What is a Megahertz (MHz)?
Formal Definition
The megahertz (symbol: MHz) is a unit of frequency equal to one million hertz (10⁶ Hz), or one million cycles per second. The prefix "mega-" denotes a factor of one million in the SI system. The megahertz is the standard unit for expressing frequencies in the VHF (Very High Frequency) radio band, FM radio broadcasting, television transmissions, and the clock speeds of early personal computers.
The megahertz range (1 MHz to 999 MHz) encompasses FM radio, VHF and UHF television, amateur radio bands, aircraft communication, early cellular telephony, and many industrial, scientific, and medical (ISM) applications. These frequencies propagate primarily by line-of-sight, making them ideal for local and regional communication.
Scale Context
One megahertz represents a frequency far above the audible range (which tops out at about 20 kHz = 0.02 MHz). The electromagnetic wavelength at 1 MHz is about 300 meters, while at 100 MHz it is about 3 meters — corresponding to the practical antenna sizes used for FM radio and television reception.
Etymology
Origin of the Prefix
The prefix "mega-" derives from the Greek word "megas" (μέγας), meaning "great" or "large." It was adopted as an SI prefix meaning one million (10⁶). Combined with "hertz," "megahertz" means "one million hertz" or "one million cycles per second."
Historical Notation
Before the adoption of "hertz" by the CGPM in 1960, the equivalent unit was "megacycles per second" (abbreviated Mc/s or Mc). Radio engineers of the mid-20th century described FM stations as broadcasting at "100 megacycles" rather than "100 megahertz." The transition to the hertz-based nomenclature occurred through the 1960s and 1970s.
The Megahertz in Computing
The megahertz became a household term in the 1980s and 1990s when personal computer clock speeds were marketed in MHz. The IBM PC (1981) ran at 4.77 MHz, and by the late 1990s, processors had reached 400–500 MHz before crossing into the gigahertz range. The phrase "megahertz myth" was coined to describe the misconception that clock speed alone determined computer performance.
Precise Definition
Exact Definition
One megahertz equals exactly 1,000,000 hertz (10⁶ Hz) or exactly 1,000 kilohertz. In SI base units, 1 MHz = 10⁶ s⁻¹.
Key Conversions
1 MHz = 1,000,000 Hz; 1 MHz = 1,000 kHz; 1 MHz = 0.001 GHz. For electromagnetic waves in vacuum, a frequency of 1 MHz corresponds to a wavelength of approximately 299.792 meters (using c = 299,792,458 m/s).
Measurement at MHz Frequencies
Frequencies in the megahertz range are measured using RF (radio frequency) instruments: spectrum analyzers, vector network analyzers, RF power meters, and frequency counters. Crystal oscillators, which provide stable reference frequencies in the MHz range, are the most common frequency references in electronic devices. A typical quartz crystal oscillator provides a reference frequency of 10–50 MHz with stability of ±20–50 parts per million.
History
FM Radio and Television
Edwin Howard Armstrong invented frequency modulation (FM) radio in the 1930s, demonstrating that broadcasting at higher frequencies (in the megahertz range) with frequency modulation could deliver dramatically better audio quality than AM radio. The FM broadcast band was established at 88–108 MHz in the United States in 1945. Television broadcasting also occupied the megahertz range: VHF channels 2–13 used frequencies from 54 to 216 MHz, while UHF channels extended into the higher MHz range.
Computing and the Megahertz Race
The personal computer revolution of the 1980s made megahertz a mainstream term. The original IBM PC (1981) had an Intel 8088 processor running at 4.77 MHz. Throughout the 1980s and 1990s, processor speeds climbed steadily: 8 MHz, 16 MHz, 33 MHz, 66 MHz, 100 MHz, 233 MHz, 400 MHz, and finally breaking the 1 GHz barrier in 2000. Computer advertisements prominently featured MHz numbers, and consumers learned to associate higher MHz with faster computers.
The Megahertz Myth
Apple Computer popularized the phrase "megahertz myth" in 2001 when marketing its PowerPC G4 processors, which performed comparably to higher-clock-speed Intel Pentium 4 processors. The argument — that architectural efficiency mattered as much as raw clock speed — was valid but self-serving. Nevertheless, the debate educated the public about the limitations of using MHz alone as a performance metric.
