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Post by Progenitor A on Oct 26, 2010 12:09:43 GMT 1
Here's an oddity that STA will waffle around (or keep very silent) and will befudle poor dim Olmy (oh My!).
Path losse for em waves are well established in vacuo for isotropic radiation, and the path loss to a satellite, for example is about 140dB.
Path loss for em isotropuc radiation is proportional to the inverse square of distance (betwen the transmitter and receiver)
Thus as we move away from a transmitting source, we can easily attain a path loss of, say 60dB, which means that if our isotropic radiator is transmitting 1W we recieve 10-6 W.
Fair enough (the equations are readily available)
The path losses can be reduced by putting atennas with gain at the transmitter and receiver. A common antenna is a parabolic antenna - very much like the car headlamp reflector which increases the power received quite enormously.
Parabolic antenna gains are commonly in the 33dB (2000) area.
Now, if we take our path loss of 60dB and stick two parabolic antennas each with a gainof 33dB at each end of our link then the path loss is reduced by the combined gains of the antennas.
In fact we get a path GAIN of 6db!
So if we are transmitting 1W we receive 4W!
Something for nothing!
Now this cannot be right, can it?
What IS wrong?
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Post by speakertoanimals on Oct 26, 2010 12:56:45 GMT 1
This nicely illustrates the difference between understanding physics, and naively messing about with equations..........
SO, lets take light. Suppose I have a light source of fixed power output, which radiates isotropically. Now suppose I have a receiver (lets take it to be a perfectly-absorbing disc placed perpendicular to the direction of propogation). Obviously, the larger the area, the more of the transmitted radiation I intercept, although the amount only goes linearly with area for small areas. Obviously, if I have an infinite disc, I mange to intercept 50% of the total transmitted, because I intercept everything within a hemisphere.
Now lets change the emitter, and have same power, but emitted as a parallel beam of area A. If I place my disc centrally in the beam, I can increase the area and increase intercepted power, until at receiver area A, I now absorb it all.
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Post by Progenitor A on Oct 26, 2010 13:05:08 GMT 1
This nicely illustrates the difference between understanding physics, and naively messing about with equations.......... SO, lets take light. Suppose I have a light source of fixed power output, which radiates isotropically. Now suppose I have a receiver (lets take it to be a perfectly-absorbing disc placed perpendicular to the direction of propogation). Obviously, the larger the area, the more of the transmitted radiation I intercept, although the amount only goes linearly with area for small areas. Obviously, if I have an infinite disc, I mange to intercept 50% of the total transmitted, because I intercept everything within a hemisphere. Now lets change the emitter, and have same power, but emitted as a parallel beam of area A. If I place my disc centrally in the beam, I can increase the area and increase intercepted power, until at receiver area A, I now absorb it all. My you are a silly-billy! Everyone knows that the equations are wrong and we cannot receive more than is transmitted The point is this (and typically you totally miss the point): The equations are perfectly OK for as long as the path loss is greater than the antenna gains - engineers use them all the time! AND it is possible to get a path loss of 60dB AND antenna gains of 33db. WHAT is going wrong is the question? (You have already been told that something is wrong) Waffler.
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Post by speakertoanimals on Oct 26, 2010 13:19:52 GMT 1
Wrong again. The equations are not wrong, what you have missed out and are trying to claim is something deep and mysterious, is the conditions under which they do apply. As I said before, physics is more than just mucking about with symbols, as some idiots always try to 'do physics' by lifting equations off wikipedia and applying their knowledge of basic algebra to supposedly derive some nonsensical result.
Like the above.
So, an equation in physics is more than just a bunch of symbols, it is what the symbols mean AND under what conditions the relationship encapsulated in that equation applies. If you violate the latter, then you get daft results as you did.
And my example shows quite simply how and why the simple assumptions break down.
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Post by olmy on Oct 26, 2010 13:37:30 GMT 1
Your understanding. According to the 'reasoning' you have employed, as soon as you stuck the parabolic antenna on the transmitter you got much, much more 'for nothing'. www.marcspages.co.uk/tech/antgain.htm
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Post by speakertoanimals on Oct 26, 2010 13:52:36 GMT 1
Exactly Olmy -- and if you focus all your radiated signal into such a narrow beam, then your gain by transmitter equations won't work either, because these are based on the gain you can achieve for an isotropic signal (the simple make the area bigger for light case). If the signal isn't, then equations don't apply.
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Post by Progenitor A on Oct 26, 2010 16:35:13 GMT 1
Your understanding. According to the 'reasoning' you have employed, as soon as you stuck the parabolic antenna on the transmitter you got much, much more 'for nothing'. www.marcspages.co.uk/tech/antgain.htmWhat a foolish reaction! Engineers use such calculations every day You do not really have a clue Yo are so DIM that you do not know when to keep your mouth shut!
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Post by speakertoanimals on Oct 26, 2010 19:45:13 GMT 1
No one said they don't -- they just know when the situation is such that these equations are applicable, and when they have a situation where they are not.
