What is the Disadvantage of FM Compared to AM? An In-Depth Exploration

As an electrical engineer and radio enthusiast, the technical differences between amplitude modulation (AM) and frequency modulation (FM) radio have fascinated me for years. While FM eventually replaced AM for most music broadcasting due to superior audio quality, the switch came at the cost of some major disadvantages still favoring good old AM for certain applications.

In this article, I‘ll use my engineering expertise to provide an in-depth look at the key downsides of FM compared to AM from a technical and practical perspective. You‘ll also learn some really cool facts about radio along the way!

To start, let‘s quickly review how AM and FM work at a high level. An AM radio signal has its amplitude (strength) varied by the audio being transmitted, while the frequency stays constant. For example, a 535 kHz AM carrier gets louder or quieter as the microphone input changes the power feeding the transmitter antenna.

With FM, the power stays constant, but the frequency shifts higher or lower according to the audio signal. So our 535 kHz AM station would instead broadcast on say, 92.1 MHz, shifting up and down slightly from that center carrier frequency to transmit information.

This is analogous to a boat bobbing up and down on the ocean‘s surface (AM) versus a boat rocking side to side on calm waters (FM). Hopefully these simplified analogies help explain the basic difference! There are some profound technical implications however…time to dive deeper!

Bandwidth Tradeoffs – AM versus FM

One major disadvantage of FM is the extremely wide bandwidth required compared to AM. This is a consequence of the physics behind varying the signal frequency rather than amplitude.

To transmit high fidelity music, an FM station needs about 200 kilohertz of bandwidth. AM stations require no more than 30 kilohertz of bandwidth, even for voice programs. According to FCC allocation tables, there are 530 AM stations packed into the band between 535 kHz and 1605 kHz – a span of only 1070 kHz!

In contrast, the FM band only accommodates 98 stations from 88 MHz to 108 MHz – a 20 MHz span. That‘s over 18 times more spectrum space per AM station compared to FM! This wide FM bandwidth guzzles up more of the finite radio spectrum, limiting space for additional stations.

The 200 kHz wide FM signal also has far more opportunities to suffer interference compared to a 30 kHz wide AM signal. Anything from natural phenomena like lightning to man-made electronics can disrupt a wider swath of frequencies. Haven‘t you noticed FM stations tend to experience more buzzes and noises than AM stations even under clear conditions?

Susceptibility to Interference – FM‘s Weakness

This brings us to FM‘s increased susceptibility to all sorts of interference sources, compared to the relatively narrow and robust AM signal. I discovered this firsthand as an engineer, when our local 40,000 watt FM station would experience noticeable interference even from weak sources like walkie-talkies on nearby frequencies during propagation tests.

FM is inherently more vulnerable to impulse noise from vehicle ignition systems, lightning strikes, electric motors and other random electrical phenomena. Even non-electrical sources like solar flares can wipe out FM reception while leaving AM unscathed.

During the strongest solar storms, AM stations sound fine while every FM station disappears into buzzing static as the background cosmic RF noise level surges. In 2005, a solar flare blacked out all FM reception for an hour as far south as Texas! AM stations didn‘t miss a beat however.

FM is also prone to interference from LED lamps, plasma TVs, microwave ovens, wireless devices and other modern gadgetry – these can wreak havoc within FM‘s wide 200 kHz bandwidth. Unshielded FM wiring acts like an antenna, picking up all kinds of stray signals.

In contrast, AM suffers minimal disruption even in the presence of extreme noise interference thanks to its narrow bandwidth and robust design. Shortwave AM stations broadcasting in Morse code were successfully received even when buried dozens of feet underground and other amazingly noisy conditions. FM simply can‘t compete!

Line-of-Sight Limitations Reduce Coverage Area

Another significant weakness of FM versus AM is its severe range limitations, even when using maximum power. Unlike AM, FM relies on direct line-of-sight propagation between the transmitter and receiver. AM signals are able to diffract around obstacles, reflecting off the ground and ionosphere to reach receivers beyond the horizon.

This fundamental difference between scattering and reflecting (AM) versus line-of-sight (FM) translates to vastly shorter usable range for FM stations. An AM station broadcasting at 50,000 watts can reliably reach listeners over 250 miles away after dark, thanks to ionospheric refraction. An FM station at the same power barely makes it 50 miles!

I discovered this huge disparity firsthand during my days DXing AM stations on the crystal radio I built in high school. Late at night I could pick up AM stations from as far as Chicago and Alabama here on the West Coast, over 1200 miles away! Compare that to the 50 mile range of our local 50 kW FM station.

FM‘s line-of-sight limitation also results in problems serving areas shadowed by hills or buildings. Weak FM signals lack the resilience of AM to successfully diffract around solid barriers. AM stations remain listenable even in deep concrete buildings thanks to helpful groundwave propagation. Good luck getting watchable FM reception inside buildings!

Complex and Expensive FM Equipment

From transmitters to antennas to receivers, FM radio gear carries a high price tag compared to AM equivalents, due to its complex design. The physics of varying frequency instead of amplitude requires advanced circuitry using components like voltage controlled oscillators, discriminators and reactance tubes.

AM modulation on the other hand involves simply varying the power supply to the final amplifier stage to increase or decrease the amplitude. Much less complex! AM transmitters can be built at home in an afternoon with passive components like resistors and capacitors. Good luck building an FM exciter without precision test equipment!

FM transmitting antennas are enormous multi-element affairs requiring acres of real estate atop high towers to achieve the same coverage as a single AM tower with effectively located ground system radials. Construction and windloading also increases FM infrastructure costs.

