1 MHz to Hz

One megahertz is exactly 1,000,000 hertz

Hertz is the SI unit of frequency. The BIPM SI Brochure, 9th edition (2019) defines one hertz as one cycle per second, written s^-1, so a 1 MHz signal completes one million full cycles in each second.

Mega is the prefix that supplies the million. The same brochure gives mega the symbol M and the factor 10⁶, with kilo at 10³ below it and giga at 10⁹ above. One megahertz is therefore 1,000,000 hertz by definition, not by measurement, and the conversion is a six place decimal shift.

Flip the number over and you get the period. One cycle at 1,000,000 Hz lasts 1 / 1,000,000 second, which is 0.000001 second, or one microsecond. That single number explains a lot of radio hardware: a 1 MHz signal spends one microsecond on each cycle, so timing circuits, filters and pulse generators in that band work in microseconds.

Write the symbol as MHz, with a capital M and a capital H, and keep a space between the number and the unit in formal SI style: 1 MHz, not 1MHz. Unit symbols do not take a plural, so 4 MHz stays 4 MHz.

The hertz ladder around one megahertz

The hertz ladder is regular, and a table makes the pattern visible. Every step upward is a factor of 1,000, so moving from megahertz to kilohertz adds three zeros to the count and moving to hertz adds six. Moving toward gigahertz removes three zeros.

Read the table from left to right and the decimal point moves in a predictable way. A 100 MHz clock is 100,000,000 Hz, not 10,000,000 Hz. A 0.1 MHz signal is 100 kHz, not 10 kHz. The period column is the quickest independent check, because 1 microsecond always belongs to 1 MHz and 1 nanosecond always belongs to 1 GHz.

Only the first row sits below the megahertz mark. Everything from 1 MHz upward is the part of the spectrum where aviation radios, weather broadcasts, industrial heaters and MRI scanners do their work.

Aviation and weather radio sit just above 100 MHz

Civil aviation talks in the VHF airband. The FAA spectrum chart for air traffic systems (2021) places VHF air to ground communications at 118 to 137 MHz, and FAA spectrum management guidance lists the working range as 118.00 to 136.975 MHz with 25 kHz channel steps. In hertz that is 118,000,000 to 137,000,000 Hz.

The emergency channel is 121.5 MHz, a frequency reserved for aircraft in distress. It is 121,500,000 Hz, and one cycle lasts 8.23 nanoseconds. The international distress tone on that channel is an audio tone, not the carrier itself, so the microsecond timing belongs to the radio wave rather than to what a listener hears.

Weather radio sits a little higher. NOAA Weather Radio broadcasts on seven channels from 162.400 to 162.550 MHz, spaced 25 kHz apart. The whole weather band spans 0.15 MHz, which is 150 kHz, and each channel is 0.025 MHz wide. A receiver tuned to 162.550 MHz is listening at 162,550,000 Hz.

These bands are narrow in percentage terms. A 25 kHz channel at 121.5 MHz is about 0.02 percent of the carrier, which is why aircraft radios need tight frequency tolerance and why a cheap scanner can drift off channel.

ISM bands at 13.56, 27.12 and 40.68 MHz

Industrial, scientific and medical equipment gets its own set of frequencies. 47 CFR 18.301 (2020) lists 6.78, 13.56, 27.12 and 40.68 MHz with tolerance bands around each one. The point of those slots is to keep intentional radiators away from the same spectrum that carries communications.

The applications follow the physics. Lower frequencies in this group heat conductive material through induced currents, so 13.56 MHz drives induction furnaces, RF welding presses and semiconductor plasma chambers. The same 13.56 MHz carrier is used for near field communication cards, where the reader and the card couple through a magnetic field over a few centimeters rather than through a radiating antenna.

Wavelength explains the hardware size. At 13.56 MHz a full wave is 22.11 meters, so a coil or a matching network can be built on a bench. At 40.68 MHz the full wave is 7.37 meters and the half wave is 3.685 meters, which fits inside a piece of industrial equipment but not inside a phone.

