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Narrowband question about how it works

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Jan 10, 2003
Grove City, Ohio, USA
I know that the change from 25 to 12.5 is what narrowbanding is all about, but what is that in terms of?

Say we use a given frequency of 462.725 (repeater output) and a standard offset of +5Mhz for a repeater input of 467.725

When narrowbanding is completed on the repeater will a radio that is still 25mhz not be able to key up and access the repeater?

I know that a wideband radio will still be able to receive a narrowband but the audio and sensativity will be down..but what is the 12.5 and 25Mhz difference actually at?..that's where I'm lost... Given the example repeater pair above, what is the 12.5 and 25 in refference to?

Any helpful explination would be greatly appreciated to help clearify the difference in narrow and wide band operations and how the mandated change to narrowband will effect our systems.

~Thanks, Tom
Dec 19, 2002
Wichita Falls, TX
12.5 kHz and 25 kHz (not MHz) "bandwidth" are 2-way industry terms used to describe the bandwidth occupied by signals modulated at 2.5 kHz deviation and 5 kHz deviation.
It isn't even the real life actual signal bandwidth or receiver filter selectivity.

The rule of thumb formula to determine bandwith is bw=d+ax2
bw = bandwidth in Hz
d=peak deviation in Hz
a=max audio frequency in Hz

The bandwidth of an FM signal with a maximum audio frequency of 3000 hz modulated at 2.5 kHz peak deviation is 11 kHz and at 5 kHz deviation the bandwidth would be 16 kHz.

Listening to a wideband transmission on a narrowband receiver will make the audio louder and possibly distorted. If the peak bandwidth is wider than the receiver filters the audio may chop out and the squelch may close on voice peaks.

Listening to a narrowband transmission on a wideband receiver will result in low audio.

Any tones or data sent at the wrong level may not decode properly at the other end.
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Jul 25, 2004
In simple terms, narrowbanding is like taking a highway that has two lanes, each 20' wide and changing it so it is now 4 lanes, each 10' wide. The highway can now handle twice the amount of traffic as before, but older trucks that needed the extra width can't drive on them without taking up two lanes. Since most vehicles aren't that wide (the new narrow-band radios) things work fine, but if a large extra-wide load (the old wide-band radio) tries to use the new lane, both lanes are taken up and traffic doesn't flow correctly.

What you'll end up when a wide-band radio transmits on xxx.250 MHz after everything has narrowbanded is interference on xxx.125 and xxx.375 since the signal is twice as wide as it should be. Chances are good that radios on all three narrowband channels (xxx.125, xxx.250, and xxx.375) will hear the xxx.250 transmission and that the wide-band radio will hear users on the three narrowband channels as well, but there may be distortion and volume issues.

You'll note that I indicated in my example above that xxx.250 (the old wide-band channel) was listed as one of the narrowband channels. This is correct, since the old wide-band channels will also be used as narrowband channels. In nearly all cases, when an agency is moved from wide-band to narrow-band, their frequcncy doesn't change, only the allowed emission (the width of their allowed signal) is changed.
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