Last Updated: 12/07/2012
As the airways become increasingly busy, requirements for narrow band operations have been implemented for many activities. These requirements involve output deviation and frequency step of the radio for performance on certain frequencies.
While most amateur radios have an option to change output deviation from 5kHz to 2.5kHz, this change does not help the radio access new commercial narrow band frequencies on 2.5 kHz steps.
When you transmit, the signal emitted from your antenna radiates with a 5 kHz deviation. This has long been considered "normal" FM or just FM mode. In the Programmer for any amateur radio, a designation of FM as operating mode implies the 5 kHz deviation..
This is not Step. Step is the difference between two frequencies. The two, Step and Deviation, are mutually exclusive. Narrow deviation will not make it possible to tune to a particular frequency (that is what Step is for) and Step will not limit your deviation on a given frequency (that is what FM Narrow or Tx Narrow is for.)
Radios handle the setting for FM narrow in several different ways. The Programmers for each radio handles the setting in a way the radio can understand. In many, you will find an option for FM Narrow under Operating Mode. In other there is a TX Narrow column to be checked. Each of these is designed to affect one frequency on one memory channel. The chart shown in the attached PDF details how the Programmer for each radio handles that radio's Narrow setting. Click here to download
In general, if a repeater is setup for Narrow FM and you use FM (normal 5 KHz deviation) to access it, your signal will over drive the repeater and sound garbled to others listening. You will be putting 5 kHz signal into a system that wants only 2.5 kHz. The extra causes interference. If you will repeatedly get the report that your signal is noisy or garbled, try changing to FM narrow for that repeater.
If a repeater is setup for normal FM and you use Narrow FM to access it, your signal will be weak to others listening. You will be putting 2.5 kHz into a system that wants a full 5 kHz. It's like putting a trickle through a fire hose. If you repeatedly get the report that your signal is weak or audio is soft, try changing to FM for the operating mode.
This is much more complicated since many amateur radio are used in the commercial world for which they are not designed. The design of commercial radios has been changed to allow frequencies that amateur radio cannot tune to.
Basically, if the frequency you are given has more than 4 digits after the decimal, there is a really good chance that your amateur radio will not be able to tune to it. Try entering the frequency into the Programmer. The Programmer is designed with the algorithms of each particular radio to know if a frequency will work or not (this varies by radio). If you repeatedly get the "Invalid frequency" message, you have exactly that... a frequency that will not work in your particular radio. There is no modification to the radio that will make it possible for this model to access that particular frequency.