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.
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