Conversion from 2 Megahertz to Terahertz

Information about the Megahertz (MHz)

The megahertz is a unit of frequency equal to one million hertz. Its symbol is MHz. It covers the part of the radio spectrum that carries most broadcasting and short-range communication, and it was the unit in which computer speed was measured for two decades.

FM radio occupies 87.5 to 108 megahertz almost everywhere, a band chosen after the Second World War because it was high enough to allow the wide channels frequency modulation needs and low enough for transmitters and receivers to be built cheaply. Terrestrial television took the neighbouring bands, and the switch from analogue to digital broadcasting freed large parts of them for mobile telephony.

Aviation communication sits just above FM radio, from 118 to 137 megahertz, using amplitude modulation rather than the clearer frequency modulation. The choice is deliberate: when two aircraft transmit at once, amplitude modulation produces an audible heterodyne squeal that alerts everyone to the collision, whereas frequency modulation would simply capture the stronger signal and silently lose the weaker one.

Computer processors were rated in megahertz from the late 1970s until around 2000. The original IBM PC ran at 4.77 megahertz, and the megahertz race that followed ended when clock speeds stalled near four gigahertz because of heat, after which manufacturers competed on core count instead. Memory bus speeds and display timings are still commonly quoted in megahertz.

Medical ultrasound works between about 2 and 18 megahertz. Higher frequencies give finer resolution but penetrate less deeply, so an abdominal scan uses a lower frequency than an examination of a superficial structure, and the operator chooses the probe accordingly.

Radio-frequency identification, garage door remotes, wireless microphones, marine VHF and amateur radio all live in this range. So do the harmonics that make poorly shielded electronics interfere with each other, which is why regulatory testing concentrates on emissions in the tens and hundreds of megahertz.

Mobile telephony has made the unit a matter of money. Spectrum is auctioned in blocks measured in megahertz, and governments have raised sums in the tens of billions from bands at 700, 800, 900, 1800 and 2600 megahertz. Physics sets the value of each: lower frequencies travel further and penetrate buildings better, so the 700 and 800 bands cover rural areas cheaply, while the higher ones carry more data over shorter distances and suit cities. An operator's coverage map and its data speeds are therefore both consequences of which megahertz it managed to buy, which is why the auctions are followed as closely as any other national asset sale.

One megahertz equals one million hertz, 1000 kilohertz, or 0.001 gigahertz.

Information about the Terahertz (THz)

The terahertz is a unit of frequency equal to one million million hertz. Its symbol is THz. It occupies the gap between the highest radio frequencies and the lowest infrared light, a region that for most of the twentieth century was so hard to generate or detect that it was known as the terahertz gap.

The difficulty was practical rather than theoretical. Electronic oscillators run out of speed above a few hundred gigahertz because charge carriers cannot respond fast enough, while optical sources such as lasers become inefficient below the infrared. The band between them had no convenient technology on either side, and for decades it was the least explored part of the electromagnetic spectrum.

That changed with ultrafast lasers and photoconductive antennas, which generate terahertz pulses by illuminating a semiconductor with a laser pulse lasting a few femtoseconds. Terahertz time-domain spectroscopy now measures the response of materials across the band, and the technique has become a standard tool in materials science.

Security screening is the best-known application. Terahertz radiation passes through clothing, paper, plastic and ceramics but is stopped by metal and absorbed by water, and its photon energy is far too low to ionise anything, so it can image concealed objects without the risks of X-rays. Body scanners at airports work on this principle.

Molecular spectroscopy uses the band because many large molecules have rotational and vibrational modes here. Explosives and drugs have characteristic terahertz signatures, and the technique is used in pharmaceutical quality control to identify crystal forms that look identical by other methods. Art conservation applies it to see beneath paint layers without touching a canvas.

Astronomy observes at these frequencies from high, dry sites or from orbit, because atmospheric water vapour absorbs terahertz radiation strongly. Cold dust and molecular gas in star-forming regions radiate here, which is why observatories are built at high altitude in Chile and at the South Pole.

Communications research has begun to move into the band. Data rates rise with available bandwidth, and there is far more room above 100 gigahertz than below it, so laboratories have demonstrated wireless links carrying hundreds of gigabits a second at frequencies approaching a terahertz. The obstacle is that water vapour absorbs these waves strongly, limiting a link to a few hundred metres in clear air and much less in rain. That confines the technology to short hops: connections between rooftop antennas, links inside a data centre, or the last stretch between a lamp-post and a building rather than anything resembling a broadcast.

One terahertz equals one million million hertz, 1000 gigahertz, or a wavelength of about 0.3 millimetres.