On Aug 25, 2009, at 12:21 PM, Charles Scott wrote: > Nate: > > As often happens, I find myself agreeing with what you say. > > I'm wondering, however, if anyone has configured D-Star systems with > separate antennas and no duplexers. We have a UHF analog system here > that's astounding in how it performs that's built this way with a > high-level preamp at the RX antenna and that shares transmission line > with the VHF at the same site. I've been looking for combined VHF/UHF > antennas to consider for our district D-Star project to see how the > pricing comes together. >
My only comment on this is that -- yes, it's possible, just like an analog system. You need appropriate isolation between TX and RX still, and at UHF this is usually done by vertically separating the antennas by roughly 100'. Another problem is in measuring that isolation. With no convenient/ inexpensive D-STAR test equipment, you might have to build up an analog repeater or at least use a separate TX and RX on the antenna system, just to test the antenna setup with commonly-available test gear. As far as dual-band antennas go, they can also be problematic, and there's only a few high-quality commercial options. Many hams run repeaters on things like Comet and Diamond dual-band antennas, but I find they simply don't survive long enough to make the pricing difference worth my time, especially when major tower climbing is involved to swap them. A single-band Sinclair folded-dipole type antenna (or Decibel, or whoever) when properly installed will last 10+ years on the tower *without maintenance* and without performance degradation if water is kept out of the feedline, etc. > Chances are we'll be required to have circulators at the sites we're > considering anyway, so we're looking at the following possible > configurations > Circulators/Isolators are often required by commercial sites, always consider that they're passive IM generators (they do create noise) and should be followed by proper filters. Many companies sell assemblies called "isolator panels" that combine the circulator/isolator(s) and filters on a rack mount unit. > The advantage of this would be that we'd only run one large > transmission > line for both VHF and UHF TX (suppose we could also share with 1.2G if > we were doing that), it would use cheaper and lighter transmission > line > for RX, doesn't need duplexers, does have a bit more antenna, but also > has about 12 dB better receive and less transmit loss. > Remember and "splitter" type device is going to have loss. The PA's on the Icom gear aren't exactly "beefy" 100W PA's. I think individual feedlines are smarter, plus add a bit of "backup" if a feedline were to be damaged by something ... at least the other repeaters not on the damaged feedline will work. Basically it's the KISS principal. While single feedline LOOKS like it follows that principal, the fact that you now have a device (di- plexer or other form of combiner, like a cavity combiner for low-loss) in-line that everything has to pass through, you end up with a single- point-of-failure problem. One lightning strike that makes it to that device and all repeaters go off the air. So far, W0CDS's achilles heel has been Icom's poor design decision to feed DC power through the RP2C and then to all modules. Power outages on the DC side of things have taken down the entire stack, twice. We're redesigning the power distribution to handle the RP2C and each module as separate powered devices. That way, the RP2C and it's power are still a single-point-of-failure in the design, but not an individual RF module shorted or otherwise down. Better chance that something will be operational if a single module drops off-line for some reason. Speaking of power... these RF modules aren't built anywhere CLOSE to commercial quality, and I highly recommend protecting your incoming AC to your DC supply with MOV's or similar in lightning-prone areas (we take more damage to our repeaters over time from the power line than we do from antenna/tower lightning strikes), etc. Other things we haven't mentioned to the new person just starting out... many people found the quality of the internal RF jumpers inside the repeaters to be of exceedingly low quality and their repeaters on certain frequencies were exhibiting internal desense. Replacing the internal cabling with proper silver-teflon double-shielded jumpers (just like you'd use to get to the duplexer from the repeaters) helped many folks extend the range of their repeaters. (Due to ownership issues and paranoia, W0CDS hasn't had this done on any modules, sadly.) If you're at a VERY high RF site with lots of signals, the shielding of the boxes themselves is pretty piss-poor, too. But those are "advanced" topics beyond just getting a system on-air and operating. -- Nate Duehr, WY0X [email protected]