An MRI magnet is rated in tesla but tuned in MHz

An MRI scanner is sold by its magnetic field, in tesla, but its radio chain is tuned in megahertz. The link is the proton gyromagnetic ratio. NIST CODATA (2022) gives 42.577478461 MHz per tesla for a proton, so the precession frequency is 42.577 MHz multiplied by the field strength.

The arithmetic is short. A 0.3 tesla open scanner runs at 12.77 MHz. A 1.5 tesla scanner runs at 63.87 MHz, which is why technologists call it a 64 MHz machine. A 3 tesla scanner runs at 127.73 MHz, and a 7 tesla research magnet runs at 298.04 MHz.

Those numbers set the design of the transmit coil, the receive coil and every filter in the receive path. A coil cut for 63.87 MHz will not work in a 3 tesla bore tuned to 127.73 MHz, even though the two scanners look alike from the outside. The field strength sets the frequency, and the frequency sets the hardware.

The same constant appears in spectroscopy. A bench NMR instrument with a 9.4 tesla magnet works near 400 MHz, which is 42.577 times 9.4. The unit on the front panel may say 400 MHz while the magnet is specified in tesla, and both labels describe the same instrument.

One microsecond, 299.79 meters, and radar range

One microsecond is the period of a 1 MHz wave, and it is also a distance. Light and radio waves travel 299.792458 meters in a vacuum in that time, which rounds to 299.79 meters. A radar pulse that returns after 1 microsecond has made a round trip of 299.79 meters, so the target sits about 149.9 meters away.

The same conversion runs in reverse for timekeeping. A 10 MHz reference oscillator produces a tick every 100 nanoseconds, and a 100 MHz oscillator produces one every 10 nanoseconds. Test equipment that counts microseconds is therefore counting cycles of a signal in the low megahertz range.

Radar engineers use a rough rule of 150 meters per microsecond of round trip time. It comes straight from dividing the speed of light by two, and it holds for any pulse length as long as the pulse is short compared with the distance being resolved.

Antenna length from MHz: the 149.9 rule

Antenna length follows from wavelength with one division. A half wave dipole in free space is half of 299.79 meters divided by the frequency in MHz, which is 149.9 / f. A quarter wave whip is half of that again, or 74.95 / f.

The numbers show why low band antennas are large. A half wave dipole for 1 MHz is 149.9 meters long, which is longer than most city lots, while a half wave for 915 MHz is 16.38 centimeters. That size gap is the reason shortwave receivers use long wires, loops or loaded whips rather than full size dipoles.

Real antennas come out a little shorter than the free space number. Wire has a velocity factor near 0.95, coils add loading, and nearby objects detune the element. Build the antenna slightly long, then trim it with an SWR meter rather than trusting the arithmetic to the last centimeter.

Reading MHz labels without losing a factor of 1,000

Most megahertz mistakes are decimal point mistakes. A 100 MHz signal is 100,000,000 Hz, with eight zeros after the one. A 0.5 MHz signal is 500 kHz, not 50 kHz. A 1 MHz signal is 1,000 kHz, not 100 kHz.

Bandwidth labels cause the next cluster of errors. A 25 kHz channel is 0.025 MHz, and a tolerance of plus or minus 7 kHz is plus or minus 0.007 MHz. Mixing kilohertz and megahertz on the same spec sheet is common, and adding the numbers without converting first gives answers that are off by a factor of 1,000.

Older documents use megacycles per second, abbreviated Mc/s or mc. That unit is the same as the megahertz, so a 14 Mc/s crystal is a 14 MHz crystal. The modern symbol is MHz, and the SI prefix table gives it a fixed factor of 10⁶.

A period check catches almost every slip. If a datasheet says the period is 1 microsecond, the frequency is 1 MHz. If it says 10 microseconds, the frequency is 100 kHz. Enter the number in the converter above and compare the period row with the datasheet before you trust the hertz value.