Threaded...
William E. Janes wrote:
> Just turning down the deviation circuitry to 2.5 kHz
> will reduce the amount of deviation, but not the bandwidth of the
> emission overall. Turning down the deviation is simply not a legal
> option.
Huh?
NBFM BW = (2x5)+(2x3)=16 kHz BW
SNFM BW = (2x2.5)+(2x3)=11 kHz BW
Seems that lowering the deviation lowers the bandwidth to my calcs!
> There is a second caveat to turning down the deviation… since
> a lot of transmitters encode either digital or tone squelch injected
> without components (pots) allowing adjustment of the level, the
> transmitter will be sending those at +\- 600-800 Hz before
> narrowbanding. That equates to approximately 15% of the available
> modulation. When you narrowband, the transmitter will still be cranking
> out tone or digital signals that will now be at about 30% modulation.
> Obviously this is gonna have a horrible effect on transmitted audio quality.
Easy on three fronts:
1. Most of the radios inject the CTCSS/CDCSS before the deviation
adjustement. These will 'auto-adjust' to the correct specs.
2. For those with separate pots, just turn them down as you did with the
deviation
3. For those that are fixed, add a resistor to lower the CTCSS/CDCSS
deviation.
It's not rocket science. You're doing the exact same thing the SNFM
radios are doing minus the RX bandwidth which has no effect on your
transmitted signal.
Joe M.
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