Scatter is being used by an increasing number of people to work long distances.
For those that do not understand this means of propagation fully, or would
like to improve their chance of completing QSO's by this mode, the following
article will help put you on the path to success.
are small particles of various compositions, They are classified into 3 main
Stony meteorites, composed mainly of silica and magnesium oxides.
Siderites, which contain mainly iron with a small percentage of nickel.
Siderolites, containing mineral and metallic elements in varying proportions.
mainly carbon monoxide, nitrogen and hydrogen are also abundant and are liberated
when the Meteor vaporises during Itís passage into the Earths upper atmosphere.
mass of these objects vary considerably and range between fractions of a milli-gramme
up to 1 kilo-gramme. The physical dimensions range from the size of a grain
of sand to a tennis ball, but does not include the numerous micro-meteorites
which have such small masses that they do not burn up but slowly settle down
through the atmosphere as very fine dust like particles. It should be appreciated
that this is a very generalised statement and objects outside these dimensions
do exist, the point being made, is that in general, Meteors are very small
particles of material being attracted towards us by the Earths gravity. Many
of these particles are of Cometary origin and this is certainly the case for
the Meteor is attracted by the Earths gravitational pull it begins to collide
with molecules of air, which become entrained in the surface. The heat produced
evaporates atoms and it is the collision between the air molecules and the
atoms moving off the Meteorite which produce the familiar sight of a 'shooting
star'. This action produces heat, light and ionisation and in general takes
place around the level of the E layer at a height of approximately l00Km above
the surface of the Earth.
trails extend between 15 and 8OKm depending on the mass, and whether they arrive
vertically or at some other angle to the Earths surface. For the Meteor Scatter
operator it is these 'tubes' of highly ionised particles that can be used to
reflect radio signals very effectively at VHF frequencies, but only when an
electrically conductive condition exists i.e. when free charge carriers (ions)
exist. By the time the Meteor has reached an altitude of 70-80Km the air density
has increased sufficiently to completely vaporise the particle, unless it is
very large and survives to be found on Earth as a Meteorite.
stated earlier it is the ionised trails produced by Meteorís that are used
to Scatter and reflect radio signals. The nature of their make up suggests
that the condition for optimum reflection will not last for very long, this
can be witnessed by the very rapid make up and disappearance of a 'shooting
star' (Meteor).† In some, the ionisation density is very low and Scattering
of the signal takes place rather than, reflection.† These are known as under
dense trails and because of the low electron density signals pass through the
trail and the total received energy is the sum of the individual reflections.
However due to the rapid change in phase angles caused by multiple individual
reflections bursts from under dense trails are very short. It is this condition,
which produces the familiar 'ping' with signals audible for only a fraction
of a second. Other trails produce high levels of ionisation and are known as
over dense. With this type of Meteor trail the high levels of ionisation cause
total reflection of the wave giving much longer bursts of information which
sometimes last for 90s and on rare occasions 2-3 minutes.
reflections will occur it the Meteor trail electron density exceeds the value
for total reflection from an ionised gas. This requires the trail to exceed
1014 electrons per metre of length.
signals reflected from Meteor trails are often subject to considerable fluctuations
in strength. There are two main reasons for this, both of which are due to
more than one reflection being received at the antenna, sometimes in phase,
and adding to the signal and at other times in anti-phase and cancelling.
first are rapid fluctuations directly proportional to the frequency used. These
have been measured by professional pulse methods and found to correspond to
a fluctuation of approximately 22ms at 144MHz. It is caused by a series of
maximum and minimums, which occur during the making up of the Meteor trail,
and is best explained with the aid of the 2 simple diagrams shown in the figures
the Meteor travels along the axis T.T1 insufficient Scatter is produced before
point AZ is reached. Reflections between ZA and ZA1 will travel the return
distance from the observer between 2R and 2(R + Lda/2) but waves Scattered
between AB and A1B1 will travel return distances between 2(R + Lda/2) and 2(R
+ 1Lda). Thus if the reflections from ZA ZA1 are positive in amplitude, those
in AB and A1B1 will be in anti-phase and cancel. Those in BC and B1C1 are in
phase with ZA and ZA1 and so add to the received signal strength. This gives
a characteristic rise and fall in received signal strength. When the trail
is complete the signal will level off and slowly decay as the ionisation is
dispersed in the upper atmosphere by high altitude winds. The second reason,
simplest case is believed to be caused by distortion of the ionised trail due
to severe turbulence encountered in the upper atmosphere.
is quite common when receiving a long burst from distant MS stations to find
periods of several seconds when no signal is present, or at a very low level.
