Peter Johnstone of Wyndham asks :-
I am informed that the human body acts as an aerial, and picks up any wavelength that matches a person's height. Is this so?
John Campbell, a physicist at the University of Canterbury, responded.
The human body conducts electricity weakly, so it does act like an aerial - sort of.
In a good electrical conductor, such as the metal alumunium, there is about one electron per atom which is free to move about the metal. When an electric field, as in a radio wave, impinges on the metal the free electrons move so as to cancel out the force due to the electric field. Hence there is no electric force inside the metal, and the electric field is mostly outside the metal with some in a very thin surface layer. The free electrons in the surface layer oscillate with the same frequency as the impinging wave so reradiate an electromagnet wave. It is this reflected wave which we observe in a radar return from a metallic aircraft.
Metals have resistance to the flow of electricity in them so in poorer conducting materials some of the incoming energy is absorbed and merely heats the surface, and the reflected wave is much weaker than the inpinging one. The ultimate is a stealth aircraft which returns very little radar wave, partly by its very absorbing (lossy) surface coating and partly by being made of flat surfaces which dont give a mirror reflection back to the radar station.
Radar came about by a question posed to British radio scientists in the 1930s, when it was clear bombers were going to be a lethal machine of war. "Could a pilot be incapacitated by beaming a strong radio wave at him?" At the time the answer was no, the then known methods of generating and radiating a radio beam were nowhere near powerful enough to raise a human body a few degrees to fever temperatures. So the reflection method (radar) was developed to give early warning of approaching bombers.
If the metal strip was half the wavelength of the radio wave it was resonant, and reradiated the wave very efficiently. (In oscillating along the strip and back the electric current stayed in step with the radio wave driving it.) The British manufactured aluminium foil strips equal in length to half the wavelength of the German radars and these were dropped whenever it was required to swamp the German radar detectors. (German civilians used them as free Christmas tree decorations.)
Radio waves travel at 300 million metres per second so a wave of wavelength four metres (twice the height of a human) has a frequency of about 75 MHz, the frequency of early television signals.
If you look at an old TV aerial you will see that the longest rods are about the length of a human. Of course the human body isn't a good electrical conductor so is not a good aerial. The very lossy nature is shown by microwave ovens being very efficient at heating anything with water molecules in it. And we are mostly water.