It is well-known that panel type antennas can be used to construct multi-channel antenna arrays which have almost constant electrical characteristics over a great bandwidth. The radiation pattern of these antennas can be shaped by optimizing the position, the magnitude and the phase of the feed. But it is also possible to use panels to construct multi-pattern antennas which simultaneously have several different radiation patterns with each pattern corresponding to a different channel.

Today, broadcast transmitter networks are very dense and it is almost impossible to install a new transmitter with an omni-directional hrp, as in former times. The frequency regulation authorities insist on the use of directional radiation patterns to protect existing stations. Even where just one site is concerned, the requirements are not the same for different frequencies and so it is almost unavoidable to construct separate directional antenna systems, even though a single broadband panel antenna array could cover the whole frequency band. Therefore, plans to start up a new station may fail because the mast has no free antenna aperture or because the mechanical capacity and the costs of erecting a new mast are too high.
Kathrein's antenna engineers have developed and sucessfully installed antenna systems which produce different horizontal radiation patterns for different frequencies but using the same antenna aperture and panels. So they can be called multi-pattern antenna systems.

Principle of operation

The radiation pattern of panel antenna arrays can be optimized through the proper alignment and feeding of the individual panels.
For a multi-pattern antenna the feeding must vary with the frequency to produce different radiation patterns because the position of the panels cannot alter with the frequency.
In practice, a special network consisting of power splitters and combiners is set up on the ground and separate feeder lines are installed for groups of panels pointing in the same azimuth direction. The power splitting network on the mast, which is usually organized according to the bays, then has to be changed because only the panels in one vertical row are fed by the same signal.

Limitations on multi-pattern antenna systems

There is no general restriction on the number of frequencies which can be transmitted by a multi-pattern antenna, but the number and shape of different horizontal radiation patterns which can be produced by one set of panels is limited.
Panels covering a 90ø or 120ø azimuth sector are available for constructing 3 or 4-sided arrays to radiate all around the mast. To adapt the horizontal radiation pattern to the actual require-ments, the position of the panels may be varied. For different patterns it is, of course, necessary to look for a compromise similar to searching for the smallest common denominator. So the number of frequencies which can be radiated by a multi-pattern antenna depends on the individual ERP limitations and the tolerances which are acceptable.
Multi-pattern antennas can be constructed for both FM and TV broadcasting.

Multi-pattern Antenna System at MARLOW

The German Telekom wanted to install seven FM transmitters at the "MARLOW" station, each with different ERP limitations which could be sorted into two groups of frequencies with similar horizontal radiation patterns. The standard approach of constructing two antenna systems was abandoned in favour of the more economic multi-pattern system developed and offered by Kathrein. Because there was no need to split the available aperture, the multi-pattern antenna system could be designed to have greater gain, thus allowing the use of smaller transmitters. The number of feeder lines required for a multi-pattern system is not more than for two independent antennas. The only extra costs involved with multi-pattern systems are those for some additional combiner units and power splitters. Figure 1 shows the set-up of the MARLOW multi-pattern antenna system which was optimized to reduce the complexity of the feeding network. The four transmitters which had to have the radiation pattern type 1 were combined using a standard multiplexer consisting of directional filter couplers. Then the power was split to feed all the panels with an equal power. The power of the other three transmitters was split to feed the panels unequally in order to produce the directional radiation pattern type 2. With every frequency a pair of identical directional filter couplers was used to add the signal. The outputs of the combiner chains were connected to a patch panel permitting switching to half antenna operation. Each half antenna consisting of four bays was fed by one pair of cables. The power was distributed to the panels by one 4-way and one 8-way splitter.
The complete combiner network was assembled and tuned in our factory (see figure 2) and then transported to the station.
After installation, the performance of the complete multi-pattern antenna system was tested by measuring the radiation properties with the help of a helicopter. The results comformed well with the predicted patterns and thus pved the possibility of constructing not only multi-channel, but also multi-pattern antenna systems.

Figure 1: Multi-pattern Antenna System of 7 FM Channels for MARLOW

Figure 2: Specially designed Combiner Network during Factory Test

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KATHREIN-Werke KG Rosenheim
last update 14.03.2000