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In article ,
wrote: In article , says... SNIP In your train analogy above, what is to keep the leading part of the echo from being QRMed by the trailing part of the transmitted signal? The analogy is good in so far as it shows the timing, but I have to admit it is poor in that I used a solid object, the train, to represent a wave and their properties are very different. For example, if two trains hit head on, you are going to have a mess. That is not the case with waves. If you throw two rocks at the same time in a pond of still water so that they land some distance apart, the waves from each impact point move out in concentric rings. When the rings from one impact point spread out enough to meet the spreading rings of the second, there is however no "wreck". The rings of waves of one appear to pass through the rings of the other with no harm done to either wave. I don't think that is true. I believe the waves would add, subtract or be somewhere in between depending on their phase relationship. What you say is true, but what I said is also true. The waves are not "wrecked" and after the waves pass each other, they are are the same as before they overlapped. I found a reference on the web (http://www.smgaels.org/physics/amsco.../chapter04.pdf) that says it much better than I could. On page 109 the author states: (Beginning of Quote) "Two or more waves may pass through a medium at the same time. When this occurs two rules apply. First, the total displacement experienced at any point where waves meet is equal to the sum of the displacements of the individual waves at that point. This is known as the principle of superposition. Second, waves pass through each other, with each wave unaffected by the passage of the others. After meeting, the individual waves continue traveling in their original directions and with the same characteristics as before." (End of Quote) SNIP What you say is interesting, but...wave theory notwithstanding, what we have are TWO RF signals on -essentially- the same frequency (ignoring Doppler, libration, etc), the incident and the reflected. And I think we both agree that the signals "overlap" for .75 seconds (as so aptly stated in your 'train' analogy). Actually they overlap for very close to 2 seconds. Thinking back to the train analogy, the instant the pulse hits the moon (cow catcher on the engine) and starts back the other way as an echo, we have an overlap. That means 99.99% of the pulse (train), or almost 2 seconds worth, still has to hit the surface and reflect before there is no longer an overlap. Just like the old USSR jamming the VOA -- two signals on the same frequency. Why wouldn't they interfere for the .75 seconds in question? I think they do to the extent of the "superposition" of the waves, and if you were in a space capsule with a receiver 1000 miles from the surface of the moon, I think you might indeed hear that interference. Superposition is the concept. The EM waves in space do not interact. Yes they certainly would interact in an antenna where they would create a vector sum. Since it is the same signal the interaction in the antenna would be to weaken if out of phase to strengthen if in phase. It would not sound like interference. -- Telamon Ventura, California |
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