[Milsurplus] NBFM/WBFM?

[Glen Zook]

The "olde tyme" wideband FM equipment had deviation of at least +/- 15 kHz.  Then, starting in 1957 the FCC "split" the FM channel spacing and the deviation was reduced to +/- 5 kHz.  This was called "narrowband".  Now the FCC is dropping the deviation to +/- 2.5 kHz and people who have not been around that long are calling this "narrowband".

Since most FM operation in North and South America on the VHF and UHF bands (and on the 10 meter band) utilizes +/- 5 kHz then the "narrowband" equipment will work fine with amateur equipment.  You can use "narrowband" equipment for transmitting to "wideband" equipment but the volume will be reduced.  However, you CANNOT use a "wideband" transmitter to transmit to a "narrowband" receiver.  The signal will "chop out" because the deviation is in excess of the bandwidth of the filter in the "narrowband" receiver.

Also, FM/PM is perfectly legal on all frequencies that phone operation is allowed by the FCC except for the 60 meter band (which is USB only).  However, the requirement for FM/PM operation below 29.0 MHz is that the modulation index cannot exceed 1.  That means that the absolute value of the deviation cannot exceed the maximum audio frequency of the transmitted signal.  Since the vast majority of amateur radio transmissions are limited to 3000 Hz maximum audio frequency the deviation of the signal cannot exceed +/- 3 kHz.  There are "modern" transceivers that have a "narrow FM" setting which reduces the deviation to +/- 2.5 kHz which meets the FCC requirement of the modulation index not exceeding 1.  This produces a modulation index of 0.8333.  When +/- 5 kHz deviation is used the modulation index is 1.66667 which is legal only above 29.0 MHz.

This per 47 CFR Part 97 Section 97.307(f)(1).

Glen, K9STH

Website:  http://k9sth.com


--- On Thu, 5/28/09, W2HX <w2hx at w2hx.com> wrote:

A radio I am interested in acquiring, PRC-351 is a narrow band fm transceiver I have read +/-5 5kHz deviation 30-79MHz.   I currently own a PRC-25 and PRC-68A which I believe are wide band FM? 
 
Can someone enlighten me if wide band and narrow band FM can interoperate (perhaps with better or worse fidelity?) and what can interoperate with amateur gear?