How to calculate electrical power needed to produce a given electromagnetic RF field?
I'll try and help a little. An EM wave in air or a vacuum meets an impedance; the impedance of free space and, that impedance is 377 ohms approximately. This means that the ratio of E field to H field is 377:1. So if your E field is 60 volts per metre then your H field will be 0.159 amps per metre.

The power will be E.H = 9.55 watts per square metre. In other words, from your antenna (if it were a perfect isotropic antenna that emits power in all directions) you would measure 9.55 watts per square metre at some distance but at what distance?

Well, the distance that an EM wave forms is defined in many ways but most people accept (with a bit of hand waving and assuming low directivity antennas) that at about one wavelength from the antenna, an EM wave has properly formed. So, if your frequency is (say) 300 MHz, then one wavelength is 1 metre and, for the perfect (but impossible) isotropic antenna, the area of a 1 m radius sphere that surrounds this perfect antenna, is \$4\pi r^2\$ or 12.57 metres.

This leads us to make a reasonable stab at the amount of power you need to feed to your antenna. You want 9.55 watts per square metre, this means pushing that power through all the other square metres at 1 metre distance from the antenna therefore, the input power to the antenna (ignoring losses) is 9.55 x 12.57 watts = 120 watts.

Clearly, as you go down in frequency then the area of that perfect sphere becomes bigger in order to satisfy the greater distance that an EM wave takes to properly form from the antenna. At 80 MHz, one wavelength is 3.75 metres and the area of that sphere is now nearly 177 square metres and this means pumping a power into an isotropic antenna of 9.55 x 177 = 1688 watts.

So you then you need to think that the real antenna you might choose is something that is much more directional like a yagi but you also need to take into account that a more directional antenna needs a bigger than one-wavelength distance to properly form an EM wave. However, the power does drop and it gets easier at higher frequencies.

I note in your question that you mention the concentration of the field within a 20 cm x 20 cm square and, you might be thinking that you are going to be using a directional antenna to achieve this. Either way if you are looking to achieve 60 volts per metre, you need to be pushing 9.55 watts through each square metre.

Hope this helps you start your search for a more complete answer.