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  • Antenna tuner


    Antenna tuner front view, with partially exposed interiorAntenna tuner, matchbox, transmatch, antenna tuning unit (ATU), antenna coupler, and feedline coupler are all equivalent names for a device connected between a radio and its antenna to improve power transfer between them by matching the impedance of the radio to the combined impedance of the antenna and feedline. Antenna tuners are particularly important for use with transmitters. Transmitters are designed to feed power into a resistive load of a specific value, very often 50 ohms. However the antenna and feedline impedance can vary depending on frequency and other factors. If the impedance seen by the transmitter departs from this design value, the output power can be reflected back towards the transmitter, a condition called "standing waves". In addition to reducing the power radiated by the antenna, this can cause distortion of the signal, and in high power transmitters may overheat the transmitter. Because of this, ATUs are a standard part of almost all radio transmitting systems.

  • Radio


    The Alexandra Palace, here: mast of the broadcasting station receiver dialRadio is the technology of using radio waves to carry information, such as sound, by systematically modulating properties of electromagnetic energy waves transmitted through space, such as their amplitude, frequency, phase, or pulse width. When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form. Radio systems need a transmitter to modulate (change) some property of the energy produced to impress a signal on it, for example using amplitude modulation or angle modulation (which can be frequency modulation or phase modulation). Radio systems also need an antenna to convert electric currents into radio waves, and radio waves into an electric current. An antenna can be used for both transmitting and receiving. The electrical resonance of tuned circuits in radios allow individual frequencies to be selected. The electromagnetic wave is intercepted by a tuned receiving antenna.

  • 33-centimeter band


    The 33-centimeter or 900 MHz band is a portion of the UHF radio spectrum internationally allocated to amateur radio on a secondary basis. It ranges from 902 to 928 MHz and is unique to ITU Region 2. It is primarily used for very local communications as opposed to bands lower in frequency. However, very high antennas with high gain have shown 33 centimeters can provide good long range communications almost equal to systems on lower frequencies such as the 70 centimeter band. The band is also used by industrial, scientific, and medical (ISM) equipment, as well as low powered unlicensed devices. Amateur stations must accept harmful interference caused by ISM users but may receive protection from unlicensed devices. The 900 MHz frequency is also used as a reference band e.g. to express the total power or impact of the electric field "E" - expressed in V/m - or the power density "S" - expressed in W/m2 - of the overall cellular frequencies emission caused by all frequencies s.a. the four bands 850 / 900 / 1,800 / 1,900 MHz - which many GSM phones support and mobile phone operators use - used by all mobile phone operators at the same time to a certain space where e.g. humans are exposed to these frequencies over a certain span of time. More: Mobile phone radiation and health section. In ITU Region 3, New Zealand domestically allocates 915 MHz to 928 MHz to amateurs. In Australia, this spectrum is allocated to radiolocation and scientific-medical services.

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