On Apr 20, 2010, at 5:43 PM, Daniel G. Thompson wrote: > Bob is definately correct that the typical 4 can setup used > on the average FM repeater will cause you nothing but > headaches, and that includes the ultra nice TX/RX stuff. > You either have to go with a 6 can setup using pass/reject > or like Bob said, throw an DCI filter between the > duplexer and receiver. This is also true of the VHF as well. > We fought with this problem for a couple of weeks before we > went on the multiplexer with about 40' between the antennas. >
The "traditional 4-can setup" often doesn't work well with Analog systems either, but the owners are too clueless to test it and see. :-) Most 4-can setups also really need a large hi-Q pass cavity on the Receive side to reject out of band and close-in crapola at multiple-transmitter sites. > Bottom line I think for this thread, is repeaters are fussy when > it comes to a good feedline and antenna system, and D-Star > is even fussier. On D-STAR it's just damn hard to test it properly. So you do the pants+suspenders thing and over-filter it a bit. The receivers are more sensitive than what most analog repeater operators are used to working with (60's vintage through 80's vintage receivers that had nice tight front-end filtering built in, vs. a badly shielded aluminum box that blocks nothing, bad quality coax internally -- which can be fixed -- and a really sensitive but not very SELECTIVE receiver... inside the "pretty" D-STAR aluminum box). I'll take the front-end helicals of a MASTR II any day of the week -- even the loss! -- over the Icom RF design that looks like it was built out of two mobiles in a box... oh wait, it was... One was ENGINEERED, the other one was ASSEMBLED. There's a big difference between those two schools of thought. One doesn't require a school at all, in fact. ;-) Again I'll rant lightly that without real numbers for when the CODEC itself falls apart (bit-error rate) and a way to test it... testing these things properly is an exercise in a whole lot of ASSUMPTIONS and trial-and-error style antenna/filter system design. Sharing real-world stories of what works and what doesn't, kinda fixes that, but it sure would have been easier if it wasn't left to the users of the repeaters to beta-test them for the manufacturer. A couple of models of "expensive as hell" Service Monitors can now record an off-air digital signal and then play it back, so you add attenuation and reverse-engineer any type of digital repeater, but expect to be in the $40K range for them, list. Probably $20-$25K real-world, pricing. Not something we're going to see too many hams doing, I suspect. -- Nate Duehr, WY0X n...@natetech.com
