ORIGINAL: iflyj3
Highplains and Abel,
Let me see if I can nut shell what you two have said.
The system picks any two available channels out of 80 channels available. It picks two channels due to antenna orientation problems presented by an aircraft.
Within each 1 MHz channel the data is spread 1 MHz wide.
Questions.
If 4.2 billion codes are available, who assigns these codes?
Since the system picks two of 80, does that not mean that there are actually 40 times 4.2 billion or 160 billion possibilities?
Since two receivers are used to receive the two channels, why didn't they use only one frequency if it was just an antenna orientation problem?
I am not trying to bore anyone or be a smart A.., I am just trying to learn.
I was within a few days of buying the AeroSpectra scanner, but if SS is real close then I don't want to waste $900.
Hi Dan-
I'll take a shot at the easy part and and leave the hard parts to HighPlains.
I mentioned the antenna directivity and need for a LOS path between Tx and Rx in upper UHF/lower SHF part of the spectrum, and I'm sure your ham background contributes to understanding that. I'm making a guess that your question might be rephrased "why not simply connect various antenna elements together to provide for full spherical coverage and feed the composite signal to the Rx." One reason is that by so doing you are opening up the window of the antenna to noise, equivalent to saying the antenna gain is reduced. S/N is generally of at least as much concern to detection of the signal as is signal strength per se. Due to both practical and statutory limits on power transmitted, that alternative doesn't fit into limited S/N budget the designer has to work with. Another reason is that it would cause phase jitter in the resultant signal in much the same way that multipath does in the lower bands, with similar effect. The spacing between elements would be bounded on the lower end by the size/shape of the vehicle, and thus at least on the order of a wavelength and more at SHF. The respective lengths of the RF path to each antenna element would thus differ constantly as the vehicle changes attitude. The phase differences between pieces of the signal represent another element of noise that can be ill-afforded.
I suspect the design compromise (2 rx's each with their own antenna) made was primarily driven by economics. A viable product must be affordable, and that likely infers a highly eclectic approach to designing it. Modules developed for established appliance markets like cell phones are adapted wherever possible. If those other applications did not need more than one antenna input, a custom approach has to be pursued. Obviously a decision has to be made somewhere in the process as to where the better signal is coming from, and select that path. The design being discussed opted to provide for that selection at baseband, rather than upstream at RF. I would find that option easier myself, but though I have engineering background, it's been a long time since I did it regularly at the 'dirty hands' level.
Abel