On 8/28/2010 7:54 AM, nj902 wrote: > > --- In Repeater-Builder@yahoogroups.com, Matthew Kaufman<matt...@...> wrote: > > " Reducing the modulation index and simultaneously reducing the receiver > bandwidth from 5 to 2.5 kHz results in a situation which requires ~6 db more > signal level for the same demodulated quality (ex. 12db SINAD)" > ... > Thus, the actual impact of changing an analog system from wideband to > narrowband is not 6dB but 3dB. In fact, 3dB is the value that has been > stated by APCO engineers in their narrowbanding presentations. You are correct, I mis-spoke here... you need 6db more C/N, which is 3db absolute signal level (assuming perfect filters) [and this is exactly what I was alluding to when I pointed out that the improvement from narrowing the receiver isn't 1:1 with the loss from the reduced deviation]... Imperfect receivers, the impact of other issues like CTCSS/DCS deviation, and the impact of multipath and flutter at the edges of the coverage area actually mean you need at least 4 and sometimes 6 or more db increase for the same intelligibility at the fringes. Which often, especially for terrain-limited systems, means "more transmit sites" and not just "crank up the transmit power".
But the short answer is the same. Even for optimal devices, on perfectly flat terrain, the coverage is reduced [approximately equal to halving the transmit power] by going narrowband. In the real world, and especially in the area of the country I live in, the coverage (meaning "the area in which the transmission is intelligible to the listener") is reduced *significantly*. Matthew Kaufman
