Re: [44net] OFDM Modem

A couple of decades ago, the "GRAPES" WA4DSY 56kbit modem kit was available for a moderate price. They weren't too difficult to put together. Alignment did take a scope, but took only a few minutes. About five people in San Diego had them. As far as I know, only three ever made it onto the air here. It generated and received at 29 MHz, and you used an off-the-shelf transverter to put it on whatever band you wanted to actually use -- in my case, 70cm. We operated them on 433.05 MHz to put them in the (barely)-suppresed lower sideband of the ATV folks on 434 MHz. You talked to it with KA9Q NOS. Once you got the multipath problems worked out, it worked rather well when the military radar wasn't eating the band. There's an archive page for the device at http://www.wa4dsy.net/rfmodem.html if you want to know more about it. But you had to build it. It didn't come all put together in a cardboard box. You couldn't just buy one, some coax, and an antenna and put it on the air. I built two of them. We put one up on at our repeater site on a 3500 ft mountain, hooked to a very sophisticated multiport packet controller, which also had 9600, 4800, and 1200 bps ports, on 70cm (439.05), 6m, and 2m. I tried to get an internet connection working on the 56k port via UCSD, but the 70cm multipath problems smeared the symbols enough that it never really worked. Eventually I turned my interest to other things. A few years later, when the controller succumbed to an excessive accumulation of rat urine, no one noticed it wasn't there for months because the 1200 bps digipeater function still worked. Eventually it all went to the metal recyclers. Perhaps I'm wrong, but I don't believe there is any future for a 70cm 56kb radio in the ham market. It's too slow and too-range-limited to be interesting. The only people I can see who would buy them are foreign military and government users. - Brian