How can the range of wifi be extended to several kilometers?

Thanks for asking this question. Any electromagnetic signal carrying any power, whether a few nanowatts or terrawatts, will travel to the end of the universe if it's unobstructed.

EM waves don't lose energy or power in free space.

It is only when EM waves interact with matter that they get reflected,refracted or absorbed. Any transfer of energy between wave and matter can take place only when they interact with each other.

With that out of the way let us say that you shine a very high energy radiation, a terrahertz wave like visible light ,from a flashlight , on a 10 floor building would it be detected on the other side? Unlikely. That means that the wave has stopped propagating after a few meters. Some of it reflected, some transmitted ,other absorbed.

Now increase the power of radiation 10 watts,100 watts,1000 watts and so on. With increased power more of the light will try to penetrate through the building but then rapidly lose its power and only faint signals will be visible,if at all from the otherside.

Now just for fun, imagine that you place the sun in front of the building, It will melt the entire structure in front of it and light will happily move along its path. Even in this case some light, the part which is used to melt,will be lost.

Let's switch back to our flashlight. Now if we move to the roof of another building that is say 20 stories high and shine the light downwards it will be detected from the otherside of the building even when operated with modest 3.2 volt batteries.

Increasing the power certainly does help in extending the range of the EM wave. Power increases the penetration ability of the wave but has no effect on how far it can go. Unless you manage to acquire the power to melt obstructions in your way, like the sun it won't get you far.

Choice of antenna and its careful placement to move the wave around obstructions is far more important.

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Notes by Akshat Jiwan Sharma

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