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Antennas for MMDS - Multichannel Multipoint Distribution System

2000-05

MMDS started in the USA a number of years ago when the frequency allocation was 2.5 to 2.7GHz. The original plan for this frequency was for Pay TV, for use where people could not get cable and digital or satellite was not available. The 200MHz of spectrum could offer up to 12 channels of TV carrying local and national programmes as defined by the local broadcaster. Over time the number of people using MMDS has not been particularly large with only a small percentage of US households having access to the system.  The system was eventually adopted in other countries, mainly in Eastern Europe, Ireland and South America.

MMDS base station antennas were usually high gain (11 or 12dBi) vertically polarised omnis antennas into which would be put several hundred watts of RF power. On the receive side households would have a very high gain yagi or grid pack style parabolic aerial at least 3ft in diameter, the size of which depended on the range. The system was broadcast only. Generally, because of the emergence of other broadcast methods the system languished. However, the availability of  200MHz of spectrum has caught the attention of companies who see this as a major asset, especially at these relatively low frequencies.

In 1999 the US government permitted the use of the MMDS spectrum for 2-way digital data communication. This has opened up opportunities for major companies to buy the spectrum licences and use them for other applications. Because of the potential for the use of this spectrum in densely populated areas and probable restriction on power levels from the base station antenna, and the fact that this would now be a 2-way system, the omni directional antennas originally used for TV broadcast are likely to be replaced with sector antennas within new digital systems.

Cobham Antenna Systems (Microwave Antennas) has been produced very slim 60 degree sector antennas with 17dBi gain. These have been used for 'beam bending' or where the general omni antenna cannot reach the last sector or valley. A signal would be picked up off air using a high gain directional antenna and would be re-broadcast through the beam bending or sector antenna.

For digital applications that are likely to go into more densely populated areas where tower space is at a premium, the benefit of having a slim, low profile panel sector antenna becomes very important. As a result of having the products and technology readily available for these new applications as and when they emerge, Cobham Antenna Systems (Microwave Antennas) has been approached by equipment manufacturers to provide medium and high gain 60 degree sector antennas with vertical, horizontal or dual vertical and horizontal polarisation. The company has also put two dual polar (V/H) antennas into one housing for diversity applications. All of the proposed panel antennas are no more than 12mm deep (0.5 inch) and have been designed to integrate directly with the radio transponder to minimise internal and cable losses. Working closely with these customers has ensured that all mechanical and electrical interface requirements are met.

In order to make best use of the RF frequency spectrum, frequency reuse is vitally important. Base station antennas have been developed with very low sidelobes and front-to-back ratios in order to help optimise the use of each broadcaster's frequency allocation.

On the subscriber side of MMDS, the use of parabolic reflector antennas is likely to be reduced significantly in favour of printed circuit antennas that can be fully integrated with the consumer transmitter receiver unit located on a wall or roof mounted. The benefit of this is the aesthetic nature of a fully integrated box with minimal visual impact when compared to that of a dish. Similarly to the base stations, the reduction of internal losses due to either very short or no cable interfaces helps to increase system performance and reduce the volume of the consumer unit.

Cobham Antenna Systems (Microwave Antennas) has developed proprietary techniques for the manufacture of integrated subscriber antennas to ensure that prices remain competitive when the products become a consumer requirement. The initial applications for digital MMDS will be high data rate, broadband internet access and digital video.