Electromagnetic radiation

For a radiated power of 5 kW, how to calculate the voltage intensity at a distance 20 km from an isolated antenna?
should we find the RMS value of voltage? plz tel me the equation.

hi
what do you mean by isolated antenna?
do you want to measure practically or this is just a question?
I meant do you have mesurment systems like VNA?

For the general problem, you would want to refer to Friis equation. In particular, you need to know the involved antenna gains.

Friis transmission equation - Wikipedia, the free encyclopedia

this is not a practical thing. just a question from a paper. im just confused with the term intensity

As long as the question isn't clear in terms of eletrocmagnetical theory, it can't be answered. If voltage intensity can be translated to electrical field strength, you at least need to know the transmitter antenna characteristic, e.g. omnidirectional, isotropical or something different.

Without specifying the pickup antenna capture area and termination impedance all you can do is calculate the field intensity at the the receive site.

You can write an equation in terms of ERP, distance, Rcv antenna capture area, and Rcv ant termination impedance.

Receive antenna capture area is based on antenna gain and frequency.