On Nov 27, 2009, at 10:09 AM, Radioman wrote: > Has anyone run into this problem with the RP2000V repeater: > > at 25 watts i have the worst desense that I've seen in many years, I've > owned and built many repeaters over the years > for my-self and many groups but this is the worst, with a 4 can > duplexer's and a pass cavity i still have major issues > I'm wondering if anyone else has had this issue and what were the fixes > > Tony > NN1D > Trustee for K1RFI
I'm sure someone has. ;-) What does it do on a dummy load directly? How about a dummy load AFTER the cans but before it goes up the tower? In other words, have you done your homework to find out if the "desense" is really desense, or possibly an external mix or something plastering your receiver with out-of-band (or in-band) signals? Is the desense "in the building" or "outside"? What kind of interconnect cabling are you using to the duplexer? What kind of cabling out of the duplexer up the tower/to the antenna? There were some people who replaced the INTERNAL cable jumpers from the mobile rigs hiding inside the module, to the rear N-connectors, but that problem (I thought) was mostly limited to the UHF modules... the cable used inside the modules isn't high-quality double-shielded cabling... as if Icom has zero idea how to build a REPEATER... But I haven't heard of anyone who MEASURED and CONFIRMED they had an internal (to the module) desense problem on VHF. Generally, the receivers are too broad/non-selective, but have "decent" sensitivity. From various reports, some folks have successfully utilized low-gain pre-amplifiers only after doing massive filtering for out-of-band signals, and the performance goes up a bit (hard to measure in dB, since this is digital... in fact, a bit hard to measure AT ALL, but relative performance measurements still work, like on any RF system)... others have found their site noise on their chosen frequencies to be too high to utilize them. And almost everyone I've talked to with wicked-good performance is filtering the receiver more than a typical Band-pass/Band-reject 4-can duplexer... especially at VHF. That duplexer is probably right on the edge of being "enough" isolation. What (specific) duplexer is it? What's your duplexer rated for for isolation numbers in dB? Since it's REALLY hard to measure performance on these things compared to analog FM, where there's plenty of high-quality test gear floating around, and a simple SINAD measurement tells the tale of whether or not your receive side of the system is working properly, you need to shoot for a LOT of isolation in the design... a "belt an suspenders" approach of a duplexer and a High-Q bandpass cavity really isn't a bad idea... along with PERFECT highly shielded RF cabling, but you'd have to describe (in excruciating detail) your entire setup, to get it "right" via e-mail from afar... This concept is no different than any other repeater, it's just more important with D-STAR because of the lack of a good way to measure performance. Repeater builders "get away with" a lot on analog FM repeaters because you can still "pick the person out of the noise" if performance isn't what it's supposed to be, by the numbers. Thus, D-STAR repeaters require a bit more "engineering thought" and design of the RF side, up-front... or you sit and scratch your head wondering "What's wrong?". Real-world examples: Our VHF module here is definitely the weaker-performer over the UHF, but there's a bunch of reasons for that including a requirement that we use a combined antenna system with some budgetary constraints on how the antenna is shared, the fact that the only open pair was 145.25 which is avoided like the plague by analog FM folks because of CATV leakage, and more. It's about "as good as it's going to get", but our UHF outperforms the VHF. And both underperform the best analog FM repeater at the same site on the same antennas... if you're looking for "perfect copy" including the non-error-corrected DV low-speed data. We still get 80+ miles of voice coverage on both, but the low-speed data is often gobledeegook for low-power (HT) users who are mobile and/or not line-of-sight to the antennas, and once in a while HT user's audio garbles with some on-site intermittent interference, and/or CATV noise (can't easily tell which) on VHF. 1.2 GHz on the other hand, I hear is ROCK SOLID, using the very nice TX/TX duplexer/combiner setup they offer for D-STAR 1.2 GHz data/voice mixed modules that a local ham "donated" by spending the big bucks. It was a priority because no one wanted to use the "triple frequency" trick and try to do it with UHF cavities, for sure... which HAS been done successfully on analog FM 1.2 GHz repeaters in the local area in the past and present. Someone also popped for a VERY nice directional panel/corner reflector style antenna from W6OAL at Ye Olde Antenna Labs in Parker, CO for the 1.2 GHz system. Another analog FM repeater had been using one of a former (larger) design in the area for a very long time, and the performance was great -- so it was a "why mess with success?" type of decision. And Dave makes nice heavy-duty antennas that last a long time. And while 1.2 Data requires a LOT of RF power and/or antenna gain to use it consistently, the 1.2 GHz band for audio is great. I just refuse (even as a system admin/tech guy) to spend $1000 for the radio until some winfall comes along that gives me an "excuse" to spend that kinda money on a rig that's a one-trick-pony... "It ain't worth it." Hopefully this starts you down the path of "the right questions to ask" -- definitely start with the high-quality dummy load and double-shielded jumpers to it, to find out if the desense is happening "indoors" or "outdoors"... and go from there! (Remember also that SOME antennas simply don't "duplex" well. If you're not using commercial-grade antennas, be forewarned. Some people get away with that, too. And others fight and fight and fight until they give up and buy a solid, DC-grounded, folded-dipole style antenna and their "noise" just disappears...) It's a "you get what you pay for" type of thing, quality-wise. I've seen a SINGLE folded-dipole from Sinclair out-perform a supposedly "high gain" co-linear style cheap Amateur grade vertical antenna at high repeater sites... again, need more info about Height Above Average Terrain, etc... to be able to speak to that. -- Nate Duehr, WY0X n...@natetech.com facebook.com/denverpilot twitter.com/denverpilot
