Now let's to untangle the first (symmetric) piece. This will turn out to be a remarkably unpleasant job. In fact it is my nefarious and sadistic plan that it be so unpleasant that it properly motivates a change in approach to one that handles this nasty tensor stuff ``naturally''.
We have to evaluate the integral of the symmetric piece. We get:
The vector potential is thus:
To get the fields from this expression by taking its curl, and then the curl of its curl, is - ahem - most unpleasant. Jackson wimps out! Actually, taking the curls is no more difficult than it was for the magnetic term, but untangling the integrals with the result is, because of the tensor forms that appear. Consequently we too will wimp out (in the comforting knowledge that we will shortly do this right and not wimp out to arbitrary order in a precise decomposition) and will restrict our attention to the far zone.
There we need only consider the lowest order surviving term, which
always comes from the curl of the exponential times the rest:
Note that the ``vector''
(and hence the fields) depends in
both the magnitude and direction on the direction to the point of
observation n as well as the properties of the source. With these
The angular distribution is too complicated to play with further unless you
need to calculate it, in which case you will have to work it out. The total
power can be calculated in a ``straightforward'' way (to quote Jackson).
First one changes the cross product to dot products using the second relation
on the front cover and squares it. One then writes out the result in tensor
components. One can then perform the angular integrals of the products of the
components of the n (which is straightforward). Finally one term in the
resulting expression goes away because
is traceless. The
For certain symmetric distributions of charge the general quadrupole moment
tensor simplifies still further. A typical case of this occurs when there is
an additional, e. g. azimuthal symmetry such as an oscillating spheroidal
distribution of charge. In this case, the off-diagonal components of
vanish and only two of the remaining three are independent.
We can write
At this point it should be clear that we are off on the wrong track. To quote Jackson:
The labor involved in manipulating higher terms in (the multipolar expansion of A()) becomes increasingly prohibitive as the expansion is extended beyond the electric quadrupole terms.Some would say that we should have quit after the electric dipole or magnetic dipole.
The problem has several roots. First, in the second and all succeeding terms in the expansion as written, the magnetic and electric terms are all mixed up and of different tensorial character. This means that we have to project out the particular parts we want, which is not all that easy even in the simplest cases. Second, this approach is useful only when the wavelength is long relative to the source () which is not (always) physical for radio antennae. Third, what we have done is algebraically inefficient; we keep having to do the same algebra over and over again and it gets no easier.
Understanding the problem points out the way to solve it. We must start again at the level of the Green's function expansion, but this time we must construct a generalized tensorial multipolar expansion to use in the integral equation. After that, we must do ``once and for all'' the necessary curl and divergence algebra, and classify the resulting parts according to their formal transformation properties. Finally, we will reassemble the solution in the new vector multipoles and glory in its formal simplicity. Of course, the catch is that it is a lot of work at first. The payoff is that it is general and systematically extendable to all orders.
As we do this, I'm leaving you to work out the various example problems in Jackson (e.g. section J9.4, 9.5) on your own. We've already covered most of J9.6 but we have to do a bit more review of the angular part of the Laplace operator, which we largely skipped before. This will turn out to be key as we develop Multipolar Radiation Fields properly.