About 20 years ago , when the civil services introduced the still current digital comms system ,I was looking after GB3SZ , the UHF 1.6MHz split Repeater in central Bournemouth.
Being based on the usual Pye deck + Amateur band tweaks ,the RX squelch was hammered when the local Police started the new system from 150m LOS.
The late Nick Foot G4WHO and I had to spend considerable time adding cavity filtering and tweaking the system using R&S sig gens , comms Rxs and Spec Anns on top of a 10 storey office block.
We got our own back when some months later I was rung up by an officer from the RF investigations division in Bristol .He asked if I knew about an RF device sending "something one of our lot thinks is Morse " and after several mins thinks it sends GB3SC .
I was able to confirm being the Keeper and that it was a VHF Repeater .
He said they were getting occasional breakthrough on a newly installed multi-site repeater , which had a node 150m away .....
I referred him to Nick ,who just happened to be the Owner/MD of AQL EMC .
Nick had a chat and agreed to go to the Bournemouth site for alook .
Turns out a contractor had gone through the Control Room and removed some cylindrical objects that had " nothing to do with the new system " .i.e. they had dissed the antenna filtering components used to provide selective performance from out of band high level sources of RF .
As it turned out they had kept the items in a store room and Nick was able to refit - the complete multi-site system performance improved considerably and Nick picked up another Consultancy contract .
70cm is a shared Secondary allocation and has been for many years . I hope the current crop of QRM crap fails fast .