Choose clean channels for a new service, check what they do on a shared site, and audit and clean up a plan that has grown dirty over the years.
The discipline
Frequency coordination is choosing transmit and receive frequencies that work at the site without interfering with, or being interfered with by, the services already there. For land mobile radio that means intermodulation first. When several transmitters share a site, sums and differences of their carriers land at predictable frequencies, and a product on a working channel masks weak signals and causes interference that is hard to diagnose once the network is in service.
The arithmetic is exact, but the grading is a choice. Whether a product counts as a hit depends on how close it lands to the victim channel, and that threshold should follow the channel width: a wideband carrier has wider hit zones than a 12.5 kHz narrowband one. ITU-R SM.1134-1 and ACMA RALI LM 08 Annex D draw the boundary differently, so a result should always say which rule it was graded to.
Underground is different again. On leaky feeder in mines and tunnels every carrier can be a victim, and 5th order products are left out, because the three signal family cannot be planned around and the two signal product sits below the noise floor once coupling loss and amplifier intermodulation rejection are counted.
Most sites also grow dirty over time as channels are added. The tools below cover both jobs: choosing new channels, then auditing and cleaning up a plan that already exists.
The workflow
Search your TX and RX ranges exhaustively, apply the split, guard bands and exclusion zones, and rank every candidate pair by the intermodulation it creates: to 5th order on the surface and 3rd order underground.
Do it withFrequency Coordination Tool Pro Open the tool →Import the licensed services on the site from the ACMA register, mark which carriers are your proposed change, and see what you would put into someone else’s receiver separately from what the site would put into yours.
Do it withIntermodulation Calculator Pro Open the tool →Submit an existing channel list and grade every product (to 5th order on the surface) against ITU-R SM.1134-1, RALI LM 08 Annex D Table D2 or a custom separation, with the worst sources, the most affected victims and the most dangerous pairs listed.
Do it withFrequency Plan Validator Pro Open the tool →Instead of a redesign, shift each channel within its tunable steps to leave the fewest products on the carriers, with priority weights that hold safety and dispatch channels to the smallest shifts. The output is a before and after comparison and a CSV reprogramming table for the technician.
Do it withFrequency Plan Optimiser Pro Open the tool →From other disciplines
Further reading
Free tools are free on a Standard account, with no time limit. Pro tools are included in the 14-day trial on every new account. No credit card.
