Can Antennas Really Emit Zero Radiation?

[tfleming]

Because of its direct link to self-fields such as described in the EM self-field theory, I want to talk about how antennas can be designed to emit zero nett radiation. Although antennas were shown by Hertz in 1888 to 'always' emit radiation he used only one antenna, a wire halfwave dipole with a gap at its centre; if we use two antennas working in sych with each other we can in fact design the system to emit ZERO NETT RADIATION. We are assuming here that there are two antennas (they are in fact magnetic loop antennas designed to operate at half-wavelength. i'll walk you through it verbally for a start and if I can I'll try to find out how to insert maths equation (help required mr moderator-how can I show Maxwell's eqns?);

Assume that the fields are NOT point-charge to point-charge. This is a classical concept that was born out of the experiments of Coulomb, Faraday and others around 175 odd years ago. The form of the inverse square followed Newton's gravitational law, and the experiments in electricity and magnetism are MACROSCOPIC; but we are interested in atoms say (to begin with). Looking at OTHER ways that people have dealt with such issues, and if you've done a masters thesis in axisymmetric antenna structures, you tend to learn a lot of good real world maths including some of the older techniques, Von Hippel, "dielectrics and waves" Wiley, 3rd printing, 1962, uses a rotating vector. He solves the problem of far-field radiation from a dipole antenna. In my phd (bioelectromagnetics), I studied this in regards a similar problem where it was desired to obtain ZERO RADIATION in the far-field. The way to do this is by adjusting components so that the RADIATION IN is equal to the RADIATION OUT (remember we are treating the field as ubiquitous and infinite, which turns out to be incorrect, but for this case there's heaps of energy residing IN the field, stored in the infinite field).

So we CAN in fact have zero radiation antennas; this leads to a realisation of exactly what is an 'imaginary' field and what it means physically. the antenna structure needs to be a cross dipole where there is a phase difference of pi/2 (or "j" between the ttwo dipoles. This is NOT an electric dipole but a ring dipole, a magnetic dipole. and so we have two toriods which have to 'access' each other, so most conveniently we have a solid sphere of metal in which two oscillating fields are established (no mean feat, but nice theoretically)-so much for lecture 1! see you tomorrow

 

[tfleming]

ok, so let's just over a couple of things from what i said yesterday; Hertz used ONE rotating vector exp(jwt) to show how energy being radiated is "always positive" in the far field of an antenna. He set up a loop across the room from a half-wave dipole and found that the loop picked up a current in the same direction all the time. Ok, fine but how did he use the rotating vector?? Well we can imagine an X-Y plane in which a unit vector is rotating. The X plane can be though of as the E-field being generated by the antenna and radiated into the far-field. So what is the Y axis? We talk about this as the imaginary field that allows us to construct the "physical" or real fields being generated which we can measure in the real world and that we see is oscillating.

This experiment must have looked like something out of a Frankenstien movie with brass/copper balls across the antennas gap. In the darkened room the gap between the spherical balls was 'sparked' by contact with an electrostatically charged rod. The currents across the loop were seen to attenuate in time as the charge ran down. Hertz applied his well-known potential theory to solve the maths and the rest as they say is history! But as Von Hippel explains we can obtain the far-fields via the rotating exponential (which is just what we do when we solve ordinary and partial differential equations). BUT Hertz only used one such exponential, and so did Von Hippel (in 1962). So this tenet of physics that the radiation must always be in the one direction is based on a limitation of the oscillation itself!

When we can use TWO such oscillations we find that the limitation is removed and just like ode's and pde's, we find that the maths heads in BOTH directions depending on the physical arrangement.

ok, see you tomorrow for more, we'll talk more about those 'imaginary' fields.