Dave VanHorn wrote: >> Anyway, you may have to get a receiver and build something to decode > the >> yelp. If you find a simple way, I'd be interested in adding it > again to my >> repeater network. > > Why would you need more than a receiver, and the equivalent of a VOX > ckt? If you have carrier on 121.5, and there's audio persisting for > more than a few seconds, then it's likely an ELT. If not, listening to > the audio for a few seconds will resolve that.
City noise has proven this wrong here in Denver before. All sorts of crud hits a high-mountain 121.5 AM reciever. The best was the bad power transformer on top of a downtown Denver building... that took quite a group of DF'ers to find... and then you'd get into downtown under the building and the signal would fade... The L-Per receivers for mountain-top use provide a way to monitor for both carrier and signals that have the appropriate sweep tones. I believe they just have a typical "zero-crossing" type circuit tied to the audio and count pulses... but I never looked at one that closely. Sweep tones vary wildly in speed when ELT batteries get weak, etc. Most of these will still trigger the logic output in the L-Per. It's pretty good. We did miss one that was sweeping so slowly that it wasn't heard until the audio from the receiver was commanded on via DTMF on the repeater after SARSAT hits in the area were reported. Then of course, it was there, sounding like a very sick dog... Also - if an aircraft flying is experiencing an emergency and you're really only monitoring for ELT's, having the sweep detector is nice. You don't have to listen in on that or have it tripping any alerts that way. It's also nice if you're setting one up to WAIT a couple of minutes before alerting, to see if it's just someone testing an ELT. Many people don't test at the appropriate legal times, or don't even realize there is an appropriate test time in each hour, but if they're just testing, the signal's only there for a minute or two... usually. The best ELT story here is probably the one in the USPS rail car... passing through the city... USPS Inspectors don't like being called out at late hours to unlock mail cars on trains, we learned... but they do show up. The occasional ELT in a car trunk, set off by the rough ride, is also always a good time to be had by all DF hunters. And the glider that was folded up and on its trailer, being towed from Boulder, CO to Golden, CO down the front of the Front Range foothills (Colorado Highway 93) provided some REALLY interesting bounces/reflections to triangulate... and of course it was also a moving target for an hour and a half or so... The Denver area is well-covered... http://www.fredf.org Multiple ELT receivers at various airports, with low-power UHF links back to a UHF repeater with various alert modes. All of which have receiver signal strength indications in voice, and the ability to monitor or alert on either 121.5 or 243.0, fully commandable by DTMF, as well as various alert and "SHUT UP" modes... (that latter one is important with airborne ELTs, as well as a way for them all to do collision-detection when they're all trying to alert at once!). They also all have discreet DTMF codes they send during alerts which allows the repeater controller on the mountain to have macros that respond to the DTMF and does other things (like put gated receiver audio from the mountaintop receiver into the "background" on the repeater... so you can hear what it's hearing, but gated so if you talk over it, it gets muted.), as well as announcing the airport identifier in voice and the frequency if that airport has both... Here's kinda what it sounds like from memory... ELT AIRPORT MONITOR DEVICE: (key) "<DTMF DIGITS> B J C JeffCo 1 2 1 POINT 5 10 seconds of audio from receiver at that airport (callsign in CW)" (unkey) REPEATER CONTROLLER: "E L T ALERT!", then two-tone warble and ELT in CW every 30 seconds or so... receiver on mountaintop gated audio placed in background... which if no one is talking, is foreground... Then various commands available to listen longer at each site (or open up other sites to see if they're hearing it also), switch to 243.0 if it's on 121.5 or vice-versa, or "sleep" the receiver at that site for a set period of time. (Early versions had on/off and people would forget to turn them back on... they still have on/off, but "sleep" is preferred.) Then eventually when it's all over... some "cleanup" or "event over" codes to reset everything. The controllers of the remote monitors are also "polite" in that if the search team is using the darn repeater, the site alerts wait for a break in the conversation, and get slower over time. Since it's all DTMF floating around triggering all of this "stuff", the guy that designed the remote receivers also took a Linux PC with a DTMF decoder on it and had it post activity from the mountain or the remote monitors to a web page... those links on the web page that say "Daily ELT Info" or similar -- that's all live. If there's an ELT going off in the Denver area and one of the receivers is hearing it, those links on the web page are near-real-time. Useful if no one heard the darn thing alerting and all the alerts quieted down and reset after a good long while, to know that we all missed it. Very good ham radio ingenuity. All of the above other than the AM Aircraft receivers (surplus old ones) and L-Per's... are home-brewed, using PIC Microcontrollers, voice synth chips, Mitel DTMF chips, and a cheap commercial UHF HT at each site. Credit also due to the sharp guys that programmed the repeater controller to react to all this "stuff" out at the remote receivers. Kent N0PSR, Ed N0MHU, and Glenn WN0EHE are all sharp guys! It all just kept growing and growing until it hit critical mass... start with one receiver... and eventually... a network of them erupts! Nate WY0X Yahoo! 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