Although this is not always the case it may be attributed to those reasons
STRENGTH AND DURATION
considering scattered signals from under dense Meteor trails the duration is
proportional to the square of the wavelength. In other words, a 1 second burst
on 2m will only have a duration of 0.11s on 70cms. The received energy is proportional
to the third power of the wavelength, which corresponds to a 27:1 reduction
on 70cms compared with 2m. A signal 15dB above noise on 2m will only be ldB
above on 70cms, a 14dB reduction.
For over dense
trails where most of the incident wave is reflected, the duration is still
proportional to the square of wavelength, but the received energy is directly
proportional to the wavelength.† In real terms this means that a burst of 10s
duration, 10dB above noise on 2m would be 1.1s long and 5.5dB over noise on
70cms. On 4m (70MHz) the values would be increased to 42s duration and 16dB
over noise compared with those on 2m. These figures indicate why 70cms is a
much more difficult band to work using this mode of propagation.
must be said that some dedicated 70cms operators have had successful QSO's
on this band, (before WSJT ! ) but compared with 2m, the combination of reduced
received energy and signal duration, make the completion of QSO's very difficult
for all but the very best equipped stations.
distance can I expect to work using Meteor Scatter? It is a frequent question
and one, which depends considerably on the conditions prevailing at the time.
Generally it can be said the ranges expected are very similar to that obtained
when working single hop Sporadic E, normally between 600 and 2000km, although
during levels of high Meteor activity ranges of 3000km are possible.
Scatter is normally considered a weak signal form of communication, Particularly
when stations are placed towards the limits of range.
attempting schedules with stations, at ranges. In excess of 1800km signal strengths
can often be very low ó only a few dB above the receiver noise floor with long
periods of no signals at all. It is under these circumstances that the utmost
patience is required as a burst of information may come along that is sufficient
to complete a QSO when hope is running out. Shorter range stations, those between
1000 - 1500km, can often provide regular, strong signals above S9 when using
some of the major showers and correct timing. This is where most people start
Meteor Scatter operating and develop an interest in this most fascinating form
of VHF propagation.
OF THIS TEXT CONCERNING DOPPLER SHIFT ARE BEING RE-WRITTEN AT THE MOMENT.
maximum velocity of a Meteor entering the atmosphere from within the solar
system is 72km/sec. This limit is made up of two components and is the sum
of the Earth's velocity around the Sun (30km/s) and the escape velocity from
the solar system (42km/s). The value of 72km/s is attained by the November
Leonids and gives a theoretical maximum Doppler shift of 34.5khz These figures
are theoretical maxima and assume the reflecting medium to be moving at this
of course, is not the case in practice because it is only the Meteor head which
is moving at this velocity. The trail of ionisation produced by the Meteor
is stationary except for relatively small atmospheric disturbances, and as
this is the reflecting medium Doppler shift should not normally be evident.
in some instances Doppler shift can be heard and although personal observations
seem to indicate that the bursts are all very short, this may not necessarily
be the case, as often the Doppler shift moves the received signal completely
across the passband of the receiver and the true duration and the amount of
shift are never discovered. It is possible that this phenomenon is due to reflections
taking place from the ionisation surrounding the Meteor head which
may also have a trajectory far from ideal for the path being worked.
