Radar and microwave links extend to around 100GHz. Above 100GHz there are very few if any users of the radio spectrum.
Frequencies above 30GHz are not widely used for communication between satellites and ground stations as these waves suffer too much attenuation by the atmosphere. However, this band is very suitable for communication links between satellites as the beams travel through the vacuum of space (which does not absorb any electromagnetic radiation).
Radio astronomers make extensive use of the EHF bands, but atmospheric absorption is of course a problem, so it works best at high altitudes.
As we saw earlier, microwave bands are designated by letters. As opposed to the SHF bands, the EHF bands overlap with one another. The Ka, U, E and F bands form one nonoverlapping series and the Q, V, W and D bands form another nonoverlapping series. The spectrum above 170GHz is not even divided into bands.
Any frequencies in the EHF range are absorbed by moisture and especially by rain, so no EHF frequencies are very suitable for very long range communication through the earth's atmosphere. Frequencies around 60GHz are strongly absorbed by the oxygen in the atmosphere, so these frequencies are absorbed even more and are thus completely unusable for long range communication. In the USA these frequencies do not require a license and they can only be used for short range applications.
EHF bands are used for radar, especially for short range applications. The 34GHz band is sometimes used for traffic speed radar. Radar detectors capable of picking up these frequencies are probably the highest frequency radio devices that are ever used by consumers.
In The Netherlands, several bands are allocated to terrestrial microwave links. The highest such bands are 32 and 38GHz. In America they use even higher frequencies, up to 90GHz. Anyway, any frequency bands above 100GHz are hardly is ever used.
There are five amateur bands in the EHF spectrum. The 47GHz band is somewhat regularly used, but the others are not. From time to time, the radio amateur magazines make mention of an amateur who manages to transmit and receive a signal in one of the higher bands, but these events are rather rare, especially in the 1.2mm band, which is pretty much out of range of conventional electronic amplifiers.
Interestingly enough, all of these bands were already allocated thirty years ago and any usage of these is still considered a rarity.
At EHF frequencies (especially the top end of the band) and beyond, conventional electronic circuits can no longer be used.
At lower frequencies, the radio frequency signal is generated in an electronic circuit containing oscillators, filters and amplifiers. Inside the transmitter we have high frequency voltages and currents, but not electromagnetic waves. The transmitter output is an electric signal source with a specific voltage and impedance. The signal leaves the transmitter via a wire (coax cable) in which an electric current flows. As specifically constructed piece of wire, the antenna, radiates the radio frequency signal as electromagnetic waves and only then it becomes electromagnetic radiation.
At the receiving end, the receiving antenna picks up electromagnetic radiation and converts it to an electric signal with the same frequency. Inside the receiver there are electronic circuits, which handle electric voltages and currents, but not electromagnetic waves.
Not all radio transmitters work as described above however. The common household microwave oven uses a special purpose vacuum tube, the cavity magnetron to generate high power microwave energy. The special thing is that electromagnetic energy is generated directly inside the vacuum tube without there being an electrical signal of the same frequency in a wire first. Cavity resonators are pipes of a specific length that act as resonators for electromagnetic energy. These cavity resonators act as the frequency determining element in the magnetron tube. Cavity resonators are the electromagnetic equivalent of organ pipes for acoustic waves.
Of course the magnetron tube in a microwave oven generates energy at only 2450MHz, much lower than EHF. However, there are other, more sophisticated vacuum tubes that work at much higher frequencies and offer much better frequency stability. An example of these is the klystron. A klystron can be used as an amplifier and (by providing positive feedback) as an oscillator. It uses cavity resonators to amplify a specific frequency. Typically the signals into and out of a klystron are electromagnetic energy carried by wave guides. Other types of vacuum tubes, such as traveling wave tubes and backward wave oscillators can also act as oscillators and amplifiers of EHF frequencies.
Receivers of EHF frequencies use the superheterodyne principle. They employ a low power oscillator (e.g. a klystron) and mix it with the received signal. The downconverted signal is at a much lower frequency and can then be processed as a conventional electrical signal.
The ITU allocates frequency bands to various users. Those frequency bands run from 9kHz to 275GHz. Above 275GHz no bands are allocated for radio communication. So 275GHz, just below the top end of the EHF band, can be considered the top end of the radio spectrum. However, the ITU is considering to regulate usage of the frequency spectrum beyond 275GHz.
The band between 300GHz and 3000GHz is called Terahertz radiation. This band is between microwaves and infrared. In some literature it is considered part of the far infrared spectrum. Terahertz radiation penetrates clothing, but not human skin, so it can be used in body scanners to find objects concealed under clothes.
Generally the atmosphere absorbs all wavelengths in the Terahertz radiation range and far infrared range, but there are a few small sub-bands that are absorbed much less. These are used for remote sensing (satellites observing the earth) and for radio astronomy (earth stations observing space). Any proposed ITU regulations will probably aim to protect these services from interference.
Anything between 3THz (0.1mm) and 400THz (750nm, where visible light starts) is considered infrared. Infrared radiation is emitted by just about any object that is not extremely cold. Therefore it is also called heat radiation. In fact such object also emit radiation at lower frequencies, but the radiated energy below 3THz is negligible. Infrared frequencies close to the visible spectrum are called near infrared. Near infrared is used by TV remote controls, most fiber optic links and the laser in a CD player.
Visible light has wavelengths between 390 and 750 nanometers, frequencies of 400 to 770THz. The higher the frequencies of electromagnetic radiation, the more it behaves like particles (photons) instead of waves. Radio waves are nearly completely described by Maxwell's electromagnetic wave equations. Photons have an extremely low energy, so for all practical purposes, radio waves are a continuous phenomenon. For visible light this is not the case, much less for X-rays and gamma radiation.
Radiation with frequencies above the visible spectrum is called ionizing radiation, as a single photon is capable of kicking an electron out of its orbit around its nucleus. Separating an electron from an atom or molecule is called ionization. Ionized molecules undergo chemical reactions that they would not undergo if they would not be ionized. Ionizing radiation causes damage to biological tissues. Ultraviolet, X-rays and gamma rays are ionizing radiation. See the table below for an overview of the complete electromagnetic spectrum.
|Gamma||30EHz and up||10pm and below|
Now I have reached the end of my exploration of the radio spectrum. I hope you will enjoy it.
I added a band diagram.