HF (high frequency) radio signals in the
range of 1.8MHz - 30MHz enable Amateur
Radio transmissions to travel World-wide, provided that the right conditions
exist to reflect the signal sent from the transmitting station (from a layer of
the Earth's atmosphere called the Ionosphere)
back to the Earth's surface and back again, perhaps several times before it
reaches
the Receiving station which could be many thousands of Km away on the other side
of the World.
Ionization levels vary
greatly between Day (Left) and Night (Right) causing HF radio waves to reflect
off different layers of the Ionosphere
or travel straight through them depending upon frequency and angle of
transmission. Under certain conditions multiple reflections can occur between
the Ionosphere
and the ground or Sea to enable Worldwide communications. The
Ionosphere
and therefore also the
Maximum Useable
Frequency (MUF) for HF Radio Communication is greatly affected by the Sun's
on average 11 year Solar
Cycle. In 2008 the Solar
Cycle is thought to be at minimum, with HF radio communications much
degraded from Solar Maximum, forecast to occur next in around 2013.
Near Vertical Incidence
Skywave (NVIS) is a radio wave propagation method that provides
usable signals in the range between
Groundwave
and
Skywave distances (usually 30 to 400 miles, or 50 to 650 km). It is
used mainly for military communications and by radio amateurs. The
transmission frequency is selected close to, but not over, the critical
frequency (the highest frequency which the F layer will reflect at a
maximum--90 degree--angle of incidence)
travel at very high angles approaching 90
degrees upwards into the
ionosphere, where they are
refracted back down and can be received within all distances from 0
km to 650 km from the transmitter. If the frequency is too high,
refraction fails to occur and if it is too low absorption reduces the
signal strength.
Click on the
following link, or the thumbnail below, to view the latest
NVIS
Europe foF2 Ionospheric Map. This
displays the highest Critical frequency for NVIS Communications and in
the example shown below for stations based in the UK 4 MHz is the maximum, so
radio amateurs would use the 80m
(3.5-3.8 MHz) band for the time and date shown for NVIS Regional UK HF
Communications.
The usable
frequencies for NVIS
communications are between 1.8 MHz and 15 MHz. The most common bands
used in amateur radio are 3.5 MHz and 7 MHz, with experimental use of 5
MHz (60
meters) frequencies. Military NVIS communications mostly take place
on 2-4 MHz at night and on 5-7 MHz during daylight. The lowest layer of
the ionosphere, called the
D layer, causes attenuation of low frequencies during the day. This
layer disappears at night enabling improved communications at the lower
frequencies during the night.
An
NVIS antenna configuration
is a horizontally polarized (parallel with the surface of the earth)
antenna placed low to the ground to ensure a high angle of takeoff,
Vertically polarised antennas are not suitable for NVIS as they produce
low angles of radiated signals and therefore the skip distance will be
too great for local HF communications.
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Latest HF DX cluster spots |
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The conditions
that affect HF signals are in the main determined by the current Solar
Cycle, which peaks every 11 years on average, unfortunately in 2007 we were nearing the Solar
minimum, which means that conditions were poorer and many of the
higher frequencies above 14 MHz could not be reliably used.
Whether the path is in daylight
or darkness also affects which layers of the Ionosphere
will reflect HF signals and at what frequency. Amateur
Radio FAQ.
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Near-Real-Time MUF map |
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The following map
shows Maximum Usable Frequencies (MUFs) for 3000 kilometre radio signal
paths. More importantly, the current sunspot number (SSN) and Planetary
A-index are updated every 30 minutes on the bottom of this image.
Additionally, the grey line position, Auroral ovals, and sun position are
provided. Click on the map for more details on how to use it.
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Spot Database Search |
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160m band (1.8 MHz)
Not much shorter than medium waves, at
daytime the "Top Band" is deeply affected by D-layer absorption and
only radio waves entering the ionosphere at a very high angle can be
reflected to the ground. At daytime this band is thus mainly dedicated
to local QSOs by ground waves, up to distances reaching about 120 km.
It is thus relatively quiet compared to the other HF bands. At night
the D-layer disappears, allowing low-angle signals to reflect more
easily on via the F-layer, and DX contacts are possible at several
thousands of kilometres. This is a band to mainly use at night.
80m band (3.5 MHz)
This band is similar to 160m but as
the frequency increases the D-layer absorption begins to decrease as it is
proportional to the inverse square of the frequency. At daytime most contacts
are worked with near countries, up to about 2000 km from your QTH. At night or
using the gray line it is possible to exceed 9000 km (e.g. Europe to W, UA or JA).
If you wake up in winter a few hours before the sunrise to avoid atmospheric
noises you can work most DX stations (e.g. VK, ZL from Europe). In Region 1 the
frequencies ranging between 3775-3800 kHz are usually dedicated to DX hunters.
