I live in Maplewood and monitor the PD on 154.815 with a Pl of 114 I think, if memory serves. Ive noticed that a Hunterdon country dispatch uses the same frequency at a different Pl tone, it's so close that on my home base it comes in full scale and on HT about S3. Why would they use the same frequency so close to one another? Isn't it bound to cause some interference?
It's not your imagination. I just ran a model on Hunterdon's three repeater sites and Maplewood's two repeater sites. The "service contours" of both come very close to each other - but don't touch - in the eastern tip of Somerset County. The "interference contours" of both intersect with each other.
What about coverage inside the intended areas?
There is no nice way to say this, so I'm just going to say it. I don't mean it in an inflammatory way because this is my hobby too: public safety communications systems are designed to put signal into a defined area. Items like cost and coverage balance at the top of the list of concerns and hobbyists usually don't make that list at all. In other words, if Maplewood gets a good signal inside Maplewood and Hunterdon gets a good signal inside Hunterdon, mission accomplished, regardless of what happens outside.
Seems that there will be interference outside the jurisdictional boundaries, but what's inside is manageable - or at least acceptable to the licensees.
Why do "they" do this?
What I wrote above touches on a very difficult balancing act. VHF spectrum is loaded with legacy users who implemented their system with much larger coverage than is needed. Very big "effective radiated power," very tall antennas. For example, the footprint of Maplewood's two sites not only covers Maplewood, but also most of Essex, part of Passaic, just about all of Hudson and Union, the tip of Somerset, and a good part of eastern Morris Counties. Hunterdon's aggregated service contours cover parts of Lehigh, Northampton, Bucks, Mercer, Middlesex, Somerset, Morrris, and Warren. Radiowaves look like drawn circles and usually defies political boundaries.
New Jersey is one of the most dense environments in the country for VHF high band and UHF spectrum and close-spacing is usually avoided, but sometimes inevitable.
Sometimes this is done because no other frequency will work at a site (combiner limitations or intermod). When an agency says they "have to have this frequency," for whatever reason, another agency can sign a "letter of concurrence" for the frequency coordinators and the restrictions of non-intersecting contours go away.
But 15X.XXXX is clear on my scanner, why don't they use that?
It may be the input to another system somewhere else, it may have a user on it that gets affected worse than whomever is on the frequencies that were ultimately picked. There is a reason, but many times, that reason is not obvious without doing a lot of digging into usage, records, and FCC rules. It also might not work at the site, as I mentioned above. Or, it might be something someone missed or an agency that migrated to another frequency band or on to a larger jurisdiction's 700 or T-Band trunked system but has decided not to "give back" the frequency (which, in that case, it cannot be taken for the greater good).
What happens in the future?
In modern systems, such as Region 8's (Southeastern NY, Northern NJ, and Southwestern CT) 700 MHz plan, a new licensee would have to define where they want to cover. Then there's a small buffer area drawn around it. The engineering proposal has to show how antenna design and placement will keep 80% of the signal inside the jurisdiction and buffer area, or they don't get the frequencies and license. The engineering proposal also has to demonstrate how there will be no signal incursion on co-channel (agencies or county areas who are on the same frequency) or adjacent channel re-use.
These systems concentrate the energy into the defined areas, and these overlaps are proactively controlled. If you plot those out, you will see lots of smaller footprints with directional antennas pointing signals in or notching them from getting out. It's a big change in design strategy. Sometimes all these overlaps inside contribute to simulcast phase distortion and you might hear that on a scanner, although actual public safety radios are designed to minimize the effects, especially in digital modes.
Sometimes special directional patterns are required in VHF, too, to concentrate signals into their intended areas and to "protect" other incumbent licensees. Some states have licensees that are implementing VHF trunking systems or VHF simulcast sites. You'll see exotic antenna patterns on them (look up Bergen County's sites, power levels, and directional patterns on 155.5500).
Sorry I couldn't speak to the specifics, but hope this answers some of the questions of how it's done.
For grins, program in 154.1600 and 155.7300.