At the risk of hijacking an old topic, I'll mention some experience that we have had here in Utah when installing a new D-Star stack.
Several months ago, antennas were installed on Farnsworth Peak - one of the busiest broadcast sites in the area, complete with most of the FM and DTV transmitters - plus most of the still-remaining analog TV transmitters. (Yes, there are some still operating!) One may read about another repeater located at this same site here: http://utaharc.org/rptr/62rpt.html Because the D-Star gear wasn't yet ready, an analog repeater - a Kenwood TK-740 - was installed using the same duplexer, feedlines and antenna, on the same frequency (145.125) as the planned D-Star. (Utah has a plan that uses 12.5 kHz spacing in a contiguous segment reserved for D-Star - one that maximizes the number of available channels.) The purpose of this was twofold: - To verify that the antenna/cabling/duplexer itself were working properly and that the coverage was as-expected. - To analyze site noise. This latter point is more important that you might realize as trying to use D-Star gear - whether it be user or repeater - is a disaster when trying to diagnose link problems. Without any built-in tools (ones that could have easily been built into the software/firmware) diagnosing a problem of any sort is a significant challenge. The repeater gear is arguably more difficult to use when diagnosing problems as, unlike the portable gear, one can't simply switch to an analog mode to "hear" what's going in. What we learned - and we could ONLY determine this easily by having installed an analog repeater in the first place - was that the site noise raised the noise floor of the receiver by a mere 10-15dB. This might sound terrible, but in actual practice, one can still achieve very good coverage considering that this site is at the top of a 5000-foot tower made of rock, so HT coverage from 80 miles away is still possible - if you are using a good mobile antenna. After several months of operation, the time and equipment became available and the D-Star stack was installed. The VHF didn't work. Fortunately, John, K7JL who was doing the install, had the foresight to install RSSI and Discriminator test jacks on the VHF, UHF and 23cm gear: This modification is mentioned elsewhere on this thread ("bosshard's" mods.) ANYONE who plans to put up a D-Star repeater on ANY band is well-advised to make these modifications and complain to Icom about their having been left off the gear in the first place! In testing with an Iso-Tee, the service monitor could NOT output enough signal to register ANY quieting on the discriminator output, indicating an effective desense of AT LEAST 40dB. Thinking that something was wrong, the cables were moved back to the Kenwood TK-740 and everything measured out fine. Hmmm... Fortunately, I was on site with another repeater project (the one mentioned above) and we had a 2-meter bandpass cavity (a 4" DB Products DB-4001) and that made all of the difference, reducing the desense some immeasurably high value (at least with the service monitor's available output and the Iso-Tee's coupling) to something on the order of 20dB or so. Because this cavity was one that was being pulled for rebuilding (having been in rough service for 25 years or so) it was quickly replaced (on the next trip) with a pair of 6" Sinclair bandpass cavities, each one set for 1dB coupling. It took BOTH of these cavities to reduce the desense of the Icom VHF repeater to the same level that the Kenwood analog experienced WITHOUT these two bandpass cavities - that is, down to the site noise floor. It should be noted that the Discriminator output - while a useful diagnostic tool, was only partly helpful in diagnosing and quantifying the problem: The real clue came about using the RSSI output, which was noted to be nearly "pegged" when it was on the duplexer alone, the discriminator output just showed normal "noise". Once the extra bandpass cavities were added, the RSSI dropped to the 1.3 volt area with no signal. (Note that RSSI values vary from radio to radio and depend on the site noise. Having said that, different radios should still be "sort of" similar in their responses.) One of the most important things to note that most duplexers - even those marked or marketed as "Bandpass/Band-Reject" are *REALLY* only Band-Reject types when one moves very far away from the design frequency. The duplexer used is a brand new 4-cavity TX/RX brand "BP/BR" type (costing >$2k). If one assumes that "Bandpass/Band-Reject" automatically means that everything off-frequency will be filtered out, you would be WRONG, WRONG, WRONG! While