Many times, the terminology we use for ham radio can be a bit confusing. For example, the terms wideband FM and narrowband FM, can be used to mean different things. Let's take a look at the bandwidth of a typical FM signal and shed some light on whether it is wideband or narrowband in nature.
FM Bandwidth
The bandwidth of an RF signal is the amount of frequency spectrum the signal uses. Radio amateurs often talk about being on a particular frequency such as 146.52 MHz, but the actual signal being transmitted is wider than just that frequency. In other words, our typical FM signal on 146.52 MHz will actually spread out across 146.514 MHz to 146.526 MHz (and maybe further). For a review of modulation types, including FM, take a look at Loads of Modes.
The bandwidth of the signal will be determined primarily by the peak frequency deviation of the FM signal. The peak frequency deviation is the maximum amount of instantaneous frequency change, relative to the carrier frequency. (The carrier frequency is the "resting frequency" of the signal with no modulation applied.) In the early days of FM mobile radio, higher peak frequency deviations were used, typically plus or minus 15 kHz. Later, the standard frequency deviation was reduced to plus or minus 5 kHz, considered "narrowband" at the time, which remains the standard today in the US for ham use. (I mention this because you may find older technical articles referring to 5-kHz deviation as "narrowband.")
You might think that the bandwidth of an FM signal is just twice the peak frequency deviation. That is, for a 5-kHz peak deviation, the instantaneous frequency swings 5 kHz in the positive direction and then 5 kHz in the negative direction. The total frequency swing is 10 kHz. Ah, but the world is not quite that simple, as the frequency content of the signal spills out beyond this 10-kHz frequency swing. This behavior is quantified by Carson's Rule. Our typical 5-kHz deviated signal actually has a bandwidth of about 16 kHz.
What is Wideband FM?
Today, there are three FM deviations that you are likely to encounter in modern ham radio gear. The widest of these is used for FM broadcast with a whopping 75-kHz peak deviation. This is definitely wideband by most standards and is used to support the high-quality audio normally associated with FM broadcast stations. Many VHF/UHF ham transceivers have the capability to receive FM broadcast stations (88 to 108 MHz) and may refer to this receive mode as wideband FM. (We will see later that this terminology is fading.)
The other two FM deviations commonly available on ham gear are 5 kHz and 2.5 kHz. The 5-kHz deviation is considered standard in the US and is often referred to as wideband FM. Similarly, the 2.5 kHz deviation is often called narrowband FM. You can see how this can be confusing: what used to be narrow is now considered wide.
This table summarizes the three frequency deviations, their resulting bandwidth, and channel spacing. Channel spacing is roughly consistent with the bandwidth of the signal but may be different depending on how tight the channels have been scrunched together.
Name | Peak Frequency Deviation | Bandwidth | Channel Spacing |
Narrowband | 2.5 kHz | 11 to 12.5 kHz | 12.5 kHz |
Wideband (US Amateur Standard) | 5.0 kHz | 13 to 16 kHz | 15 to 25 kHz |
FM Broadcast | 75 kHz | 180 to 200 kHz | 200 kHz |
Other Radio Services
It is important to note that these are the norms for the US Amateur Radio Service. The FCC has required most commercial and public safety radios using analog FM to adopt the narrower 2.5 kHz frequency deviation. This is one reason we are seeing this narrowband option showing up on ham radio gear. In fact, you may encounter some radios intended for the commercial market that only do narrowband FM.
The Family Radio Service (FRS) uses the 2.5-kHz narrowband deviation, while the General Mobile Radio Service (GMRS) uses a mix of narrowband and wideband signals, depending on the particular channel.
Practical Equipment Examples
A review of some of the more recent handheld radios reveals that the manufacturers are using relatively consistent terminology in their products.
The Yaesu FT-4XR has a W/N.DEV setting (Wide/Narrow Deviation) in the menu system, with Wide being 5 kHz deviation and Narrow being 2.5 kHz. The FM broadcast receive feature is kept separate from the normal ham use. So when you turn on the "FM radio," it automatically goes into the much-wider FM mode.
The Yaesu FT-5DR is a more advanced radio that includes Yaesu's C4FM digital mode. It has many modulation types to choose from, including analog FM. The extensive menu system has a selection for "FM Deviation" which can be set to "Wide" or "Narrow", meaning 5-kHz or 2.5-kHz deviation. Again, the FM broadcast receiver function is handled as a separate feature of the radio, automatically using a wider FM mode.
The ICOM ID-52A is an advanced D-STAR radio that includes multiple operating modes, including analog FM. The transceiver has 5 operating modes, FM, FM-N, AM, AM-N, and DV.
FM corresponds to 5-kHz deviation (wide) while FM-N corresponds to 2.5-kHz deviation (narrow). AM and AM-N are amplitude modulation modes while DV is D-STAR.
The low-cost Baofeng radios (UV-5R and others) have many different models and firmware variations. However, they typically keep the FM broadcast receive feature separated and provide a Wide/Narrow selection for the FM deviation (5 kHz and 2.5 kHz) in the menu system.
The Anytone AT-D878 radios also keep the FM broadcast receive separate and have a bandwidth setting in the Settings menu: "Wide Band" for 5 kHz and "Narrow Band" for 2.5 kHz.
Note that all of these radios keep the FM broadcast function separated out and consistently use the term Wide for 5 kHz deviation and Narrow for 2.5 kHz deviation.
What if I Use The Wrong Deviation?
The vast majority of the analog FM repeaters in the US are set up for 5-kHz deviation. (This is because so many pieces of ham gear in use can only do 5-kHz deviation.) Unless you have good reason to do otherwise (maybe your repeater trustee tells you to use Narrow) you should set the FM deviation to Wide (5-kHz) deviation.
If you happen to set your radio to 2.5-kHz deviation, your signal will have a lower modulation level and will sound quieter on a typical repeater or simplex channel. If the repeater you are using is set up for narrowband deviation and you are using wide deviation, your signal will sound louder and may be distorted. (You can compensate for this by talking quieter but it would be best to change the setting to the desired deviation level.) Generally, wideband and narrowband radios can talk to each other, but you may encounter annoyingly quiet audio or loud, distorted signals. So try to have your radio set correctly but the world won't end if you make a mistake.
I hope this helps you understand your choices concerning FM deviation.
- Bob KØNR
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