Meteor Scatter propagation

Meteor 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.

WHAT ARE METEORS

Meteors are small particles of various compositions, They are classified into 3 main types.

1. Stony meteorites, composed mainly of silica and magnesium oxides.

2. Siderites, which contain mainly iron with a small percentage of nickel.

3. Siderolites, containing mineral and metallic elements in varying proportions.

Gases mainly carbon monoxide, nitrogen and hydrogen are also abundant and are liberated when the Meteor vaporises during Itís passage into the Earths upper atmosphere.

The 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 major showers.

METEOR TRAILS

As 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.

Meteor 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.

METEOR TRAIL REFLECTIONS

As 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.

Strongest 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.

SIGNAL LEVEL FLUCTUATIONS

Received 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.

The 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 below.

As 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, in the simplest case is believed to be caused by distortion of the ionised trail due to severe turbulence encountered in the upper atmosphere.

It 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 given above.

SIGNAL STRENGTH AND DURATION

When 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.

It 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.

EXPECTED RANGE

What 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.

Meteor Scatter is normally considered a weak signal form of communication, Particularly when stations are placed towards the limits of range.

When 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.

DOPPLER FREQUENCY SHIFT

PARTS OF THIS TEXT CONCERNING DOPPLER SHIFT ARE BEING RE-WRITTEN AT THE MOMENT.

The 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 speed.

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.

However 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.

Sporadic 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.

ANNUAL VARIATIONS

There 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.

DIURNAL VARIATIONS

Owing 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

Earth 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.

 

METEOR SHOWERS

At 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.

THE RADIANT POINT

The 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.

Although 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.

The 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.

When 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.

The 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 site.

 


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 !

WSJT recordings 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>..

Just click on the button once..


Go to the Home Pages of OH5IY at http://www.saunalahti.fi/~oh5iy/

Go to the Home Pages of K1JT for WSJT & JT44 at http://pulsar.princeton.edu/~joe/K1JT/

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