notes-electronics

 elihu 1 day ago [-]

According to the inverse-square law, doubling the distance will reduce the power of the signal by 1/4th. The moon is about one light second away, whereas Voyager is about 70,000 light seconds away. If we round down to 64k light seconds, that's about 16 doublings. If each doubling represends a 6db loss, that's about 96 decibels.

So, starting from a system that can communicate from Earth to the moon, if you can find a way to add 64 decibels then it can work from Earth to Voyager. Ways to add decibels include using more directional antennas on one or both ends or transmitting with more power. Alternatively, you can make up some of those decibels by communicating much slower.

The interesting thing about the inverse square law is that it's insensitive to the scales involved. For instance, going from 10 meters to 20 meters results in a 6db loss, and going from 1 light year to 2 light years also results in a 6db loss. This is much different from, say, light in a fiber optic cable, which would experience a 6db loss from impurities in the glass each time the light traveled some constant distance.

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FabHK? 16 hours ago [-]

> According to the inverse-square law

I think that's the crucial point. While transmission through a cable etc. has exponential decay, transmission through vacuum has quadratic decay, so much more feasible.

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tempestn 1 day ago [-]

That's assuming you're firing the signal out in all directions though, right? For example, a laser doesn't drop in power in this way. I would think a tightly focused radio signal could also avoid such extreme degradation.

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elihu 23 hours ago [-]

It's really hard to make electromagnetic radiation perfectly parallel. If you shine a laser pointer at something far away, the dot gets bigger with distance.

If you could send a perfectly parallel beam, it would effectively be an antenna with infinite gain. As far as I know, that's not possible but getting as close as you can is a good strategy. There's also antenna aiming limitations to consider -- it's possible to have too much gain if it exceeds your ability to point in the right direction.

The opposite extreme is an isotropic radiator, which emits equally in all directions. (That isn't possible either, but it's a good theoretical baseline.) Antenna gain is usually described relative to an isotropic radiator. So, an antenna with a gain of 12dbi means that in the direction it sends its strongest beam, it's 12 decibels stronger than it would be if the antenna were an isotropic radiator.

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