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passive repeater experiment
Wednesday, July 1 2009
Some weeks ago when I'd been connecting the greenhouse to the 120 volt power grid, I'd also run four Cat-5e cables in the same trench. These wires had just been hanging out of the wall down in the greenhouse on one end and balled up in the shop area of the garage on the other. Today I routed those wires up into the laboratory, another step along the road of implementing some sort of remote control and monitoring system similar to what is in place for my basement-based solar hydronic system. Once I had the cables reaching from where I wanted them to start to where I wanted them to finish, I could start figuring out which ends in the laboratory corresponded to which in the greenhouse and then color-coding them on each end. Then, once I knew what wires was what, I could experimentally attempt a use for one of the sixteen twisted pairs now at my disposal.
My recent work with building a J-pole FM broadcasting antenna had led me down a rabbit hole of high frequency cable terminology and theory, stuff that, like setting up Windows 2003 Group Policies, I'd just as soon have left outside of my limited brainspace. But now it's in there in some imperfect form, stripped of the many essential mathematical formulæ but with plenty of color illustrations and at least one video clip of a chimpanzee giving a bath to a duck. Part of the new knowledge is a general familiarity with the difference between balanced and unbalanced signals and how to convert between the two. This led me to understand why (and that) twisted pairs are fairly immune to radio noise, suggesting a theoretical use for one of the twisted pairs going to the greenhouse.
Gretchen and I have a set of DECT 6.0 telephones in our house which not only provides a land line signal to six handsets through a single telephone jack, but voicemail messages can be checked from any handset and those handset can page each other to serve as a network of walkie-talkies. The only problem with this setup is that the signal has trouble reaching from the first floor office (where the base station and telephone jack connection constitute a point of presence) to a handset in the greenhouse. The signal is unreliable after passing through sheetrock-clad carpentry walls, across 70 feet of open air, and through a concrete block wall. If the KX-TG1033S system were more advanced, I could set up a cell in the greenhouse, but I've already tried this by attempting to register a phone to two different base stations and it does not work.
So today I tried making a passive repeater system that picked up the 1.9 GHz DECT 6.0 signal in the laboratory with a homemade yagi antenna, and then sent it down a twisted pair the the greenhouse to be rebroadcasted by a dipole quick-built for 1.9 GHz. Not surprisingly, this didn't seem to do anything to improve phone reliability in the greenhouse. But there were too many variables involved in this experiment for me to write off the basic theory of the technique. Sure, the loss in the 125 feet of twisted pair might have been too high, but the yagi, it turns out, was built for the wrong frequency (800 MHz instead of 1.9 GHz).
At some point today I joined Gretchen to watch part of a movie called Fanboys, the road trip movie adventure of a group of Star-Wars-obsessed young adults trying to get a rough cut of the first prequel of the Star Wars franchise. (It's the first "period movie" I've seen that is deliberate set as recently as the late 1990s.) Fanboys had obviously been made with the approval of the actual Star Wars franchise, as it had plenty of authentic Star Wars memorabilia and included cameos by the likes of Carrie Fisher (as well as William Shatner, representing the rival Star Trek stream in SciFi fandom). For me the movie was most amusing when its scenes and dialogue referenced scenes and dialogue in Star Wars. I kept waiting for the trash compactor scene that seemed certain come.
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