How to measure RF Power density (a probe)

I want to make simple device to measure RF Power density. I want it to output just the value for the power and be frequency independent. Is such a thing possible?

I read in some places
(here for example: http://www.ndt.net/article/nde-india...df/1-08C-4.pdf )
That voltage on terminals of electrically short dipole exposed to electric field is simply field strength (in V/m) multiplied by length of dipole (m) and it seems to be frequency independent.

If this is true I can take square of the voltage, divide by impedance of free space (377 Ohm) and get the power (in W/m2).

Does this mean the concept of antenna gain/efficiency does not apply?

Field probes with l<<λ are often used in EMC laboratories to monitor field strength in susceptibility tests. Their output can be interpreted as power density under certain prerequisites (e.g. far field or quasi far field conditions).

The field probes are terminated with high (ideally infinite) impedance, in other words there's no received power respectively they have zero efficiency.

There is a gap between theory and practice.

The mere presence of the antenna distorts the electric field. The results of such measurements can be considered approximate at best.

A strong electric field and a very small dipole and a low conductivity will give the best results.

But suppose you want to measure the RF power density coming from the space to be detected with a radio telescope, then you are on your own...

After further reading found more evidence of the same approach and practical equipment examples using it. Still did not reconcile all the theory. The key to such measurement is near infinite impedance on dipole's terminals. Stepping out of 50 Ohms universe ...