This invention relates to antenna polarizers .and in particular to ferrite rotators of the type which may be used in UHF or microwave antennas.

For reception of satellite transmissions, e.g. television signals, an antenna, typically a parabolic dish, is pointed at a satellite. Many satellite transmissions use linearly polarized signals and cxarπionly both vertically and horizontally pol.arized signals are radiated. The receiving equipment therefore requires same means for selecting which polarization to receive.

One way of selecting the desired polarization is to physically rotate the antenna pick-up-probe in the waveguide.

Another way of selecting the desired polarization is to use a ferrite rotator which consists of a piece of ferrite within the antenna horn with a coil wound .around the outside of the horn. This is known as a Faraday Rotator. When the ferrite is magnetized by a current in the coil, the polarization of the received radiation changes frc 0 to 90 degrees provided the ^" correct current is used for the frequency in question.

In a satellite receiver for television signals the frequency range may typically be 10.95GHZ to 11-75GHZ and it is desirable to have a 0 degree or 90 degree polarization rotation selectable by varying the current in the coil. The problems encountered with such a system are typically a high loss of signal and the fact that the polarization rotation varies significantly with frequency. If rotation changes significantly from the desired angle then there can be problems with cross polar interference if another signal is

being transmitted on the other polarization at the same frequency.

One object of the present invention is to provide a polarization rotator which requires only two values of current to select 0 or 90 degrees polarization throughout a predetermined frequency band e.g. 0.amps give 0 degrees polarization and N amps gives 90 degrees polarization.

According to the invention there is provided an antennna horn selectably responsive to each of two perpendicularly polarized signals, the antenna horn comprising a waveguide, the waveguide containing a ferrite rod within a dielectric sleeve, and a coil wound around the outside of the waveguide, the arrangement being such that the horn is selectably responsive to signals polarized in a first direction and a second direction substantially perpendicular to the f rst direction, the selection being made by setting the current in the coil to either a first or a second predetermined value respectively.

The invention will now be described in more detail by way of example with reference to the drawings in which:

Figure 1 is a side-sectional view through an antenna horn embodying the invention;

Figure 2 is an end-sectional view .along the line X-X in figure 1; and

Figure 3 is a perspective view of the horn of figures 1 and 2.

The horn shown in the figures comprises a tapered waveguide 2 closed at one end with a pick-up-probe 4 entering the waveguide 2 close to the closed end. Fitted to the opposite

open end of the waveguide is a choke ring 6, the purpose of which is to kill currents circulating in the open end of the waveguide.

Within the waeguide is a ferrite rod 8 which has tapered ends 10. This rod 8 is contained within a plastics sleeve 12 which also has tapered ends 14 and is of substanti.ally the same length as the ferrite rod 8. The plastics sleeve has a known dielectric constant. The ferrite rod 8 is a push fit within the plastics sleeve 12.

Around the outside of the waveguide 2 is a multi-turn coil 16.

The ferrite rod 8 is magnetised by passing a current through the coil 16. With no curent passing through the coil the pick-up-probe 4 is responsive to signals polarized in a first direction and when current passes through the coil the magnetised ferrite rod causes the polarization to rotate through 90 degrees.

In the eπibodiment described here the arrangement is such that the antenna horn is to be used in the 10.95 GEE to 11.7GHZ frequency band. The dimensions and shape of the waveguide 2, ferrite rod 8, and plastics sleeve 12 are chosen such that for no current flowing in the coil 16 the polarization of the received signals will be 0 degrees and for a chosen current, the polarization will be 90 degrees.

In this particular freqency band the chosen dimensions are as follows. The waveguide 2 is 60mm long with an internal diameter tapering from 22rπm at the open end to 19mm at the closed end. The ferrite rod 8 and the plastics sleeve 12 are both 20πm long. The ends 10 of the ferrite rod 8 have a 15 degree taper (double cone angle) and the ends 14 of the

plastics sleeve 12 have a 30 degree taper. The plastics sleeve 12 has an outside diameter of 12mm and an internal diameter of 4.75πm. The ferrite rod 8 is a push fit within the plastic sleeve. The coil 16 has 2000 turns and switching the current passing through it from OA to approximately 100MA will rotate the polarization of the received signals from 0 degrees to 90 degrees.

Using the above dimensions in the particul.ar frequency band gives very little variation in the angle of the selected polarization across the freqeuncy band. Thus there is no need to make adjustments in the coil current according to the frequency of the signal being received. There is also very good cross polarization rejection and in this particular frequency band this is of the order of 20DB.

The above effects are due to the fact that the pl.astics sleeve increases the bandwidth of si-gnals that the ferrite rod 8 effects as well as reducing the loss of signal which occurs when the plastic sleeve is not used.

It will be appreciated that using the above construction the size of the ferrite used is smaller than is usually required in such a rotator, thus making it cheaper. The plastics material is also cheap.- The antenna can be easily produced. The use of only two values of current for polarization selection removes the need for .any circuits to vary the current according to the received frequency. And finally, the signal loss is reduced because of the presence of the dielectric sleeve.