How does this all really work? Can anyone help?

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brooklynspacecadet

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first let me thank you for the excellent illistration and explanation.<br />yes, you have explained things but it creates more questions. first of all you mention that "If you divide 230 to get the radius then square that number and multiply it by Pi you get 41,548 square feet as the area of the dish" why do you square it and multiply it by pi? what is the reason for that specific multiplication? please explain your mathematical objective and why you initiated it.<br />I also don't understand by have 41,548 as the square footage of the dish allowing each square foot to be the equivalent of reaching one mile or one square mile. how does the feet and miles go together? and then you go on to stretch the distance by the 36 million miles by square feet of the dish. <br />I also dont understand what you mean when you say "the dish focuses all of that area onto a smaller antenna that is one square foot in area. Now you have a ratio of 41,548 to 1 " if the dish that is being focused on is big or small why would it make it a ratio of 41548 to 1?<br />additionally you illustrate using a parabolic example to amplify receiving sound and signal. but how about sending signals? you said the antennae is actually very weak on the space rovers and of course the topic is it's so many millions of miles away. so how does the rover receive signals and commands without having the satellites huge disk and only having a small antennae?<br />untill i figure out how the math defines the physics or maybe it should be vice versa this will still seem like amazing magic and skepticism to me.
 
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brooklynspacecadet

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"It takes 20 minutes for a signal from earth to arrive at Mars."<br /><br />is that signal going at the speed of light?
 
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richalex

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<blockquote><font class="small">In reply to:</font><hr /><p>why do you square it and multiply it by pi? what is the reason for that specific multiplication? please explain your mathematical objective and why you initiated it. <p><hr /></p></p></blockquote>He wants to find the area of the collecting antenna. A formula from geometry states that Area = Pi * radius * radius. That is, A = Pi * r^2. <br /><br />Next, he pointed out that if you focus all the energy collected by the entire dish onto a single spot that was 1 foot square, you would have a concentration equal to the ratio of the two areas (41,548 square feet to 1 square foot). That is your amplification from focusing the signal. <br /><br />Finally, to show what effect this amplification has on the incoming signal strength, he divided the distance the signal travels by the amplification. The end result is that because of amplification, the signal from so many millions of miles distant has the same strength as a signal only 800 miles distant. <br /><br />One reason the incoming signal is weak is that the original signal generated by the spacecraft is weak, compared to a signal generated by a large transmitter on Earth. That is because we have much bigger electrical generators on Earth than we do on space craft. So, we can transmit a huge signal into space, much bigger than the signal the space craft could send to us. So, the space craft does not need such a big antenna, because we send it such a big signal.
 
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richalex

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<blockquote><font class="small">In reply to:</font><hr /><p>is that signal going at the speed of light?<p><hr /></p></p></blockquote>Yes, because the signal *is* light!
 
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sssalvi

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Regarding the long distance coverage.<br /><br />We have a constant volume of our speech so we can cover a specific distance. But we can increase this distance in a particular direction by forming a cone in front of the mouth and directing it in that direction. How??? We CONCENTRATE all the energy to travel in a particular direction. <br />Same is true for a antenna. The larger it becomes, it turns more and more directive.. that is why we point accurately the antenna to the spacecraft. ( In case of a cellphone the basic requirement is that it should cover more area so its antenna is not highly directive. )<br />Secondly if you want to reach further you SHOUT... the space antennas use high power amplifier and transmit hundreds or thousands of watts of power and in a specific direction. So transmission to any distance is not a very big problem; it is the reception from such spacecrafts that is challenging.<br /><br />Towards that we require a very special type of amplifiers called Low Noise Amplifiers in addition to the large antennas. The performance of the antenna to receive weak signals is measured in terms of G/T of the antenna. The higher the G/T the weaker the signal that it can detect. Additionally some special protocols and techniques of communication theory are used to overcome larger distances.<br /><br />=======<br />S^3
 
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holmec

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<blockquote><font class="small">In reply to:</font><hr /><p>Yes, because the signal *is* light!<p><hr /></p></p></blockquote><br /><br />Radio signals travel at the speed of light. So does electrons through wire, photons through space.....etc. <div class="Discussion_UserSignature"> <p> </p><p><font color="#0000ff"><em>"SCE to AUX" - John Aaron, curiosity pays off</em></font></p> </div>
 
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henryhallam

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<font color="yellow">Radio signals travel at the speed of light. So does electrons through wire, photons through space.....etc.</font><br /><br /><nitpick /> Actually, electrons in a wire move at relatively slow speeds, in the tens of mph if I remember correctly. However the *signal* does move at a high fraction of the speed of light. Imagine a row of billiard balls touching each other, you push the first one, the last one starts to move "immediately" so the signal has travelled through them at the speed of sound even though each ball is only moving at a slow speed. It is the same with charge carriers in a wire and the speed of light.
 
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richalex

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Electrons are massive, so they are never going to go at c. They don't go at c in a wire, either. IIRC, electrons travel in an electric current through ordinary wire at about 30k mps. <br /><br />Photons through space is light. Radio waves *are* photons through space! They are similar to light waves, except the photons have more energy and so a greater frequency.
 
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holmec

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<blockquote><font class="small">In reply to:</font><hr /><p>Electrons are massive, so they are never going to go at c. They don't go at c in a wire, either. IIRC, electrons travel in an electric current through ordinary wire at about 30k mps.<br /><p><hr /></p></p></blockquote><br /><br />Hmmm....educating the lay, eh! {I sit corrected at my console}<br /><br />Ok then, why does electromagnetism travel through space at all? <div class="Discussion_UserSignature"> <p> </p><p><font color="#0000ff"><em>"SCE to AUX" - John Aaron, curiosity pays off</em></font></p> </div>
 
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CalliArcale

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<blockquote><font class="small">In reply to:</font><hr /><p>Photons through space is light. Radio waves *are* photons through space! They are similar to light waves, except the photons have more energy and so a greater frequency.<p><hr /></p></p></blockquote><br /><br />*nitpick*<br /><br />Actually, radio waves are lower frequency than visible light. The spectrum goes radio, infrared, visible, ultraviolet, x-ray, gamma-ray as it goes up in frequency. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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henryhallam

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<font color="yellow"><br />Ok then, why does electromagnetism travel through space at all?</font><br /><br />It is not obvious, that's why Maxwell and the others were so clever and important! Essentially a changing magnetic field sets up a changing electric field which sets up a changing magnetic field etc etc. There are no electrons involved in the free-space propagation of EM waves.
 
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