Modern Relevance
Although processor marketing has shifted to gigahertz, the megahertz remains relevant for memory clock speeds (DDR4 at 2133–3200 MHz), radio frequencies, and embedded systems. Microcontrollers used in IoT devices typically operate at 8–240 MHz.
Current Use
FM Radio
FM radio broadcasting operates in the 87.5–108 MHz band worldwide (with minor regional variations). Each FM station occupies a 200 kHz channel, and the frequency is what you see on your radio dial — "101.1 FM" means a carrier frequency of 101.1 MHz. FM radio remains one of the most widely used communication technologies, with billions of receivers in use globally.
Aviation Communication
Aircraft voice communication uses the VHF band from 118.000 to 136.975 MHz, with channels spaced 8.33 kHz apart in Europe and 25 kHz apart in most other regions. When a pilot says "contact tower on one-two-one point nine," they mean 121.9 MHz. The emergency frequency 121.5 MHz is monitored by all ATC facilities worldwide.
Memory and Bus Speeds
Computer memory speeds are specified in MHz: DDR4 RAM operates at 2133–3200 MHz, and DDR5 at 4800–8000 MHz. Front-side bus, PCI Express, and USB specifications all involve MHz-range clock frequencies that determine data transfer rates.
Medical Equipment
Medical ultrasound imaging uses transducer frequencies of 1–20 MHz. Higher frequencies provide better resolution but less penetration depth: a 10 MHz transducer gives excellent images of superficial structures, while a 2 MHz transducer can image deeper organs like the liver and kidneys.
Everyday Use
FM Radio Tuning
Every time you tune to an FM radio station, you select a frequency in megahertz. The numbers on the radio dial — 88.1, 93.5, 101.9 — are frequencies in MHz. Changing from one station to another typically involves moving 0.2 MHz (200 kHz) or more, which is the minimum channel spacing for FM broadcasting.
Wi-Fi Channels
The 2.4 GHz Wi-Fi band is divided into channels at specific MHz frequencies: Channel 1 at 2,412 MHz, Channel 6 at 2,437 MHz, and Channel 11 at 2,462 MHz. When your router settings show channel frequencies, they are displayed in MHz within the GHz band.
Microwave Ovens
Microwave ovens operate at 2,450 MHz (2.45 GHz), a frequency at which water molecules absorb energy efficiently, heating food. This frequency was discovered almost accidentally by Percy Spencer at Raytheon in 1945 when a chocolate bar melted in his pocket near a microwave-emitting magnetron.
Baby Monitors and Wireless Devices
Many household wireless devices operate in the hundreds of MHz range. Older cordless phones used 900 MHz, baby monitors often use 900 MHz or 2,400 MHz, and remote-controlled garage door openers operate at 300–400 MHz.
Interesting Facts
The FM radio band (88–108 MHz) was almost lost to television broadcasting. In the 1940s, the FCC moved FM radio from its original 42–50 MHz band to the current 88–108 MHz band, rendering all existing FM receivers obsolete and nearly killing the technology.
The Intel 4004, the world's first commercial microprocessor (1971), ran at 0.74 MHz (740 kHz). A modern smartphone processor running at 3 GHz is over 4,000 times faster in clock speed alone, with vastly greater instructions-per-clock efficiency on top of that.
The international aircraft emergency frequency, 121.5 MHz, is so important that it is one of the few radio frequencies protected by international treaty. All pilots are required to monitor it, and interfering with it is a criminal offense in most countries.
A quartz crystal oscillator — the tiny component that keeps time in watches and provides clock signals in electronics — vibrates at a precise MHz frequency (typically 32.768 kHz for watches, or 4–50 MHz for digital electronics). Its frequency is determined by the crystal's physical dimensions, cut angle, and temperature.
The first transatlantic television transmission, in 1962 via the Telstar satellite, used a carrier frequency of 4,170 MHz (4.17 GHz) — well above the MHz range. But the baseband video signal it carried had a bandwidth of about 6 MHz, the same as a standard analog TV channel.
MRI (Magnetic Resonance Imaging) machines operate at specific MHz frequencies determined by the magnetic field strength. A 1.5 Tesla MRI operates at approximately 63.9 MHz, while a 3 Tesla MRI operates at about 127.8 MHz — the Larmor frequency of hydrogen protons at those field strengths.