Tuesday, July 6, 2010

The Amateur's Microwave Spectrum

The electromagnetic spectrum of microwave allocations is one of the hottest and fastest-rising frontiers in amateur communications technology. This unique frontier offers a true kaleidoscope of unlimited challenges and opportunities for today's innovative amateurs. Although a relatively uncharted area until recent times , today's microwave spectrum is gaining a widespread popularity and rapidly increasing acceptance. This trend shows no signs of waning; indeed, microwave communications are destined to mark the path of future developments in amateur communications. These communications will include all modes, from data packeting and multichannel television relays to multichannel voice links of FM, SSB, and computer interlinks. While the line-of-sight propagation associated with microwave communications would seem to restrict its capabilities , such is not necessarily the case. This situation has been commercially exemplified in such arrangements as longdistance telephone microwave links, television microwave networks , etc. These systems provide broadband cross-country and intercontinental linking. Transcontinental linking has been accomplished by geostationary communications satellites. Amateur radio is destined to progress in a similar manner; furthermore, amateur satellites capable of providing these interconnect functions are being developed at this time. The future of amateur radio looks quite promising and very exciting , and microwave communications will playa major role in its developments.

The h-f band operator of today might ponder the logic of using microwave communications. Why switch from the populated rf areas to a seemingly vast, empty, range of extremely-high-frequency spectrum, when few amateurs operate that range? One reason is that the line-of-sight propagation of microwaves affords reliable and predictable communications , independent of solar or weather conditions. Extended communication ranges are possible using one, two or more microwave repeaters. Additionally, the wide bandwidth associated with such repeaters allows multiple communications to be simultaneously conducted .

The following example may further clarify this situation: Assume two amateurs living in metropolitan areas separated by one (or two) mountains. They desire to set up a fast-sean-television repeater station. Although an in band 70-cm (420 MHz) system could be used, it would require expensive filters and duplexers for effective operation, and that operation would carry only one transmitting ' signal at a time. A crossband fast-scan repeater operating with an input on 70 cm and an output on 23 cm (1240 MHz) or 13 cm (2300MHz) would alleviate the problems and costs of special filters and duplexers. However, its operation would still be confined to only one transmitting signal at a time. Thinking ahead, the two amateurs would set up a relatively inexpensive 10 cm (2300 MHz) or 3 cm (10,000 MHz, or 10 GHz) "bare bones" repeater station for relaying their signals across the mountainous area. At any later time, other amateurs c04.1d join the activity simply by adding the appropriate microwave " front ends " to their setup . An additional microwave link could then be added at one, operator's location for further feeding the signals to other interested amateurs. Each new addition to the network would carry its own weight in equipment support/finance, causing the system to grow and expand precisely in the direction ofmost interest. The original two network-instigating amateurs are now part of a multioperator system.

Further, let's assume several amateur-radio computer enthusiasts, plus some amateur RTTY (radio teletype) operators , and a number of voice-only operators desire to join the network. The vast bandwidth capability of this system stands ready to accommodate the new group of amateur operators: only minor alterations in power levels and antenna configurations are necessary.

The network continues to grow until several communities and cities are linked in a totally reliable and predictable manner. An amateur satellite uplink/downlink is added to the network, along with electronic-mailbox and intelligent-voting systems, .plus emergency/priority interrupts for special requirements. The network ultimately spans coast to coast and continent to continent, conveying many forms of amateur-radio activity. Each new area would be responsible for its own expenditures, and thus the system carries its own weight. The original instigators, plus many fellow operators, now enjoy multimode communication from small, personal, transceivers that access the network via simple 2-meter, 70-cm, or newly introduced 13-cm units.

Science fiction? Hardly. A vision into the near future? Surely. Realizing the many beneficial aspects of microwaves, only one of which has been exemplified here, we can truly calculate that amateur operations during this and subsequent decades will flourish through utilization of all available assets-and the microwave spectrum is one of these prime assets. A simplified example of the previous discussion is shown in Fig. 1-1.

Fig. 1-1. Simplified overview of a basic microwave network that can be expanded to cover many areas and modes.

Moving in a slightly different direction, let's now consider a more personal application for which microwaves could again prove useful. An individual microwave link can be used for remote highfrequency receiving setups. Several wideband converters, for example, can be connected to respective antennas and used for reception of all hf bands. The resultant wideband spectrum may then be microwave relayed to an amateur's home location or transmitter site. Following retrieval of the h-f spectrum from the microwave receiver's output, conventional signal processing can be utilized for producing a truly optimum DXing setup, The signal diversity creates unique capabilities which thus allow a station to perform in a definite "top-gun" manner. See Fig. 1-2.

Fig. 1-2. Basic arrangement for a remote receiving site linked by 10-GHz microwave equipment.