
Ionospheric and Tropospheric Scatter systems require the use of diversity systems where the use of different paths through the atmosphere (or ionosphere for that version). Diversity systems operate through at least two physical paths and the final output is achieved through either selection or combining. The improvement is obtained by the uncorrelated nature of the signal strength.
Input provided by Brian Skingley – March 2014
We produced one Ionospheric Scatter system in all the years that I have known and some 50+ Tropospheric Scatter systems.
Ionosphere systems will operate at much lower frequencies, generally 50 to 150 MHz (I do not have precise figures but you could use the numbers used in the IOW to Malta link as a starter).
Ionospheric can often be one way putting a lot of power into the Transmit direction. On the other hand the Receiving site could contain a lot of signal processing designed perhaps for the required usage.
There is not generally such a high requirement for power as the signal is bounced off of and through the atmosphere. Whereas in the Troposphere we are guiding, bending the signal through the layers of the Troposphere. And we often need as much Power/signal as we can get; just to achieve the required range. Generally TROPO SYSTEMS operate at one of the following frequency bands, 450 MHz (brute force and ignorance) or 900 MHz which is a good compromise for long range systems with Transmit powers generally in the 10kW area.
Most of the medium range systems are achieved at the 2GHz frequency band with transmit powers in the 1 kW region. All of our North Sea systems were in the 2GHz band. Shorter systems, generally inland, use 4.4 to 5.0 GHz with whatever Transmit power is required, 1kW or less.
Ionospheric scatter systems will generally be one way, uni-directional and cater for telegraph traffic primarily. If a two way circuit is required you presumably just double everything up! But never having had to design one – I just don’t know – and perhaps we should not pretend that we do unless we find an expert!
For Tropospheric Scatter systems the balance between the 2, 3 or 4 received signal is likely to be much closer (just from the geometry) and the signals are generally combined in a Post Detection Combiner where the weight of each signal depends on its strength (inverse of noise) and an Optimal or Maximal ratio combiner can be designed to do just that.
But that is past history! And we , Marconi, have designed a range of Pre-detection Combiners where the output combined signal is generally better than the above just because the combined fading signal spends much of its time above threshold thus sending fewer bursts of noise into the output. A much improved beastie! An improvement of 6 to 10 dB results. Patent No ………….
One other thing to bear in mind, our 900 MHz Trop are all or nearly all based on the H1230 Klystron Amplifier which is smaller than the one that you are offering, neater and generally sits there all day just pushing out power!
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Description of systems and technologies
Marconi Scatter Communication systems click here
Introduction to Tropospheric-scatter systems (1965) click here
Wikipedia entry for Tropospheric scatter click here
Wikipedia entry for Ionospheric scatter click here
Scatter systems
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