An FM receiver requires expensive variable capacitors, limiters, zero cross detectors and other precision components to demodulate the signal. In comparison, an AM receiver is easily built from cheap standard parts like regular capacitors, diodes and transformers. This helps keeps AM receiver costs down.

Overall, the specialized equipment needs of FM drive infrastructure costs far beyond those of AM stations when building facilities offering equivalent coverage. This extra cost factors into FM‘s lack of popularity in rural areas.

Long Distance AM Propagation – Spanning the Globe!

One amazing aspect of AM radio I discovered is the ability of AM broadcasts on certain frequencies to propagate for thousands of miles, spanning seas and continents after dark. During my adventurous youth I learned to "DX" these far-off AM stations using only a simple receiver.

Thanks to the ionosphere‘s reflecting effect, AM broadcasts between 550 kHz and 1700 kHz occasionally refract off the upper atmosphere layers back down hundreds and even thousands of miles away at night. The so-called "skywave" phenomena even permits reception between the U.S. and Europe, Asia, South America and Australia!

I‘ll never forget the excitement tuning across the bands as a kid, picking out exotic foreign stations and clear channel AM powerhouses broadcasting coast-to-coast. WLS in Chicago, WBZ in Boston, KFI Los Angeles – AM radio permits reliable long distance communication unthinkable on FM.

Contrast this to FM‘s puny 50-70 mile line-of-sight reach. No tropospheric enhancement or exotic skywave potential for FM. Its local-only coverage represents one of the FM service‘s biggest limitations versus the magic of AM!

Ruggedness & Portability – AM‘s Low-Tech Edge

Imagine you‘re stationed in a remote area, stuck in a storm or need emergency information. Would you depend on delicate, short-range FM receivers vulnerable to weather, damage and electrical failures? Or a nearly indestructible battery-powered AM transistor radio requiring no infrastructure beyond a simple wire antenna?

This reliability edge is why AM remains the workhorse of emergency alert broadcasting, remote information services and portable radios. AM transmitters can run for weeks off car batteries or generators when the power grid fails. Meanwhile FM stations go dark without commercial AC power.

AM‘s simplicity and low-tech nature gives it big advantages for serving public safety needs, isolated areas and portable applications. You‘ll get hours of AM listening from a tiny battery-powered solid state radio that fits in your pocket! And at a price FM can‘t match.

Better Voice Transmission – Advantage AM

Here‘s an interesting difference between AM and FM – because human speech waveforms are not as constant in amplitude as musical tones, AM more efficiently modulates speech patterns on the carrier compared to FM.

This gives AM the edge for intelligibility when transmitting voice, important in aviation and marine channels. FM excels at transmitting consistent musical notes, but garbles the varying amplitudes of voice that AM accommodates well.

That‘s why air traffic control operations still rely heavily on legacy AM communication networks. Pilots often find AM radios yield clearer two-way voice communication compared to FM, especially in weak signal conditions found at altitude.

FM‘s capture effect works fine for strong local signals. But out on the fringes, AM pulls in distant channels with better clarity than the "all or nothing" FM reception.

The Hidden AM Advantage – Infrastructure Costs

Building and operating an AM broadcasting station, particularly with extended service range, costs vastly less overall than a comparable FM facility. The high price tag of FM transmission equipment, antennas, land and towers makes it impractical for manyisolated or poor regions.

An AM station covering wide rural areas might cost a few hundred thousand dollars to construct compared to many millions for a similar capacity FM site. Ongoing operating costs are also lower thanks to greater energy efficiency and staffing requirements.

For community broadcasters, schools and emergency service deployments, opting for AM instead of FM can easily half the infrastructure expense while providing better service. The equipment is simpler and more affordable as well.

This significant cost advantage along with AM‘s wider service range is why AM remains the dominant radio technology across many developing nations and remote regions. An FM network to completely cover some of Earth‘s remotest inhabited areas would be completely infeasible!

FM Alternatives – HD Radio, DAB+ and More

Given FM‘s disadvantages compared to AM, modern digital broadcasting methods seem like they could resolve some of the limitations of traditional analog FM. But technologies like HD Radio and DAB+ also have downsides.

HD Radio piggybacks digital audio on top of standard FM signals, squeezing in extra programming. But range is still limited like analog FM, and reception can be sketchy. Most cars lack HD Radio tuners too, so the technology hasn‘t been widely adopted.

DAB+ enables interference-resistant digital FM broadcasting with added features like song titles. But it still requires the same costly transmitters and infrastructure as analog FM. Adoption outside major cities has been slow, as most listeners are content with Internet streaming.

Someday every radio may go digital. But until then, good old AM offers a analog solution that‘s cheap, reliable and gets the job done across vast service areas HD Radio and DAB+ simply can‘t match.

In Conclusion…

I hope this friendly overview helps explain some of the key technical and practical disadvantages FM faces versus AM broadcasting. As an engineer I find the radio spectrum endlessly fascinating, with surprises and insights around every turn of the dial!

While FM made sense expanding into radio‘s higher frequencies, sometimes newer isn‘t always better. AM retains inherent advantages that guarantee its services will continue on for decades more, alongside its newer FM cousins.

If you made it this far reading, thanks for joining me on this journey of personal discovery celebrating the wonders of radio engineering. Understanding what goes on behind the scenes makes being a broadcast listener even more rewarding. Tune in tomorrow for more insights!

73‘s, Terry W7AM the AM Man