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Yes, it does make sense to me. I think conclusions drawn from this thought experiment are not entirely correct. Under your experiment when sending you spatially

Yes, it does make sense to me. I think conclusions drawn from this thought experiment are not entirely correct. Under your experiment when sending you spatially create a beam - there is this beam as wide as parabolic dish and no light anywhere else. With zero interference (using the word quite freely) this beam "will be just as strong no matter how far away the receiver is". With roles reversed, you do not necessarily get directional transmitter. For the sake of argument consider omnidirectional light bulb (the same bulb without parabolic dish) as sender. Classical intro/ELI5 description of transmit power is fixed number of spatially evenly distributed omnidirectional "rays". The larger the distance, the lower number of "rays" you receive per unit of area (receive power scales with surface area of transmit profile, usually r^2). By adding a parabolic dish you effectively increase receive area to size of dish instead of sensor size. When sending, the area over which power is dissipated is constant instead of scaling with distance. > You're claiming it won't matter how far away the sender is from the focus, which makes no sense. You are correct in that it makes no sense, but the claim was "every improvement in sending is completely equivalent in receiving for a system that uses the same antennas for both directions", key word improvement. While this is not entirely correct, this approximation (that antenna improvement works both ways equally) is pretty close to reality in practical far field applications.

throwaway613834 9y ago
> With roles reversed, you do not necessarily get directional transmitter. I don't get why you put "necessarily". You do not, period. > [...snip...] but the claim was "every improvement in sending is completely equivalent in receiving for a system that uses the same antennas for both directions", key word improvement. Yes, the claim is wrong: 1. With the sender at the focus and the receiver farther away, the mirror reduces the loss (due to distance, since everything else is assumed ideal as you already acknowledged) to ZERO. You literally cannot do better than reducing your loss to zero. 2. With the receiver at the focus and the sender farther away, the mirror DOES NOT even remotely reduce the loss due to the distance to zero. Loss is still ~1/r^2. Because the mirror only gets so much of the omnidirectional light. This should be very obvious. I don't care how you slice and dice this, and I'm tired of arguing something this obvious, so I won't keep commenting. 1 is not equal to 1/r^2.
- dpark 9y ago
  > I'm tired of arguing something this obvious You're arguing a scenario that is impossible both practically and theoretically and then calling others pedants for pointing out the fundamental flaws in your thought experiment. You're literally arguing against the textbook and then asserting that your thought experiment is close enough to make your point. It might be time for you to stop assuming that everyone else is an idiot and entertain the notion that you might actually be wrong.

throwaway613834 9y ago

> With roles reversed, you do not necessarily get directional transmitter. I don't get why you put "necessarily". You do not, period. > [...snip...] but the claim was "every improvement in sending is completely equivalent in receiving for a system that uses the same antennas for both directions", key word improvement. Yes, the claim is wrong: 1. With the sender at the focus and the receiver farther away, the mirror reduces the loss (due to distance, since everything else is assumed ideal as you already acknowledged) to ZERO. You literally cannot do better than reducing your loss to zero. 2. With the receiver at the focus and the sender farther away, the mirror DOES NOT even remotely reduce the loss due to the distance to zero. Loss is still ~1/r^2. Because the mirror only gets so much of the omnidirectional light. This should be very obvious. I don't care how you slice and dice this, and I'm tired of arguing something this obvious, so I won't keep commenting. 1 is not equal to 1/r^2.

dpark 9y ago
> I'm tired of arguing something this obvious You're arguing a scenario that is impossible both practically and theoretically and then calling others pedants for pointing out the fundamental flaws in your thought experiment. You're literally arguing against the textbook and then asserting that your thought experiment is close enough to make your point. It might be time for you to stop assuming that everyone else is an idiot and entertain the notion that you might actually be wrong.