The division of a radiating structure into "antenna" and "feeder" is to some extent arbitrary. Usually the feeder conveys power from a transmitter at some distance from the radiating structure, or from the antenna in receive mode to a receiver also at some distance from the structure. If this is done by means of transmission line, maybe twin balanced feeder line or coaxial cable, or perhaps waveguide, the metal of the feeder structure will often pass through the near field region of the antenna proper, and currents will be set up on it which modify the properties of the antenna considered as an isolated radiator.
For example, consider the 70 metre diameter Cassegrain deep space antenna at Tidbinbilla. The primary feed arrangement is shown here...
We see that the reflections from the 10 metre diameter subreflector impinge again on the feed structure, which will scatter this radiation into the sidelobes.
A special case may be made for feeds from a waveguide horn to a satellite reflector dish, in so-called "offset feed" arrangements. The radiation passes from the horn feed to the reflector dish through free space, where superposition of fields applies. The main beam is then directed away from the horn primary radiator and the interaction between feed and antenna is minimised.
Feeders are designed to get the power from the radiating structure to and from the electronics with a minimum of loss, a minimum of group delay distortion, and without contributing too much to the radiated fields. They may also set the polarisation characteristics of the system; differential phase shift may be used with crossed dipole structures to create circular and elliptical polarisations, and the mode properties of waveguide horn feeds may also be used for a similar purpose.
Horn feeds may also tailor the field strength profile and phase profile across a reflector antenna, to achieve desired sidelobe suppression and cross-polar discrimination.
In a balanced antenna radiating structure formed from dipoles or collections of dipoles, possibly arranged as a Yagi-Uda array, the instantaneous voltages on the two arms of the dipole may have a differential mode and a common mode with respect to ground and to objects at large distances. The radiation properties of the rods when fed in common mode will be quite different to those when fed in differential mode. If such an antenna is fed from an unbalanced feeder (coaxial cable, maybe) then there will be a mixture of these modes excited depending on how the feed is connected to the antenna structure. Objects in the near field of the antenna, which do not preserve the symmetry of the antenna structure, may also unbalance the antenna and give rise to time-varying distortions of the radiating behaviour.
An important example of the effects of unbalance in radiating systems may be seen in the hand-held mobile phone. Here, the human holding the phone takes the place of a ground plane to large extent. The antenna rod is therefore primarily a monopole over this ground plane. However, there will be currents flowing on the human which will contribute to the radiation properties. There is only capacitative coupling from the electronics in the handset to the human, and indeed, the phone handset may be placed on an insulating dielectric surface (wood table for example) in which case the antenna rod may be balanced by the equivalent length of the case containing conducting material. In this case the radiating structure looks more like a dipole.
In a typical YAGI-UDA installation, the driven element (driving element in the case of a receive antenna) is a balanced dipole. Very often the feed is an unbalanced coaxial cable; reflections at the feed-dipole junction will give rise to currents flowing down the outside of the coaxial cable braid. This contributes to the radiation and the polarisation sensitivity may be altered from the orientation of the dipole elements. It also affects the radiation pattern and the positions of the nulls. The problem can be addressed with the provision of a balun (balance to unbalance transformer). However, in many practical installations, the upper and lower rods of the dipole elements see different "near field" environments; for example, the lower rod in a vertically polarised Yagi-Uda may be close to the supporting pole, and thus unbalance is introduced into what, in free space, would be a balanced element. Thus the use of a coaxial feeder may be justified, as going to the trouble of introducing a balun may not give worthwhile improvements to the performance.
Frequently, the vertical mounting post will unbalance a Yagi-Uda antenna. To avoid this, the post may be offset from the plane containing the Yagi-Uda elements, or mounted on an extension of the horizontal bar holding the elements apart, behind the reflector element. The objective of both of these mounting methods is to reduce the induced current in the support structure as much as possible.
The performance of many primary horn feeds for reflector antenna structures is improved by applying a corrugated inner surface to the horn and its flare. The purpose of this little note is to explain the significance of the corrugations.
Considering a smooth conductor surface, possibly a plane sheet or a slowly turning (on the scale of a wavelength) curved surface, the electric field has to meet the surface at right angles, and the time-varying magnetic field has to lie parallel to the surface.
Suppose there was a parallel component of electric field. Then very large currents would be induced in the conductor surface, which effectively "shorts out" the parallel E-field component. However, E fields perpendicular to the surface can end on local surface charge. Similarly, if there was a component of changing magnetic field perpendicular to the conductor surface, then the surface would act like a "shorted turn" and very large circulating currents would be induced. These currents prevent the changing magnetic field from meeting the conductor surface at right angles.
Thus, considering the inside surface of a pipe of circular cross section, there can be no component of electric field close to the surface either circumferentially, or along the axis of the pipe. Similarly, there can be no component of changing magnetic field meeting the pipe in the radial direction.
Now consider circumferential slots cut in the inner wall of the pipe. These are a quarter wavelength deep (free space wavelength, that is) and are narrow and closely spaced in comparison with a guide wavelength along the axis of the pipe.
The slots may be considered to be 1/4 wavelength sections of shorted transmission line, and therefore in the direction of the axis of the pipe they can support voltage without drawing any current. This has implications for the effective boundary conditions for electric and changing magnetic fields on a hypothetical surface formed by the envelope of the tops of the ridges either side of the slots.
There can now be a longitudinal component of electric field; that is, a component of E along the axis of the pipe close to this surface. As before, there can be no circumferential component of electric field but there can be a radial component.
Since the guide wall currents are at right angles to the adjacent changing magnetic field, on this hypothetical surface there are no longitudinal currents and therefore no circumferential component of changing magnetic field (d/dt)H. Circumferential currents are possible, so there can be a longitudinal component of (d/dt)H. If the ridges between the slots are very thin, there can be no circulating currents and so there can be a component of changing magnetic field perpendicular to the hypothetical surface.
Summarising the boundary conditions for the corrugated surface, and assuming the slots are exactly (lambda)/4 deep, we have
Axial Electric field - - - - - - Allowed
Circumferential E field - - - - Forbidden
Radial E field - - - - - - - - - Allowed
Axial Magnetic field - - - - - Allowed
Circumferential H field - - - - Forbidden
Radial H field - - - - - Allowed
Here it is implicit that all fields are time-varying at the
guide frequency.
We see therefore that the boundary conditions are the same for both E and H fields, and therefore the propagation constant must be the same for hybrid modes formed originally from a mixture of waveguide TE modes and waveguide TM modes. This fact greatly improves the performance of feeds which generate the hybrid modes needed by reflector antenna structures.
Suppose we determine (theoretically or experimentally) the far field radiation pattern of a horn feed. Normally radiation patterns are specified in terms of power density, but it is also possible to determine the radiation pattern by specifying the complex amplitude (magnitude and phase) of the two orthogonal E field components representing the polarisations of the antenna.
If, on some spherical surface surrounding the horn feed, the phase angle everywhere is constant, then the centre of such a sphere is called the "phase centre" of the antenna (feed). It is important to know where the phase centre of the feed is, as it can then be placed with respect to the main antenna reflector dish by using geometrical optics calculations. For example, on a parabolic reflector dish we might try to place the phase centre of the primary feed at the focal point of the parabolic reflector.
Usually there is no single well-defined phase centre. However it may be possible to approximate the position of the phase centre (a position called the "apparent phase centre") for a limited range of outgoing wave directions. To determine the apparent phase centre one can find the surface in the far field on which the phase is constant. The phase centre for any particular direction will then lie at the centre of curvature of this equi-phase surface.
Copyright D.Jefferies 1997, 1998, 1999.