Huh? Run some high power into the antenna and click the shutter button. What
could be simpler (assuming you have the right camera). Obviously need a fair
amount of power.

Thermal images register differentials in temperature . The lower horizon and
ground are usually between 200-300 degrees Kelvin (with no water vapor, the
horizon looks much colder) and it is thus an issue of looking at temperature
changes that are typically a few degrees or less.

Antennas have surface areas and thus heating and cooling rates. The
instantaneous thermal temperature is indicative of the point where these rates
come into equilibrium.

The cooling rate will be affected by the surface layer of air. Usually,
convective cooling is a big factor and biases the true temperature at the
surface. Thus the actual temperature in air is different from that in a vacuum.
I don't believe you are referring to a vacuum.

Electrical thermal probes can and are used.In order to have minimal effects on
the current of the antenna, they are attached by a small but highly heat
conductive paste--an electric insulator as well-- and then choked. Usually the
leads are run normal to the radiator surface.

The amount of power you need is indeed, not small. Fairly large metal surfaces
dissipate heat moderately well through radiation and air convection, so you
could be burning up a fair amount of power and not be able to detect it with
any great precision. If the heated surface is small, i'ts much easier. Most
antennas of interest do not have small surfaces or lengths.

Hope this helps.

73,
Chip N1IR