF communications to contact a shore station in
their own country may be affected; however, ships should normally operate to the nearest
coast station for the purpose of public correspondence. Likewise, there is also a potential
interference problem if the coast station receiving transmissions from a ship, lies within
range of NVIS fixed/mobile operations. The possibility of shared use of some HF
frequency bands between localised services using NVIS techniques and medium/long
range services using low angle launch antennas and oblique angle reflection from the
ionosphere would therefore appear to be feasible.
6.6 MF - Communications
6.6.1 Frequency Allocations (international, Region 1)
The following bands are available for MF communications in ITU Region 1 and the EMA:
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