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How Tropospheric Scatter Works |
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Here is a simplified explanation of Tropospheric Scatter, as used on the North Atlantic Radio System. There are now digital systems available but I've never worked with them and the US military used analog FDM systems almost exclusively. If you're an engineer and spot any really serious errors, please call them to my attention. Otherwise, please give me a break, I don't design the things. The typical military Tropo site uses Frequency Division Multiplex (FDM) to combine a number of "circuits", which can be telephone, teletype or data lines, into one "baseband" signal, which is then applied to the modulator for conversion to an intermediate frequency (IF.) The IF signal then goes to the "up-converter" where it is amplified and converted to the final radio frequency (RF) that is used for transmission. This is typically between 600-1000 MHz but higher frequencies have been used. Finally, the signal from the up-converter is applied to the power amplifier, where it is amplified to the desired power level.
At the receiving site, the small amount of signal received from the transmitting site is directed by the reflector to the feedhorn, down the waveguide, through a filtering network and into the down-converter, where it is amplified and converted back to the IF frequency. It is then amplified again by the IF amplifier, before being converted, in the demodulator, back to the baseband frequency. Finally, it is routed to the multiplex to be broken up into individual circuits, again.
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At each site, radio frequency (RF) energy from the transmitter is
conducted to the feedhorn, via a waveguide, a form of transmission line designed for low
loss. The feedhorn directs the energy towards the reflector. In the NARS case, the
reflector is a concave "billboard" but the shape is only for practical reasons,
the effect is the same as the "dish" type antennas seen on satellite
systems. The reflector focuses the RF energy into a fairly tight "beam"
which is pointed at the "common volume". The common volume is the area of
the troposphere, up to about six miles above earth, where the beams from both sites, which
have now spread to cover a much larger area, converge. Within the common volume,
much of the RF energy passes through, into space, but enough of it is scattered back to
earth to provide a useful signal strength at the distant site.