Extremely high frequency (EHF) refers to the highest range of frequency in the radio spectrum, ranging from 30 to 300 GHz and with wavelengths ranging from 10 to 1 millimetre. It is named millimetre band or millimetre wave (MMW or mmW). Because these radio waves have high atmospheric attenuation, they have a short range and used only for terrestrial communication with a distance of about a kilometre.
Because of its short range, propagation of EHF has many limitations. Its reuse potential is usually affected by rain, humidity and other weather conditions. It is also blocked by buildings as it travels solely by line of sight. This is why it is often used in densely packed communication networks where frequency reuse is improved.
Applications of EHF
Millimetre wave bands have plenty of applications, including inter-satellite links, point-to-point communication, and point-to-multipoint communication.
- Point-to-point high bandwidth communication links use the bands 71-72, 81-86, and 92-95 GHz, as these are not affected by oxygen absorption. Propagation, however, would require a transmitting license from the U.S. Federal Communications Commission.
- Licensed high-speed microwave data links in the United States use the band 38.6-40.0 GHz, while the unlicensed short-range data links use the 60 gigahertz band.
- Wireless HD technology operates near the 60 GHz range. The same spectrum will be used for the Wi-Fi standard IEEE 802.11 with up to 7 Gbit/s data transfer rate.
Scientific Research and Medicine
The extremely high-frequency band is commonly used in remote sensing and radio astronomy. In fact, several satellite sensors of the U.S. currently operates using this frequency range, including 4 NOAA satellites, and Advanced Microwave Sounding Unit (AMSU) on Aqua (NASA satellite).
Aside from satellite-related applications, studies have also been carried out in the use of EHF as a complex therapy for elderly patients with mixed forms of bronchial asthma. The therapy applies EHF on acupuncture points to treat patients in different phases of the disease. Another therapy called Extremely High Frequency therapy or Millimetre Wave Therapy also relies on EHF, and is widely used in former USSR nations to treat a wide range of diseases. Frequency, however, is delivered in low-intensity electromagnetic radiation.
Radio waves of EHF find plenty of uses in weaponry. Short-range fire-control radar in aircraft and tanks use millimetre wave radar to track and shoot down incoming missiles. The same concept is used on automated guns on naval ships.
The U.S. Air Force along with Raytheon, a defence, civil and cyber security contractor has also developed a weapon system called Active Denial System (ADS), which is non-lethal but is said to cause extreme pain to the target. The intense burning pain is delivered by a beam of radiation with a wavelength of 3 mm.
Aside from defence, EHF also protects. It enables satellite communication systems to recover quickly from physical attack, electronic warfare and even from scintillation caused by a nuclear explosion detonated in high altitude.
Some mm-wave atmospheric windows allow newly developed imagers to screen airport passengers as though they are not wearing a stitch of clothing. Three of these scanners have already been installed in Schiphol Airport in Amsterdam. Despite privacy concerns, manufacturers prefer to focus more on the benefits. After all, the millimetre wave scanner covers a wide area, searching as far as 50 metres from where the scanner is installed.
Despite its vulnerability to atmospheric conditions, extremely high frequency can still be used in many ways that prove beneficial to the world and its population.