FINAL REPORT - Stakeholders - Ofcom

• The class of emission, in accordance with current ITU designators, is F7B.
However, any emission type may be used provided the bandwidth does not
exceed that allocated to the frequency in use.
• Any bandwidth may be used provided that the bandwidth allocated to the
frequency in use is not exceeded.
• Since WRC-2003 many frequencies in the exclusive maritime mobile bands
designated in Appendix 17 to the ITU Radio Regulations may be assigned.
These include duplex voice channels, facsimile/data frequencies and frequencies
previously assigned for Morse telegraphy. Frequencies in bands allocated to the
maritime mobile or mobile services could also be used.
• The radio used for this service should:
1. utilise Digital Signal Processing (DSP) techniques;
2. be capable of being controlled from a computer;
3. have a pass-band with no group delay distortion and a pass-band
ripple with a maximum variation of 0.5 dB;
4. be frequency stable to +/- 10 Hz;
5. be able to have its frequency accuracy, power and VSWR monitored
remotely via a computer,
6.5.3 NVIS (Near Vertical Incidence Sky-wave) Propagation Techniques
NVIS, or Near Vertical Incidence Sky-wave, refers to a radio propagation mode which
involves the use of antennas with a very high radiation angle, approaching or reaching 90
degrees, along with selection of an appropriate frequency below the critical frequency, to
establish reliable communications over a radius of around 300 km. Deliberate exploitation
of NVIS is best achieved using antenna installations which achieve some balance
between minimizing ground-wave (low angle of launch) radiation, and maximizing near
vertical incidence sky-wave (very high launch angle) radiation. Successful NVIS
communications depends on being able to select a frequency which will be reflected from
the ionosphere even when the angle of radiation is nearly vertical. These frequencies are
usually in the range of 2-10 MHz, though sometimes the limit is higher. A frequency is
selected which is below the current critical frequency (the highest frequency which the F
layer will reflect at a maximum 90 degree angle of incidence) but not so far below the
critical frequency that the D and/or E layers have an adverse impact.
NVIS techniques concentrate on the areas which are often in the skip zone. The skip
zone is the region consisting of areas of the earth's surface which are outside the
coverage area of the transmitting station's ground-wave but not sufficiently distant far to
receive sky-wave reflections. The goal is to radiate a signal at a frequency which is below
the critical frequency, at a nearly vertical angle, and have that signal reflected from the
ionosphere at a very high angle of incidence, returning to the earth at a relatively nearby
location. Absorption by the D layer, and other factors, determine some minimum
frequency below which the signal will no longer be usable, and usually some distance
beyond which signals will no longer be usable.
One of the most effective antennas for NVIS is a dipole positioned from 0.1 to 0.25
wavelengths (or lower) above ground where vertical and nearly vertical radiation reaches
a maximum, at the expense of lower angle radiation. A dipole can be used at even lower
heights, resulting in some loss of vertical gain.
The advantages of NVIS operation include:
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