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Post by Progenitor A on Oct 29, 2010 17:04:23 GMT 1
Hmmm.... thanks for that. I think I see the problem here Take the two dB values refered to 33dB and 33dB which you put in your calculator and got 36dB If we ADD logarithms the operation is a multiplication operation. 33dB + 33dB = 66dB Your calculator is not actually adding the logarithms to give a multiplication operation Here's what is happening in your calculator: 33dB expressed as a linear quantity = 10 3.3Now adding two of these as linear quntities gives: 10 3.3 + 10 3.3= 2 x 10 3.3Now converting this number back to dB gives: 10log 10 2 + 10 log 10 10 3.3= 3dB + 33db = 36dB (as you said) In other words your calculator is concerned with the addition of powers and not the addition of logarithms One result gives a straight addition and the other a multiplication A question of definitions really
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Post by Progenitor A on Oct 30, 2010 7:37:22 GMT 1
The linearity assumption for gain at the receiver. At last we are starting to get somewhere! Well done! Needs expansion though As soon as the beam is narrower than my dish, making the dish bigger gives me no gain in received power. Hmmmmm.... I do not quite see this. With regard to the numbers presented -- just because the supposed power received is less than power transmitted doesn't mean it is the right answer, just possibly not wrong. Whereas when the power received is supposedly more, it is certainly wrong. Ah, a good intelligent observation that is helpung us get there As the light example shows, you actually need more information than just these three numbers, in the general case. Not really. Consideration of the physics of what is going on - the meaning of the gain of a parabolid antenna, the practicalities of a parabolic antenna, the propagation characteristics of the antenna and takig account of Mawells equations for em wave propagation is sufficient to show why an overall 'gain' is a fallacy and that the combination of antenna gain and path loss can never be positive.
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Post by speakertoanimals on Nov 1, 2010 17:40:31 GMT 1
How difficult can it be? If you have a laser beam 1m across, you capture all of it, whether your dish is a metre, or ten metres across, as long as it is aligned.
Which is more than the 3 numbers you quoted previously........
You don't need to invoke Maxwells equations, you can understand the basics using a gun spitting out pellets and a collection net. The answer is actually blindingly obvious, and also totally obvious that what is at fault is the linear relationship which applies to gain at transmitter and receiver, and forgetting that this linear relationship only applies in special cases. So, if I have a laser beam 1m across, I get a gain at the receiver provided my dish is centered on the beam, and of width less than 1m. Wider than this, no further gain. Simple.
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Post by Progenitor A on Nov 1, 2010 17:55:22 GMT 1
How difficult can it be? If you have a laser beam 1m across, you capture all of it, whether your dish is a metre, or ten metres across, as long as it is aligned. Which is more than the 3 numbers you quoted previously........ You don't need to invoke Maxwells equations, you can understand the basics using a gun spitting out pellets and a collection net. The answer is actually blindingly obvious, and also totally obvious that what is at fault is the linear relationship which applies to gain at transmitter and receiver, and forgetting that this linear relationship only applies in special cases. So, if I have a laser beam 1m across, I get a gain at the receiver provided my dish is centered on the beam, and of width less than 1m. Wider than this, no further gain. Simple. If you are really interested, go to the equations for path loss, go to the equations for parabolic antenna gain, look at the distances involved in path losses being less than the nominal antenna gain, look at the dimensions of the antenna, look at the gross imperfections in a parabolic antenna, look at the multiple side lobes (including backward) on an antenna, look at the Maxwell near and far fields and it will become self evident that an antenna gain is not an absolute and that a nominal antenna gain is only valid when the path length >>> the diameter of the parabola. Indeed if you wish, you can find the path length where the antenna gain becomes valid within a few dB Nothing magic Just plain old physics. A good exercise for engineering students
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Post by speakertoanimals on Nov 1, 2010 18:16:34 GMT 1
Not quite -- rather an indication of how reducing a complicated physical situation to a few simple equations, suitable for people such as engineers, fails to capture ALL aspects of the problem, and can lead to daft results if you try to apply the simplified engineers description to situations where it is not appropriate.............
To any enraged engineers, my missus was a radar engineer in the Navy for 25 years, so i'll get worse off her than anyone here can dish out. Mind you, probably not as bad as what radar chappies had to say about other brands of artificers...................
And for physicists, given Oxfords theoretical bent, how do you tell an Oxford physicist? Because they pick a soldering iron up by the wrong end.............
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Post by Progenitor A on Nov 1, 2010 19:23:53 GMT 1
Not quite -- rather an indication of how reducing a complicated physical situation to a few simple equations, suitable for people such as engineers, fails to capture ALL aspects of the problem, and can lead to daft results if you try to apply the simplified engineers description to situations where it is not appropriate............. To any enraged engineers, my missus was a radar engineer in the Navy for 25 years, so i'll get worse off her than anyone here can dish out. Mind you, probably not as bad as what radar chappies had to say about other brands of artificers................... And for physicists, given Oxfords theoretical bent, how do you tell an Oxford physicist? Because they pick a soldering iron up by the wrong end............. That's better! Humour! I had no idea you were a lesbian! Well done!
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Post by principled on Nov 1, 2010 21:43:26 GMT 1
They wouldn't be the same ones who can work out the square root of a biscuit tin but can't fathom how to take the lid off, would they? P And for a few corny jokes about physicists, pop along to www.workjoke.com/physicists-jokes.html
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