Measuring relative rf signal strength
You need a RF power detector that suits your target frequency and power ranges.

You may also need some kind of band-pass filter if you want to detect a signal whose bandwidth is much narrower than the dipole antenna bandwidth and there are other interfering signals or adjacent channels that could be picked up by the antenna.

If RF is not your field of expertise, I'll recommend using COTS modules because RF is very, very tricky. However, they may be an expensive solution for your application.

Mini-Circuits RF power detectors run at $90 approx, but have huge bandwith and dynamic range. You may find lower priced, less capable, RF power detectors that may still comply with your frequency/power/precision needs. If yours is a DIY project, you could turn to AliExpress and the like to find much cheaper options, but keep in mind that you'll get what you pay for (still, it may be good enough for your needs).

You may want to take a look at this superb hack: a guy called Tim Heavens used one of Mini-Circuits RF power detectors, an Arduino, a LCD and a case and built a RF power meter.

EDIT:

You may also need to match the impedance of the antenna to that of the modules down the RF chain (usually 50 Ohm).