it is true that SOME of them have honest-to-god bandpass responses (e.g. one loop on one side of the resonator, and another loop on the other side) most do not! The biggest clue to this was that putting a wattmeter on the RX coax (along with a dummy load) showed many 10's of mW of RF coming down the pipe from the VHF (single-band) Telewave VHF stick mounted at the 60 foot level. This energy was the combined intercepted power of the FM and TV broadcasts on site. Now, admittedly it is a bit much to ask for any receiver to deal with, maybe, 1/10th of a watt of garbage coming down the coax, but it was interesting to note that the lowly TK-740 seemed to have no problem (and was not really helped by the addition of the bandpass cavities) but the Icom was totally demolished by the same RF configuration. Because we ran out of time (and bandpass cavities) we have yet to do one more important thing: Put a bandpass cavity on the TX port. While we have a 2-stage isolator there now, it is worth remembering that a high-band VHF isolator isn't going to work very well with FM-band or UHF DTV signal, so one has to "pre-filter" before applying the TX antenna to the isolator. In our testing, we have found that the UHF Icom D-Star repeater isn't being demolished by the extra signals coming through the duplexer, but we consider that to be mostly a matter of luck: This receiver will also sport at least one bandpass cavity when we get the opportunity to do so. It should also be noted that if we'd changed from a "4 cavity" duplexer to a "6 cavity" duplexer, we would probably NOT have helped the situation at all - unless those extra cavities were strictly bandpass types as it wasn't addition TX/RX isolation that was needed! With the onset of winter, it probably won't be until May or June 2010 before we'll be able to drive up to the site again and make additional changes/improvements. *** In light of all of this, we are considering adding another piece of test equipment to the D-Star repeater: An analog repeater. This may sound odd or even sacreligious, but it makes sense in a way: On a shared site such as this there are a lot of things that can go wrong once you rule out a problem with the gear itself. For example, a malfunctioning FM broacast transmitter can throw lots of garbage across the spectrum that is often transient in nature. Unfortunately, the very nature of D-Star prevents the tried-and-true diagnostics previously used for track on-site QRM (like "hearing" what the QRM sounds like) and especially on an all-digital system, determining the cause of system degradation can be particularly difficult - especially if it is intermittent and occurring only when it is inconvenient to drop everything and race to the top of the mountain, hoping that it is still happening by the time that you get there! The plan is to simply have the analog repeater commanded off, using a T/R relay to switch between the two transmitters. Because of the site noise, we have the option of simply using a splitter on the receiver and ignoring the 3-4dB hit on insertion loss - which would make no difference in receive sensitivity, anyway. If problems show up that appear to be RF-related, we'll simply command the analog repeater online and do "normal" tests to determine the possible cause. Fortunately, we have enough rack space and gear around to pull this off. Another thought was to simply install a GE Exciter board and an NHRC-4 repeater board (and a simple squelch circuit) inside the repeater's box to accomplish the same thing, using the Icom's discriminator output as the audio source - which would be more representative of the system's receive state, anyway: The 200mW or so of TX output from the exciter board would be more than enough for testing. In our brainstorming, we have also thought about having an analog repeater in which one takes the discriminator outputs of the various D-Star repeater's receivers and selecting among them, providing a way to remotely "hear" what the D-Star receivers are hearing to see what sort of QRM may be present - a valuable diagnostic tool especially if the QRM is intermittent in nature. *** These are observations that have been made JUST in the installation of this D-Star gear and we expect to learn other things as time goes on and other things happen. FWIW, in our testing of the D-Star gear, we usefully employed all of the techniques described on another page that we'd put together: http://utahvhfs.org/dstar_testing.html 73, Clint KA7OEI --- In dstar_digital@yahoogroups.com, Radioman <radioma...@...> wrote: > > > Hello Group, > > 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 >
