General RF
Radio frequency (abbreviated RF, rf, orr.f.) is a term that refers to alternating current (AC) having characteristics such that, if the current is input to an antenna, an electromagnetic (EM) field is generated suitable for wireless broadcasting and/or communications. These frequencies cover a significant portion of the electromagnetic radiation spectrum, extending from nine kilohertz (9 kHz),the lowest allocated wireless communications frequency (it's within the range of human hearing), to thousands of gigahertz(GHz).
When an RF current is supplied to an antenna, it gives rise to an electromagnetic field that propagates through space. This field is sometimes called an RF field; in less technical jargon it is a "radiowave." Any RF field has a wavelength that is inversely proportional to the frequency. In the atmosphere or in outerspace, if f is the frequency in megahertz and s is the wavelength in meters, then
The frequency of an RF signal is inversely proportional to the wavelength of the EM field to which it corresponds. At 9 kHz, the free-space wavelength is approximately 33 kilometers (km) or 21 miles (mi). At the highest radio frequencies, the EM wavelengths measure approximately one millimeter (1 mm). As the frequency is increased beyond that of the RF spectrum, EM energy takes the form of infrared (IR), visible, ultraviolet (UV), X rays, and gamma rays.
Many types of wireless devices make use of RF fields. Cordless and cellular telephone, radio and television broadcast stations, satellite communications systems, and two-way radio services all operate in the RF spectrum. Some wireless devices operate at IR or visible-light frequencies, whose electromagnetic wavelengths are shorter than those of RF fields. Examples include most television-set remote-control boxes, some cordless computer keyboards and mice, and a few wireless hi-fi stereo headsets.
The RF spectrum is divided into several ranges, or bands. With the exception of the lowest-frequency segment, each band represents an increase of frequency corresponding to an order of magnitude (power of 10). The tabled epicts the eight bands in the RF spectrum, showing frequency and bandwidth ranges. The SHF and EHF bands are often referred to as the microwave spectrum.
| Designation | Abbreviation | Frequencies | Free-space Wavelengths |
| Very Low Frequency | VLF | 9 kHz - 30 kHz | 33 km - 10 km |
| Low Frequency | LF | 30 kHz - 300 kHz | 10 km - 1 km |
| Medium Frequency | MF | 300 kHz - 3 MHz | 1 km - 100 m |
| High Frequency | HF | 3 MHz - 30 MHz | 100 m - 10 m |
| Very High Frequency | VHF | 30 MHz - 300 MHz | 10 m - 1 m |
| Ultra High Frequency | UHF | 300 MHz - 3 GHz | 1 m - 100 mm |
| Super High Frequency | SHF | 3 GHz - 30 GHz | 100 mm - 10 mm |
| Extremely High Frequency | EHF | 30 GHz - 300 GHz | 10 mm - 1 mm |
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RF wave modulation and applications
RF as defined is an AC or continuous wave signal propagated through space. This continuous wave signal is useless unless modulated by another signal (either analog or digital), which carries the information that needs to be transmitted. Popular analog modulation techniques are AM and FM while digital modulation includes FSK, QAM and QPSK. Early applications include the traditional analog TV and radio broadcasts, long-distance HF communications and portable two-way VHF and UHF communications. New applications now include Mobile Phone services (GSM, CDMA, WCDMA), Digital TV (DVB-T, ATSC, ISDB-T), Mobile TV (DVB-H, One Seg, DMB, MediaFLO), Wireless Local Area Networks ( 802.11x or WiFi), Wireless Metropolitan Area Networks (802.16x or WiMAX) and Wireless Wide Area Networks (3G, WCDMA, GPRS, EDGE, UMTS).
BTW, I have a related article regarding Radio Mobile, a free RF design software: http://dev.emcelettronica.com/radio-mobile-free-rf-design-software
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