Eric, K6ERC, and I conducted a range test on Friday, using a Motorola Astro handie-talkie to compare the transmission range between digital P25 and conventional analog. Eric was at a base station where the antenna is on a short tower and I drove into the field with the HT.
The testing was non-scientific and was on a 147 MHz. amateur channel. Five locations were tested, the most distant 11 miles from the base. A Uniden P25-capable scanner was used as the receiver.
Received signal levels were, not unexpectedly, the same for both P25 and analog signals transmitted from the same location.
The difference was in the quality of the recovered signal. Except for the very closest test location, the P25 signal was easier to listen to and understand than the analog signal. For example, at the last location where we had a useful signal, the P25 rated 4.5 (on a five-point scale) for intelligibility, while analog rated a 2.5.
At the point where P25 could no longer be decoded, the analog signal could still be detected, but was not intelligible. This squares with the concept that P25 works fine until it stops working — the digital error rate becomes too high — while an analog signal fades into unintelligibility.
Based on this small, unscientific test, P25 considerably extends the range of intelligibility of a simplex handie-talkie signal on VHF frequencies. The receiver was located at a place known to suffer from significant amounts of VHF noise, which may have impacted our findings. Nevertheless, given a choice, I would use P25 for simplex VHF communications whenever possible.