Originally Posted by KC2OBW
Measuring over the air can be a pain, How strong was the signal you where trying to measure.
Measuring over the air is indeed tricky; the main challenge is that you don't know how modulated the signal is. But there are a few tricks.
A not great way is to look at the maximum deviation over time, in the hopes that a fully modulated signal will eventually fill the passband. But that takes a while.
It's easier if there's a standard level in the signal somewhere. PL tones on an otherwise unmodulated signal can work if you know the exact specs of the transmitting radio, but the levels vary too much among vendors and even models to be a reliable method in general.
MDC code signals work well for this, and fortunately FDNY uses MDC (at least on the portables). Once you know the standard levels for MDC at various bandwidths, it's a snap to determine the bandwidth with the right receiving equipment as long as the signal peak is about 10 or 20 db above the noise floor.
My technique is to capture the spectrum P/Q with an SDR and then look at it offline with software (much easier than doing it in real time, since you can isolate the MDC part of the transmission this way.).