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[aprssig] An amusing aside (Text Pagers)

Erik Finskas lakki at iki.fi
Thu Jan 28 18:15:47 UTC 2010


Scott Miller wrote:
> Buy enough of them, and you can get them from the factory on whatever 
> frequency you want.  Though I've never seen one as low as the 2m band. 
> And getting the off-the-shelf pager hardware to demodulate AFSK would 
> likely be impossible, I suspect.

Most of the current models are frequency agile by design. The older 
models rely on a crystal or other fixed frequency source to determine 
the frequency. The national paging network in Finland (decomissioned 
about five years ago) was just above our 2m band, at 146.325MHz... 
Unfortunately 99% of the equipment was old enough to meet the dumpster 
rather than undefined amount of work to get them to 144MHz.

We purchased a batch of programmable alphanumeric pagers from a dealer 
called Mutual Aid Supplies some years ago to start up similar project, 
to do ham paging, but that did not fly due to lack of coordination and 
generally, time. The pagers were about $100, available in both VHF and 
UHF. A preliminary network design was drawn, with a hot-spot solution in 
mind to spread low-power POCSAG transmitters virtually anywhere with 
internet connection.

If the transmitters of a paging network are not frequency synchronized, 
they need to be timeslotted. (Of course, this only in case that there is 
100% coverage with overlapping coverages of transmitters). That would be 
a interresting algorithm to design to make sure there are geographically 
no transmitters near each other transmitting in a single slot. I recall 
we thought of a 10 second slots, and if retransmissions would be needed, 
they would take place in the next slot for the current TX.
The modern pagers are capable of detecting duplicate transmissions and 
disregard them.

Now I would think UHF is better place to put Ham Paging, as it has 
better penetration to buildings and UHF PA gear is generally well 
available. Additionally, the transmitters could be co-located with APRS 
nodes, and being on UHF, there would not be problems with paging 
transmissions blocking the APRS receiver.

But, VHF is good too as we have a pile of 200W transmitters ready to do 
paging :)

One good application for ham paging is also filtered DX-alerts..
http://lakki.iki.fi/~lakki/kauha-dxcluster.jpg

73,
Erik OH2LAK



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