Meteors, as the name implies, have a random distribution over the sky with
non-defined orbits. They account for the majority of particles that enter the
Earth's atmosphere although Meteor showers with well defined orbits and high
concentrations provide much improved propagation for short periods only. Sporadic
Meteors are often used by MS operators with considerable success throughout
the year, although certain times of the day, and months of the year give a
definite improvement in communications efficiency.
are certain months of the year which provide a much higher yield of sporadic
Meteors. A peak in activity occurs during June, July and August with minimum
activity occurring in February and March.
to the Earth's motion, certain parts of the day produce higher rates of sporadic
Meteors. As the Earth rotates on its axis, some parts are in sunlight and others
in darkness. During the early morning around 06.00 the observer's part of the
Earth is forward on its journey around the sun and tends to sweep up the Meteoric
particles in its path whereas at sunset the opposite occurs as the
is acting as a shield to incoming particles and only those with sufficient
velocity to overcome that of the Earth's motion will enter the atmosphere.
Any Meteors approaching the Earth towards sunset will need to exceed the forward
velocity of 30km/s whereas at 06.00 optimum conditions exist and the velocities
are additive. The figure below shows this in the form of a diagram which illustrates
the relative motions and times. Although sporadic Meteor Scatter can be used
at any time of the day throughout the Year, best results will be obtained during
the early mornings of the Summer.
certain times every year, the Earth, on its path around the Sun passes through
large areas of concentrated particles resulting in a major Meteor shower. The
distribution is uneven and contained in highly elliptical orbits around the
Sun and are inclined at varying angles compared to that of the Earth. The origins
are certainly Cometary and although the Comets themselves are now extinct in
most cases, the remains continue in predictable orbits and have celestial co-ordinates,
which allow accurate timing and positioning to be made.
radiant point is the position in the sky from which the Meteors appear to originate.
The shower name is taken from the constellation in that part of the sky, which
contains the radiant point. Hence the Geminids shower radiant appears in the
constellation of Gemini and the Orionids in Orion. The only exception to this
is the January Quadrantids, which originates in Bootes.
the Meteors give the appearance of coming from a point source it is an effect
of perspective and in fact they are moving in parallel paths towards us. This
fact can be best understood by imagining two long straight roads running parallel
to each other and stretching towards the horizon. In the far distance they
seem to converge into a single point and this could effectively be looked upon
as the radiant point.
co-ordinates for establishing this point on the celestial sphere are known
as Right Ascension (celestial longitude) and declination (celestial latitude)
angles and are quoted in degrees or time. All Meteor shower radiants have the
same apparent motion as the stars, rising in the East and setting in the West
due to the rotation of the Earth on its axis.
the Right ascension and declination angles are known it is possible to plot
the path of the radiant point onto a plane surface and determine the best possible
times and directions for Meteor Scatter communications in any given shower.
arduous task of making these plots has been eased greatly by the use of computer
predictions. Detailed information on shower peaks and expected ZHR can be easily
accomplished using software distributed by OH5IY. This is simple to use and
will provide detailed information on the optimum times for a given direction
in any shower. This DOS based software can be freely downloaded from his web
THE SOUNDS OF METEOR SCATTER
Would you like
to listen to the sounds of a signal propagated via Meteor Scatter? If you would,
there are 3 separate .mp3 files that may be downloaded here. Each file is via
a different form of transmission, one each of High Speed CW, Single Sideband,
and WSJT. All of these files can be listened to on any media player and gives
a "flavour" of just what a DX MS signal sounds like on each of these
three transmission modes. If you want to decode the HSCW recording then some
form of digital playback/recorder is required. For example, Cooledit 2000 can
be used to slow down the recording and listen to the CW in real time, or as
an alternative, the same program can be used to "see" the morse characters
on screen !
can only be properly read by playing back a .wav file on K1JT's software "WSJT".
This software can be downloaded from the link below. Why not try it? Download
the free software and listen on 144.370 (USB) and chances are you will hear
plenty going on, particularly during shower times. Should you want to download
all 3 "MS sounds" files together in .wav format, they are all zipped
up into a single 300k file.
Download .mp3 files...................All 3 files in .wav>..