Introduction
The terms wideband and narrowband are often discussed on the air, and selecting the wrong setting on your radio can have a substantial impact on the legibility of your transmission to others. In general; on GMRS, you should be using wideband unless you are using a repeater that is specifically defined as narrowband, or communicating with FRS users using the shared channels (who are required to operate narrowband by the FCC). All of the SWCRS repeaters are wideband, if you are transmitting in narrowband, your audio will be heard with reduced volume and fidelity to other users of the system. If you get recurring complaints of low audio from other users but have an otherwise good signal, this is often why.
On many radios; there is a “Wide/Narrow” menu setting that can be set, chirp uses NFM (narrow) vs FM (wide), and other programming software will have similar definitions.
So how come bandwidth has an impact and what’s the mechanism by which this works?
The bandwidth of an FM signal is the space in the radio spectrum that it occupies. It can be approximately be defined by what’s known as Carson’s Rule; a simple formula that estimates the overall bandwidth occupied by an FM signal. It states that total bandwidth is equivalent to double the sum of the transmission’s peak frequency deviation and the highest modulating frequency, as follows:
BWFM = 2 (Δf + fm)
Where Δf is the peak deviation frequency of the transmitter, and fm is the highest modulating frequency of the audio source (likely your voice). Note that the above is a rule of thumb, but it allows you to estimate actual bandwidth with reasonable accuracy; the actual formula for determining true bandwidth is much more complicated, so this will just serve as a starting point for the purposes of this article.
So how does this apply to FM 2-way radios? Two types of FM modulation are most common to 2-way radio communications:
- Wide Band, with a peak deviation frequency of 5khz, and a highest modulating frequency of 3khz (FCC designation 16K0F3E)
- Narrow Band, with a peak deviation frequency of 2.5khz, and a highest modulating frequency of 2.75khz (FCC designation 11K0F3E)
This results in an approximate maximum bandwidth of 16khz for wide band, and 10.5khz for narrow band communications. Bandwidth is often confused with channel spacing, convention for wide band is 25khz channel spacing, and 12.5khz for narrow band – channel spacing is unrelated to occupied bandwidth, but is intended as a mechanism to avoid adjacent channel interference from neighboring signals depending on their anticipated bandwidth. Some radios use the spacing setting to automatically set the permitted bandwidth.
What is peak deviation frequency and modulating frequency?
FM transmissions essentially consist of two types of information; the frequencies generated by your voice (or other source), and the amplitude (volume) of those frequencies. The human voice can be captured fairly clearly by the pass band in a 2-way radio transmitter. For a wide band radio, this pass band captures approximately 300-3000hz of human speech, and for a narrowband radio, it’s 300-2750hz of audio frequencies. The first 0-300hz is still used by your radio for PL and DPL tone signalling to the repeater or other radios, but is cut off from the speaker and the microphone’s audio range, so it’s referred to as the sub-audible band (as to not be a nuisance to the radio’s operators). The upper end of the passband sets one of the variables in Carson’s Rule; the highest modulated frequency (fm).
So off the bat, narrowband already carries slightly less fidelity than wide band, since you’re missing about 250hz of the higher audio that caries your voice, but that in itself is not all that much and is probably not all that noticeable. So why the drop in audio levels between the two signals? This has to do with how an FM signal is processed by a receiver, or de-modulated. To understand how this works, we need to look at a Frequency Modulated (FM) signal at a microscopic level. Below are two images of a spectrum analyzer monitoring FM signals carrying a 1khz audio tone, one modulated at 1.5khz deviation, and the other modulated at 3.0khz total deviation. Both signals carry the same audio information, the 1khz audio tone, but the main difference is that one is quieter than the other. The peaks are where the transmitter is generating RF power, and the spacing of the peaks in the signal are where equal to the audio frequency information modulated onto the signal; this spacing matches that of the audio tone, they are 1khz apart (a 20khz span is shown, each box is 2khz wide). The main difference is the total deviation between the two signals; note that the louder signal is much wider, and the quieter signal is much narrower.
The difference in the limitations of available bandwidth can be offset by the calibration of the receiver; a narrowband receiver will perceive a lesser deviating signal as much louder, while a wideband receiver needs a wider signal to get to full volume. Both receivers are of course calibrated for the full volume (dynamic) range of normal human speech, however a fully modulated narrowband signal is simply not wide enough for a wideband receiver to reach full volume. This also works the other way, a wideband signal received by a narrowband receiver will be excessively loud and is likely to be distorted as it’s perceived as overmodulated (the same thing a stereo sound system does when turned up too loud; you “clip”)
Advantages of Wideband vs. Narrowband
So why have two different bandwidths of modulation in FM? The reality is that there’s no hard practical limit to how much or little an FM signal can deviate, so the standard values used in radio design are somewhat arbitrary and merely chosen industry standards. FM broadcast can be up to 180khz wide as opposed to the 10.5khz and 16khz we’d expect to see for two way communications. An FM broadcast signal can of course carry a much greater range of audio frequencies, at a much greater dynamic range compared to two way communications, and at a superior signal-to-noise ratio; it’s meant to carry high grade audio, not just communication grade speech.
In communications, the difference between wideband and narrowband is a matter of needed spectral efficiency and government regulations. In a given space on the radio spectrum, you can fit more signals at 12.5khz channel spacing, compared to 20 or 25khz channel spacing. So the advantage of narrowband (11K0F3E) FM is you can fit more traffic on a small slice of band. So why not just do everything narrowband so there’s more room?
Wideband (16K0F3E) signals can carry a greater portion of the human voice, and can do so with a greater dynamic range (quiet to loudness) compared to narrowband. The greater use of spectrum demanded by wideband signals also means improved signal-to-noise ratios – theoretically a wideband signal can be more easily understood in noisy signal conditions compared to narrowband, another reason why FM broadcast signals are considerably wider than any FM 2-way signal, but in reality is also probably not all that noticeable. The other limitation on narrowband is that it demands a higher frequency accuracy from your transmitter and receiver; radio tuning becomes more critical the narrower you go. There are radios that can operate at super-narrowband (6.00khz total bandwidth, designator 6K00F3E), however they require more expensive reference oscillators to ensure frequency stability, and are even more susceptible to signal-to-noise issues.
The main GMRS channels and the 462 interstitial channels are licensed for wideband. Many commercial radio systems were forced to go narrowband by the FCC to make more room in the spectrum for other users, however GMRS and Amateur Radio (and a few select other radio services) were not affected by this requirement; so fundamentally the need to stay wide-band is to be consistent with the system’s design that you are using, so your audio levels are constant.
If you’d like to dive deeper into this topic; TRX labs has a great video explaining the subject here; it’s about 45 minutes.