We have seen how radiated far magnetic and electric fields can be calculated if the current density on the antenna is known or can be fairly assessed. For small dipole and loop antennas we can assume the current density to be constant. For thin dipole antennas we can—in a first order approximation—assume the current to be sinusoidal. For an increased accuracy, more terms must be added, complicating the analysis. For large loop antennas (circumference larger than a tenth of the free-space wavelength) we can only find a solution easily if we take measures to keep the current density uniform and in-phase. For aperture antennas, like a horn or parabolic reflector antenna, an assessment of the current density over the conducting, three-dimensional structure in general is a far from easy task. Often, however, it is possible to make a fair assessment of the fields in the opening or aperture. In this chapter, we will use these fields to form equivalent sources and calculate the far magnetic and electric fields from these equivalent sources.
When looking at an aperture, like for example the horn antenna shown in Figure 7.1, we immediately see the difficulty in assessing the current density on the metallic parts of the antenna.
An assessment of the fields in the opening or aperture of the horn antenna is fairly easy though. ...