40m band (7 MHz)
This band is still under the influence of
the D-layer and is the lowest band showing an appreciable skip distance, up to
500 km at night. At noon it is hard to work stations located over 800 km away,
while working with the gray line or at night, this band is open to DX contacts.
This band is slightly influenced by 11-year solar cycle. Atmospheric noises are
still present, but not as strong as on the lower bands. QSOs can however be
difficult during summer months, but signals are not completely overridden by
static. This is also one of the most crowded bands and do not be surprised to
find in Europe and during weekends each station within 1 kHz or so from each
another.
Recently in Region 1 this band was made
made accessible to amateurs from 7100 kHz up to 7200 kHz. Note that it is
already open to amateurs to 7300 kHz in North and South America.
30m band (10 MHz)
Conditions are similar to the 40m band,
but this band works better in summer showing some of the properties of the 20m
band. As communications reach 1600 km at daytime and 12000 km or more at night,
this band is considered open for 24 hours a day. It is also the band least
affected by variations of the solar cycle. However this band is affected by the
ionization level of E and F-layers and at night, during the minimum of the solar
cycle, it is regularly above the MUF for most DX paths, becoming thus the higher
workable frequency for night time communications. With very few exceptions this
band is reserved for digital modes and CW.
20m band (14 MHz)
This is the primary HF band most used by Radio
Amateurs and open throughout the solar cycle in daylight hours. Indeed, the 20m band is the DX band par
excellence and is considered by many hams as the most reliable band for hunting
DX stations because atmospheric noises are weak.
It is practically usable all the day long as soon
as there is propagation, and mainly in summer, and all the more during periods
of high solar activity. It is only during the minimum of the solar cycle that this band closes down in the late afternoon
and is unusable at night.
The 20m band shows an appreciable skip distance
reaching about 700 km in daytime and exceeding 1600 km at night. It is
thus not suited to local QSOs.
17m band (18 MHz)
This band is similar to the 15m band in
many respects although its activity is affected by the 11-year solar cycle, but
not as pronounced. During the maximum of the solar cycle this band is open
all day, up to well after sunset. When the solar cycle decreases this
band closes earlier, just after sunset. At the minimum of the solar cycle
this band opens to middle or equatorial latitudes, allowing north-south contacts,
but mainly around noon. This band is mainly open during daytime with the skip
distance reaching 1200 km in daytime and
disappearing at night.
15m band (21 MHz)
This band works similar to
the 17m band and is considered by most amateurs as a quieter alternative to the
20m band. It shows however a greater sensitivity to the fluctuations of the
solar cycle. It is mainly used in daytime but during the peaks of solar activity
DX contacts can extend to the night. During the minimum of the solar cycle this
band can be closed except for a few trans-equatorial paths. Like 12m and 10m
this band is also subject to weak Sporadic-E activity mainly in early summer and
mid-winter. The skip distance reaches 1300 km at daytime and disappears at
night.
12m band (24 MHz)
This band varies depending on the solar
cycle and combines the best from the 15m and 10m bands. One year before
solar cycle maximum and up to one year after this peak, this band allows DX
contacts until after sunset with practically any kind of antenna. However, when
the solar cycle descends from maximum, this band is open at mid and low
latitudes only at daytime, with very few openings after sunset. Near the minimum
of the solar cycle this band becomes unusable, except at daytime for the highest
latitudes with some North-South openings. This band is open to Sporadic-E
traffic between the late spring and early winter. The skip distance reaches 1600
km at daytime and vanishes at night.
10m band (28 MHz)
This band works in the same way as the 12m
band, but is characterized by a great variability according to the solar cycle.
During the peaks of the solar cycle DX contacts can be established with very low
power and over 12000 km away. It is mainly a daytime band, but remains open a
few hours after sunset. During moderate solar activity this band opens near noon
for some trans-equatorial communications, but is closed for the higher
latitudes. During the minimum of the solar cycle this band is straight out dead.
There are however some exceptional openings for Ionoscatter, Meteor Scatter and
Sporadic-E. This occurs mainly between April and early August allowing multihop
communication up to 4100 km. This band is relatively quiet compared to the 15m
band, for example, but it works globally very well for DX communications. The skip distance reaches 2000 km at daytime.
The holder of an Advanced (Full) UK licence
may allow an unlicensed person to send a greetings
message provided it is under the direct supervision of the licensee, or if a
club an
authorised club member, who must operate the transmitter and identify the
station. Each greetings message must not be longer
than five minutes and each person may send only one message to each station with
which the station is in contact. Greetings messages
may be sent and received only within the UK, or to and from stations in the
United States of America, the Republic of Maldives, Gibraltar, Malta and
Falkland Islands. Greetings messages may also be sent to or from stations in
Canada and Pitcairn Islands provided that each greetings message does not exceed
two minutes. When using a digital mode, anyone may type the message for
transmission.
