[Lf] LF: Diversity reception]

[Andre' Kesteloot]

Tom Boucher wrote:

> Mike, Alan, Larry and all,
>
> Surely the AR88s would be used with an external combiner? I would guess
> that the D suffix probably indicates the receivers had the necessary
> interface to an external combiner. Todays aerospace telemetry systems
> employ space, frequency and polarisation diversity, sometimes
> simultaneously which involves banks of receivers and combiners.
>
> Space diversity, using two physically seperate receive antennas, is used
> to overcome signal fades due to multi-path phase cancellation. Frequency
> diversity uses two seperate RF links on different frequencies to
> overcome radiation pattern nulls on larger vehicles. Polarisation
> diversity is used to overcome polarisation twisting of the received
> signal due to vehicle rolling or other factors.
>
> A fourth system is time diversity where the data is delayed and
> transmitted again to overcome short term signal attenuation such as on
> re-entry vehicles.
>
> Multiple receivers are necessary with a combiner which takes each
> receiver output and sums it in an optimal ratio depending on the S/N
> from each.
>
> A better system, known as Pre-Detection Combining, phase locks the two
> IF signals together and combines at IF frequency.
>
> Maximum theoretical improvement with two similar signals is 3dB but of
> course if one signal has a good S/N and the other is lousy, then the
> improvement can be much greater. Could be worth exploring some of these
> techniques for scraping LF signals out of the noise.
>
> Larry did you say you junked an AR88? What a terrible admission! I'm
> sure someone could have put it to good use, if only at the end of a long
> chain by a yachtsman.
>
> 73, Tom G3OLB.