Horn Antenna Transmission Distance

Hi

I would like to ask about how to determine the transmission distance of your Horn Antenna, provided you already have the s-parameter, directivity, vswr and gain. Thanks.

the microwave energy that comes out of your waveguide horn will travel forever.
At some point the background noise of the universe will be higher in power, and you will not longer be able to detect it.

Friis (one way radar range equation) will be helpful. Look into link budgets as a subject area.