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### What is the maximum distance between two satellites orbiting at 400 km that would still allow them to communicate with each other using radio waves?

This depends on the wavelength. While short waves spread more like a light, long waves bend over horizon and can cover distances in excess of 17,000 km even from the surface of the Earth. This is more ...
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### What is the maximum distance between two satellites orbiting at 400 km that would still allow them to communicate with each other using radio waves?

If, for example, you have only 2 satellites separated by half the distance (i.e., 21, 271 km), will they be able to communicate with each other? I guess not because Earth would be in the way. ...

### What is the maximum distance between two satellites orbiting at 400 km that would still allow them to communicate with each other using radio waves?

Imagine a circle (the circumference of the Earth) and a triangle with the angle points being the locations of the satellites and the centre of the Earth. (the centre of the circle) You can divide that ...
For line-of-sight communication it's not longer than $2\sqrt{ (6371+400)^2 - (6371)^2} = 4585$. Or $2\cos^{-1}\left(\frac{6371}{6371+400}\right)$ = 40 degrees, so 9 satellites at least.