One of the design goals for this system was to improve portable in-building coverage, which is one reason you see a concentration of tower sites in the Urban Simulcast cell. A "big" antenna (aka gain antenna remotely mounted) should not be necessary to receive this system within its intended service area. In fact, it is likely to be counterproductive because you increase the number of tower sites you will receive - which is not helpful. Reception quality of a P25 simulcast system depends on the radio equipment you are using, and the physical location of the radio. If one is lucky enough to live nearby a tower site, signal from that tower will overwhelm the signals from the other member tower sites of the simulcast cell. This will allow any P25 Phase 2 capable radio to receive the system adequately - like a 996P2, HP2, 325, 436/536 or appropriate Whistler models. However if you live in an area where several tower sites are more or less equal-distant (meaning no dominant tower in terms of signal strength) radios not specifically designed to receive P25 simulcast will perform poorly, resulting in spotty, missing or garbled transmission reception. Some people have been able to improve reception of non - P25 simulcast optimized radios by applying attenuation, moving their scanners a few feet - or even inches - while for others, nothing helps. As far as changing from a stock antenna, the strategy should be to limit reception to a single site by either detuning your antenna (using one not specifically cut for 450/460 Mhz.,) or using a Yagi antenna pointed slightly off-axis towards a tower site (but no other sites should be located along the same azimuth.) Some people have had success using a paper clip as the antenna element - or removing the antenna completely - but even this may not help because the signal strength can be comparatively high in some areas.
The bottom line is if you want to receive pristine comms on a P25 simulcast system, buy a radio specifically designed to receive P25 simulcast - like a SDS100 / 200. It will work everywhere within the system's intended service area using its stock antenna. If you don't need to be mobile and are technically inclined, you can buy some good RTL SDR dongles and use a software package like DSD+, which works great and costs far less than a SDS radio. You could even build a small radio using a Raspberry Pi 3+/4 and use OP25 and a single RTL SDR for less than $100.
Those of us who have suffered through the transition from analog repeater systems, to SmartZone simulcast, to Astro P25 simulcast (or EDACS to Harris P25) have learned the above lessons the hard way - aka through trial and error. Use our experience as Guinea pigs to inform your decisions.
*** Note that I did not mention a Unication G4/G5 pager as a possible radio to receive this specific system. Apparently EFJohnson P25 systems are not always 100% compatible with other vendor equipment. Initially, some Harris subscribers would not work on ATLAS infrastructure, as well as Unication pagers. I do not know if these incompatibilities have been rectified.
