Sunday, July 11, 2010

Microwaves And The Amateur Satellite Program

The OSCAR satellite program utilizes several amateur microwave bands, and future projections call for yet more use of these bands . OSCAR8, for example, produced a mode-] output on 70 em that could easily be received by basic amateur setups. The OSCAR 9 satellite includes beacon transmitters operating in the 13-cm and 3-cm bands, which again reflects the wave of future events. OSCAR Phase III satellites are projected to afford communication capabilities in the 23-cm, 13-cm, and 3-cm bands, thus our amateur microwave spectrum may become quite popular and commonplace during the mid 1980s. See Fig. 1-7.

Fig. 1-7. OSCAR 8, a Phase-II Amateur Radio satellite, orbits approximately 800 miles above the Earth, where it relays 7D-cm,2-meter, and to-meter signals. Future (Phase-III) spacecraft will use 432, 1260, and 10,000 MHz to provide hemisphere-wide communications capability.

The microwave spectrum, with its reliable line-of-sight propagation, is particularly appealing for future geostationary (Phase III) OSCAR satellites. Relatively large dish antennas can be directed at these satellites, resulting in very dependable communications. Through the use of earth-based microwave OSCAR links, one or two spacecraft may be interlinked for near global communications. Future OSCAR satellites are destined to be recognized as prime users of amateur microwave frequency allocations.

The microwave spectrum in its entirety promises to be a major factor in future amateur-radio pioneering. The vast bandwidth allocations, combined with computer communications and other advanced technology forms, will permit this range to be used in a heretofore unrealized manner. Dependable and reliable amateur communications with distant lands will be provided by long range OSCAR satellites, while cross-country microwave networks will provide nationwide signal linking.

Hand-help FM transceivers will also gain "seven-league boots" through microwave links and FM-to-SSB converters situated at OSCARsatellite uplink points. Also, EME systems may use moonbased microwave repeaters. Amateur pioneering efforts, however, will not cease ; a creditable rise of interest in radio astronomy will serve as proof of that situation.

The following chapters of this book describe, in easy-tounderstand form, the exciting world of amateur microwave operations. Separate discussions of the history of microwaves, getting started in microwaves, and detailed information on equipment and operations on various bands is included. This works is thus a guide for microwave newcomers. Here's your invitation and join the excitement of this challenging amateur frontier. Come on along and get in on the action! See Fig. 1-8.

Fig. 1-8. A view of the future of Amateur Radio communications? A 10-GHz Gunnplexer and 2-meter hand-held transceivers combine to expand the horizons.

Tuesday, July 6, 2010

Microwave and EME

The microwave range has, for many years , been synonymously related to amateur moonbounce activities . Centering on the 70-cm, 23-cm and 13-cm bands, amateurs have often successfully communicated over this Earth-Moon-Earth path. The parameters associated with moonbounce are many: they include considerations of atmospheric losses, faraday rotation, moon-encountered losses, galactic noise interference, etc. A general outline of these parameters is illustrated in Fig. 1-6.

The Earth-Moon-Earth distance varies between 225,000 miles (perigee) and 250,000 miles (apogee), producing fluctuations of up to 2 dB of reflected signals-a difference between communicating and not communicating via this difficult path. The EME signal is also masked by a variety of noises and requires top-notch earthstation setups plus high-gain antennas and high transmitted power levels for ensured success. The minimal acceptable rf-output power is 400 watts, and the minimal antenna-gain figure is 20 dB. These parameters do not allow any leeway for additional signal fades or noise, thus one can logically surmise that EME communications reflect extreme challenges for only the stout hearted!

Fig. 1-6. Some of the many parameters affecting uhf and microwave EME signals.

The full aspects of EME communications are beyond the scope of this book, thus the reader is referred to more specialized works in this particular area. Rest assured that additional information and equipment for EME operations will be a natural part of tomorrow 's innovations.

Higher Bands

The 15-mm and higher amateur microwave bands represent truly challenging and unpioneered frontiers in communications. Until recent times, the prime drawback to amateur operations in this range has been a lack of available gear, parts, and technical information.

Again, Microwave Associates of Burlington, Massachusetts, has recognized this situation and provided a means of ' operation. Special Gunnplexers for 24 GHz and (upon special order) 48 GHz are available for less than the cost of many 2-meter transceivers. This inspiring challenge can open new doors for amateurs, and firmly establish those involved as pioneers ini microwave history. What else could one ask? Yes, today 's Golden Age of Radio is alive and well-particularly in the unpioneered regions of microwave communications! See Fig. 1-5.

