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Spring 2003 Issue |
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HSMM
Communicating Voice, Video, and Data with Amateur
Radio By Neil Sablatzsky, K8IT |
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Did you ever want to show off that new rig of yours while talking with your buddies on the local 2-meter FM repeater? Have you ever needed to show your computer screen to an Elmer while following his verbal step-by-step instructions? Were you ever asked to provide live video back to a command center while operating portable and providing tactical voice communication? How about needing to look up a callsign from the Field Day site? Is your station capable of supporting all the above under emergency conditions with one radio while portable? I suspect that most amateurs have answered “no” to my last question, thinking that I’m dreaming! Well, not any more. High Speed Multi-Media (HSMM) communications is now here and is going to be the future rage in amateur radio. HSMM is about being able to communicate over voice, live video, and data modes at the same time! Sometimes we desire full-duplex point-to-point communications, and at other times we want half-duplex multi-point communications. No problem: The infrastructure of HSMM communications is based on IEEE 802.11B protocol and can support both communication modes. HSMM is about being able to access a computer remotely to access and retrieve callsign or other data. That is, at the same time, we are showing video of our club’s barbecued steaks to the hams who are a bit late arriving. Even remote control of your amateur station is possible over HSMM. From a technical side, HSMM is Direct Sequence Spread Spectrum (DSSS) modulation. It is legal to operate DSSS in the amateur 2400–2450 MHz amateur band. Because the amateur band allocation overlaps the U.S. Part 15 Industrial, Scientific, and Medical (ISM) band, which has eleven channels between 2401–2473 MHz, equipment is easy to purchase. In fact, since the wireless Internet has become very popular in the U.S., equipment can be purchased for less than $50 at many suppliers. Currently, the ISM band overlaps the amateur band on channels 1–6. Since the ISM channel spacing is 5 MHz center to center, and the bandwidth is ±11 MHz from center frequency, channels 1, 3, and 5 look the most promising. However, because of the satellite use on the low end of the band, we should avoid channel 1. There is no satellite activity in the channel 5 portion of the band at the moment. It is suggested that HSMM activity use channels 3 and 5 for amateur use. This also helps to avoid most ISM users, who operate on default channel 6 “right out of the box.” To minimize further interference to ISM Part 15 users, most HSMM activity uses horizontal polarization. This is because the majority of ISM Part 15 users are vertically polarized. It is important to note, however, that the ISM stations use diversity antennas to achieve both space and polarization diversity. Space diversity is used to reduce the effects of reflections and RF nulling. Polarization diversity is used to maximize signal strength when portable devices such as laptop computers are positioned without respect to the receiving access point’s polarization. For amateur use, the majority of our links will be of greater distances than from office to office. In fact, we want to communicate over many miles of separation. In this case, using a common polarization and high-gain antennas will maximize the communication distance.
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