I found a project called rpitx, works with most raspberry pi's (I think only the 2 and at present the 4 have issues but they are working on resolving the latter), you can install the whole app if you wish but just grabbing the pocsag executable and adding this "gpu_freq=250" to the end of the config.txt file in the boot folder for transmit stability reasons is all you need.
This software allows the raspberry pi to produce any transmit frequency between about 100Khz and 1.4Ghz at very low power (10mW) directly out of GPIO4, you must put a filter on it for the frequency you are using because the harmonics can be strong i.e if transmitting on 150Mhz the 3rd harmonic of that would be 450Mhz so a suitable bandpass filter is required with say a width of 50Mhz or so and a centre of 150Mhz. Once completed you only need a simple script to send a message to the pager.
Pagers can be bought quite cheaply from ebay now as pretty much all the pager networks have been shutdown and are no longer used, 10mW of transmit power will not get you far, probably 100 metres or so but more than good enough for important local alerts / alarm conditions.
Certainly here in the UK you need to have a license to transmit unless you are using a license except band and I would never encourage anyone to break the law, I was lucky and have a pager with a programmable frequency between 450Mhz and 470Mhz and here in the UK 458Mhz band is also license exempt, other countries will vary so please check although at 10mW and less then 100 metres range and a very rare transmit period if only used for alerts I dare say no one will ever notice.
Theres a few little things to remember, GPIO4 toggles between 0 and 3.3v when transmitting so any filter needs to be a "series first" design so there is a capacitor before any coils to ground otherwise you will short the DC value of the pin to GND and probably damage the GPIO port for good.
Theres' plenty of free filter design software on the web, this for instance https://rf-tools.com/lc-filter/ but if you have little RF experience remember to use coax and make sure any unscreened leads (apart from the antenna) are as short as possible.
I'm happy to help anyone who's interested.
Regards
Dave
Oh yea, and mine has been working fine for over 2 months
