At 03:16 AM 06/01/07, Milt N3LTQ <[EMAIL PROTECTED]> wrote: >Chris, > >First you need to determine if your repeater controller can generate the >tones. Two tone paging was a feature in controllers made by S-Com, ACC and >others. As long as the controller can generate the sequence it should work >well. If the controller does not support two-tone paging then if you have >logic outputs you could use an add-on board from Communications Specialists >that will generate a single tone sequence.
I'm not as familiar with the ACC or others, but the Scom has a single audio tone source. There were pagers that used 1+1, ones that used 5 quick tones, ones that used a longer first tone, then 5 quick tones, and some weird specialized pagers that used the 2+2 format (two simultaneous tones, then two more simultaneous tones). Obviously the last ones will not be usable with the Scom. Then there are the digital pagers - they use a system where the transmitter is modulated by shifting the RF frequency at a digital data rate. I've seen a few of these digital paging transmitters in amateur repeater use because they are really continuous duty, and frequently cheap - there's no receiver - just a 110w, 330w or 500w transmitter. Adding a receiver is not hard, then you add a repeater controller and just ignore the digital input and feed it into the analog input. In some cases you need to add the missing analog modulation components (just compare the analog and digital modulator schematics). As to actually generating the pager code(s), I suppose you could program a PIC or Atmel processor to generate the digital bit stream that is fed to the transmitter, then initiate the PIC or Atmel with a pulsed digital output from a regular repeater controller. BTW the data burst sounds really obnoxious, and has to be at the level of a loud voice, so you won't want to send it too often. Or if your users use PL decode (i.e. your repeater transmitter runs a PL tone) you can turn it off while the data burst is sent. >As for pagers, parts are hard to imposssible to find for most pagers. Since >the paging market dried up with the advent of cell phone based text >messaging the supply of pagers has also dried up rather quickly. Good point. If you are going to use pagers, it's probably best to get a bunch and shelve the extras. Think "lifetime supply". Some used a specialized nicad and a charger, others used AA or AAA cells. And many pager receivers are crystal controlled, and most of them won't go down to 2m or 440 amateur. Those that will still need crystal, and they won't be cheap... And those that are programmable and in surplus generally aren't analog. >A word of caution about pagers; they use an antenna that is a compromise at >best. It is physically and electrically small. Plus it is shielded by the human body. You don't think much about that, but the human body is full of water. And water shields the antenna nicely. As a simple demonstration, tune your handheld to a 1/3 quieting signal, hold it against your chest, and rotate in place. You will hear a big change in signal level. That trick is used by some T-hunters when they get close to a signal... tune the dualbander handheld to the 3rd harmonic of the 2m signal, and by deriving a direction with body shielding you can get to within a few feet of the average hidden transmitter. >Only at UHF does a pager antenna come close to being able to approach a >quarter wavelength. That's one reason why 93x MHz paging worked so well. Most pagers used 1/4 wave antennas, some actually used 1/4 over 1/4 dipoles. >As a result the performance of pagers is poor compared to an HT. (Milt is referring to high band and UHF in the above sentence) In addition to the rotten antenna, the receiver performance itself is pretty poor. The case is so tiny that the designer compromised with fewer RF stages (if any) and fewer IF stages. >Most paging systems run very high power to compensate for the poor >performance of the paging receivers. It's called "RF penetration". And that takes power and multiple sites to talk to 1uv or 2uv receivers inside steel-frame buildings and underground parking garages. Many years ago I worked for a few years for an RCC here in Los Angeles. To cover an area about 200mi E-W by 80-90 miles N-S we used 14 sites with a mix of 110w, 330w and 500w simulcast transmitters. And these were transmit-only sites, we didn't need to worry about duplexers or receivers. Plus we had multiple channels - one on 43mhz, one on 152, plus telephone channels - two 150mhz and three on 450 mhz. The 43.22 mhz channel was loved by the outdoors guys (freeway construction crews, USFS bulldozer operators, etc) as it would carry for miles. High band was the busiest... 152.24 would get busy at 5am and not get slow until 1am. UHF paging was part of the IMTS system - a mix of 2-tone, 5-tone, Secode and DTMF. When paging channels on 930mhz came along I was already out the door (my job was taking care of the home-grown paging terminals that were based on Data General computers... 44 simultaneous tasks in 64kb of memory, all in assembly code). Things have changed in the years since, analog has gone away, digital was king until the cellphones killed paging, and now you find paging transmitters abandoned in place at mountaintop sites (and being resold on eBay). Milt's comments are just scratching the surface. I know of only one or two groups that are using pagers on a regular basis. The usual situation is that someone stumbles across an RF programmable pager, discovers that it's digital, and the idea dies there. Or if it is analog, they program it for the local repeater, and try and con the owner into setting up a paging function. Then the other users get annoyed at the paging tones. If I were setting up a paging callout system for an ARES group, I'd do one of two things: 1) Get a single account on a commercial paging service, and program all the pagers for the one paging code, then program the autopatch to call that number/code. This lets someone else build and pay for the multiple simulcast systems and sites. The cost of one paging account spread across a number of pager users should be under $2 per month. 2) find an abandoned 93x mhz paging transmitter, move it and the abandoned feedline and antenna to the club repeater site, (or buy a transmitter and antenna off of eBay). Once you have it in place you can reprogram the transmitter (or buy a crystal) for 927mhz, and set up a paging encoder to trigger all the pagers. Then trigger the encoder with the repeater controller. This way all the pagers are on an optimum frequency for the pager antenna, and there are no obnoxious paging tones on the main repeater user audio channel. However you will have the coverage / penetration limitations of a single transmit site. Mike WA6ILQ
