Transmitters Used for Forward Scatter

General requirements

For receiving meteor scatter signals, a radio transmitter is needed. In amateur meteor forward scattering, existing transmitters are used. Of course, the transmitter has to fulfill some conditions to be usable.

First of all, it must transmit in the right frequency band. Lower frequencies (approximately below 30 MHz) are not usable, because more often than not there is reception of the broadcast through ionospheric propagation, masking the meteoric reflections. Higher frequencies (approximately above 150 MHz) are not suited either, as theory shows the maximal height of observable meteors decreases with increasing frequency, and so do the duration and power of the reflections.
Professional systems generally use frequencies between 40 and 70 MHz as a compromise.

Secondly, the transmitter must be distant enough to prevent direct reception of the signal. One can also use a transmitter that is shielded by a mountain. Without mountains, the minimal distance is about 600 km. The absolute maximum is about 2000 km. In theory, it is possible to observe without this requirement, but special methods must be employed.

Last but not least, the power of the transmitter has to be sufficient. Stating a lower limit is not possible, as that depends on the distance between the transmitter and the receiver, the antenna patterns of both stations, the noise figure and bandwidth of the receiver, the method of signal detection, ... As a rule, kilowatts of power are needed (though lower power and high gain antennas also work), and the more powerful the transmitter is, the best.

Some classes of transmitters

FM broadcast stations

These transmitters are very popular. Their frequency (87.50 - 108.00 MHz) is quite satisfactory, and the powers range from some watts to a few hundred kilowatts. The main problem is that an enormous amount of transmitters transmit at a given frequency within a radius of 2000 km from a given place, and that finding a clear frequency is difficult in populated areas. One possible solution is the use of RDS (Radio Data System) for the identification of the received transmitter.

Not all countries in the world use the same frequency band for FM broadcasts. In the former Soviet Union and eastern European countries, the band from 65 to 73 MHz is also used. These transmitters are extremely useful for West European observers. Note that only the former Soviet Union countries will keep using this band. From the year 2000 on, all other East European countries will probably have stopped transmissions in the 65-73 MHz band. Japan also uses another frequency band (76-90 MHz), which can be useful for observers in the neighborhood.

• FM Transmitter Radius Utility (for localising US FM transmitters for forward scatter)

TV stations

Several TV channels are located in the lower VHF band. The exact frequency allocations depend on the countries. The bandwidth of these transmitters (about 6 MHz) is wider than that of regular FM transmitters, but one can tune in to the video carrier frequency with a narrow-band receiver. This carrier signal has a constant power, and is not affected by the modulation of the signal.

Aircraft beacons

Aircraft beacons emit between 108 and 136 MHz, and transmit an identification code. However, their power is in the Watts-class (typically 6 W ERP and an elevation angle of 5° for VOR-beacons), which has been experienced to be too little for meteor scatter.

Amateur beacons

Parts of the 50 MHz and 70 MHz amateur bands are occupied by beacons. These are mostly CW transmitters, transmitting a carrier which is regularly interrupted by an ICW or FSK morse code identification, which unfortunately has a low power compared to the broadcast. Lists featuring the location, power, frequency, antenna characteristics and call sign can be found on Internet or in radio amateur publications.

Remarks

Remember that there may be (and most probably are) several transmitters active at the same frequency, and that a particular meteor echo is thus not necessarily powered by your selected transmitter. Identification of the transmitter should be tried, e.g. with RDS for West European FM transmitters.

Some transmitters are not emitting constantly, but stop broadcasting during nighttime.

For a reliable reduction of the observations, the antenna pattern of the transmitter is useful, as it seriously affects the radiated power in the direction of the meteor. This pattern can probably be acquired by writing to the broadcaster.

A good source to look for candidate TV or FM transmitters, is the "World Radio and TV Handbook". There is an updated version every year.