WHY anyone would want to claim otherwise, or what the supposed point of all this is (apart from the obvious one), we will wait and see.........
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Post by Progenitor A on Oct 26, 2010 20:31:51 GMT 1
Exactly Olmy -- and if you focus all your radiated signal into such a narrow beam, then your gain by transmitter equations won't work either, because these are based on the gain you can achieve for an isotropic signal (the simple make the area bigger for light case). If the signal isn't, then equations don't apply. What idiotic, bumbling, incoherent reasoning!
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Post by Progenitor A on Oct 28, 2010 13:42:11 GMT 1
As I thought our two 'scientists' STA and Olmy (Hons. Domestic Science) have nothing whatsoever useful to say on this subject.
Now, how did I know that?
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Post by speakertoanimals on Oct 28, 2010 14:34:29 GMT 1
What, trying to provoke with insults when we won't play your silly games?
We explained what was wrong with your scenario, job done.
WHY you thought it worth posting in the first place, you have yet to explain..............
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Post by Progenitor A on Oct 28, 2010 15:17:41 GMT 1
What, trying to provoke with insults when we won't play your silly games? We explained what was wrong with your scenario, job done. WHY you thought it worth posting in the first place, you have yet to explain.............. In fact you have not even addressed thje problem (which is a standard puzzle in electrical engineering). The problem is this: You can get a total antenna gain of 66db You can get a path loss of 60dB When we add these together, AS LOMG AS the path loss is greater than the combined antenna gains, the additions work; if the antenna gains are larger than the path loss then the additions do not work There is something fundamentally and physically going on here that you have not addressed and that I am inclined to think that you cannot address, perhaps do not even understand
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Post by speakertoanimals on Oct 28, 2010 15:41:29 GMT 1
Yeah, right. You tell us then, why it is anything other than simply applying equations in a regime where they don't apply.........
Which in is effect what you have already said, in that some times the addition works, other times it doesn't. Except this is no surprise to anyone, we have already shown that the gain equations are just a simplified view of the case that only applies to specific cases -- and if you apply them elsewhere, you get daft results.
So what's the big deal, we're all waiting with baited breath for you to tell us what the significance is, please educate us O wise one................
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Post by Progenitor A on Oct 29, 2010 8:40:40 GMT 1
Yeah, right. You tell us then, why it is anything other than simply applying equations in a regime where they don't apply......... Which in is effect what you have already said, in that some times the addition works, other times it doesn't. Except this is no surprise to anyone, we have already shown that the gain equations are just a simplified view of the case that only applies to specific cases -- and if you apply them elsewhere, you get daft results. So what's the big deal, we're all waiting with baited breath for you to tell us what the significance is, please educate us O wise one................ You are being evasive and not addressing the problem. Olmy (Oh My!) is too dim to even understand the concept of antenna gain Yeah right indeed! Are you a rebellious teenager I wonder? Explain why we can have a path loss of 60dB and combined antenna gains of 66dB, yet are not able to add them together You do not have an inquisitive mind do you?
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Post by olmy on Oct 29, 2010 8:58:21 GMT 1
Olmy (Oh My!) is too dim to even understand the concept of antenna gain Irony. Didn't you read the link I gave? If you had you would have seen the very obvious first problem with simply adding gains and then interpreting the result in terms of the ratio of input power to output power. If this were valid, it would have to be valid all the way through the system. Look what happens as you add in the various components (in the order of the physical arrangement).... Input power = 1W + 33dB antenna = 2000W + 60dB path loss = 2mW + 33dB antenna = 4W. This obviously breaks down right at the start, with the transmission antenna gain. As the article I linked says antennas do not "magically create power". You can forget the rest of the path - if your 'reasoning' (about gain giving input power to output power ratio) was correct, we have now put 1W in and got 2000W out! All the gains are ratios and you need to understand what they are ratios of.In the transmitter, the antenna gain is "The relative increase in radiation at the maximum point expressed as a value in dB above a standard, in this case the basic antenna..." (taken form the link). What a high gain antenna does is focus it's power in a given direction, rather than just radiating it out in all directions. What all this means is that if I put some 'standard' receiver next to (in the correct direction) our 1W transmitter (fitted with the fancy 33dB antenna), it would receive the same signal as if it were next to a 2000W transmitter that was fitted with a 'standard' (0dB gain) antenna that just spewed all its power out symmetrically. Now, that deals with your original question - your interpretation of the gains as simple ratios of input to output power is just wrong. There is no surprise that adding all the gains in the system and getting a net gain but this does not mean we have got 'something for nothing'. Now, depending on how the references (0dB points) are defined in the rest of the path and how the figure is going to be used, there may well be (probably are) other considerations and some good 'ol engineering 'rules of thumb' that mean that a net loss is more useful than a net gain but I don't know off the top of my head and frankly my interest in this has come to an end...... BTW you made a couple of posts without going out of your way to add some puerile personal insult to me - good to see you are back on form - I was starting to feel neglected ...
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