Fig. 1-5. Author Dave Ingram, K4TWJ, makes preliminary focal-point adjustments in a 10-GHz Gunnplexer and 3.5-foot dish antenna to be used in a microwave link . The system is capable of relaying amateur high-frequency band signals or amateur television (ATV) signals.

3 cm

The 3-cm (10-GHz) amateur band is gaining popularity at a very creditable rate. The primary equipment used for these 10-GHz activities is the Gunnplexer. The Gunnplexer has a Gunn diode located in its 10-GHz cavity , which is directly mated with its waveguide and horn-antenna system. The complete 10-GHz unit functions as a "front end" for a lower frequency unit that acts as an i-f stage. A small portion of the transmitted signal from each Gunnplexer is used as the receiver's local oscillator .

A further clarification of this technique is shown in Fig. 1-4. The two communicating Gunnplexers are frequency separated by the amount of the desired i-f, which is 146 MHz in this example. Both Gunnplexer transmitters remain on continuously, thus providing a local oscillator for mixing with the 10-GHz signal from the other unit. The ultimate result is a 146-MHz signal appearing at the i-f port of each Gunnplexer.

These 3-cm communications systems have proven their abilities over paths of 100 miles (160 km), and several European amateurs have communicated over 500 km (310 miles) on 10 GHz. An attractive plaque , sponsored by Microwave Associates of Massachusetts, awaits the first 3-cm pioneers to break the 1000-km (621 mile) range on this unique band. Gunnplexer communication networks are ideally suited for data communication links and multichannel TV relays, and as such could truly mark the direction for future .developments in amateur communications.

Fig. 1-4. A basic Gunnplexer communications system for 10 GHz. Each Gunnplexer oscillator provides energy for transmitted signal and couples a small amount of that energy into a mixer for heterody ning the received signa l down to an i·f range. The two transmitter signals are separated by the frequency of the chosen i-f.

5 and 10 em

The 10 cm and 5 cm amateur bands have received miniscule interest during the past, primarily due to the lack of effective gear capable of operation in this range. The recent escalation of interest in satellite-TV terminals capable of operating in the 3.7- to 4.2-GHz range, however, shows great promise in ratifying that situation. Since many telephone companies utilize frequencies between 5 and 10 em for broadband relays of multiple voice links, evolutions may also provide a surplus of modifiable gear for radio amateurs.

13 cm

The 13-cm amateur band holds particular appeal for future amateur activities. Its proximity to the MDS band permits use of inexpensive 2-GHz downconverter receiving systems and 2.3 GHz transmitting gear in a very cost-effective manner.

A group of amateurs in a given area can actually become operational on 2.3-GHz for a lower expenditure than on almost any other amateur band. Direct communications on 2.3 GHz typically range from 20 to 60 miles, depending on terrain and the antenna systems employed. This spectrum is especially attractive for such wideband signals as multichannel fast-scan TV, multiplexed data links, computer interlinks, etc.

A number of 2-meter repeaters could also be linked via 2.3 GHz, and the line-of-sight propagation would permit ' peaceful coexistence of several of these services in any particular metropolitan area.

MDS and Satellites

Situated between the amateur 23 cm and 13 cm bands are two particularly interesting commercial services. The weather satellite band used for studying cloud formations from approximately 20,000 miles above earth employs 1691 MHz while the public carrier service of MDS (acronym for Multipoint Distribution System) employs the range of 2100 to 2150 MHz.

Although reception of weather satellites has previously appealed primarily to commercial services, numerous amateurs are realizing the advantages of this capability,and are constructing their own receiving systems. Several inexpensive receiving kits have been recently introduced for satellite reception.

The MDS band may best be recognized by its recently dubbed nickname of "microwave TV braodcasting." Carrying restricted- type viewing similar to cable-TV programming, microwave-TV systems operating in the 2.1 GHz range are springing up across the nation. Reception of these pay-TV signals may be accomplished through the use of relatively inexpensive 2.1 GHz downconverters.

Additional information concerning this commercial activity is presented later in this book. The United States space shuttles also use the 2.2-to 2.4-GHzrange during flights. Numerous educationaltelevision services also frequent this spectrum for point-to-point relays .