RF Antenna input is typically used to connect a television antenna, cable TV wire, or satellite feed to a television, VCR, or other device that can process radio-frequency video signals, including some computers. Knowing when to use an RF signal and how it differs from other signals can be useful when setting up some computer and video systems.
RF Signals
A radio frequency (RF) signal is an electrical signal emitted as a radio wave. It travels from the broadcasting antenna to the receiving antenna. Different channels carry the signal at different frequencies. Depending on conditions, the signal can be received as far as 80 miles from the broadcast antenna. An RF signal can also be carried on wires that have been configured appropriately, either from a nearby broadcast antenna or as part of a cable TV distribution system serving an area ranging from a building to a city.
Appearance
The most common RF input resembles a short, thick bolt about one-quarter inch in diameter and one-quarter inch long. The cable cord screws into the bolt. Older television sets may have two screws side by side, designed to attach wires from a cable called a twin lead. There is an electrical difference between the two forms of connector. An adapter may be required to attach twin lead wires to a round RF input or vice versa. The adapters are inexpensive and widely available.
Receiving RF Signals
A television image encoded as an RF signal contains audio and video, both of which are decoded using a tuner. The key difference between a monitor and a television set is that a monitor cannot process an RF signal directly. Most televisions, VCRs, and cable TV boxes let you output the decoded audio and video signal to other devices. Separating the video and audio signals gives better quality than the integrated RF signal; S-video, component video, and HDMI outputs offer higher quality than RF. For a computer to process RF signals it must have a video card equipped with an RF input plug, as well as other hardware and channel-selecting software.
Sending RF signals
Using an RF signal to carry audio and video from one device to another is an option of last resort, as it is always the lowest-quality option. However, with older television sets and VCRs, or in complex setups, RF may the only option available. Most VCRs, legacy game consoles, satellite receivers, and cable descramblers can accommodate an RF signal, typically on channel 3. External RF modulators are available to generate an RF signal from the video and audio outputs widely used on computers and audio-video devices.
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Other RF Antenna Uses
RF signals are used for FM radio signals and Wi-Fi computer networks. FM radios and audio/video receivers may have an RF input for the radio antenna, or a connection for a cable system carrying FM radio signals. Wi-Fi antennas usually connect directly to the wireless transmitter.
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Effective radiated power (ERP), synonymous with equivalent radiated power, is an IEEE standardized definition of directional radio frequency (RF) power, such as that emitted by a radio transmitter. It is the total power in watts that would have to be radiated by a half-wave dipole antenna to give the same radiation intensity (signal strength in watts per square meter) as the actual source at a distant receiver located in the direction of the antenna's strongest beam (main lobe). ERP measures the combination of the power emitted by the transmitter and the ability of the antenna to direct that power in a given direction. It is equal to the input power to the antenna multiplied by the gain of the antenna. It is used in electronics and telecommunications, particularly in broadcasting to quantify the apparent power of a broadcasting station experienced by listeners in its reception area.
An alternate parameter that measures the same thing is effective (or equivalent) isotropic radiated power (EIRP). Effective isotropic radiated power is the total power that would have to be radiated by a hypothetical isotropic antenna to give the same signal strength as the actual source in the direction of the antenna's strongest beam. The difference between EIRP and ERP is that ERP compares the actual antenna to a half-wave dipole antenna, while EIRP compares it to a theoretical isotropic antenna. Since a half-wave dipole antenna has a gain of 1.64, or 2.15 decibels compared to an isotropic radiator, if ERP and EIRP are expressed in watts their relation is
If they are expressed in decibels
- 5FM example
Definitions[edit]
Effective radiated power and effective isotropic radiated power both measure the amount of power a radio transmitter and antenna (or other source of electromagnetic waves) radiates in a specific direction: in the direction of maximum signal strength (the 'main lobe') of its radiation pattern.[1][2][3][4] This maximum radiated power is dependent on two factors: the total power output and the radiation pattern of the antenna – how much of that power is radiated in the desired direction. The latter factor is quantified by the antenna gain, which is the ratio of the signal strength radiated by an antenna to that radiated by a standard antenna. For example, a 1,000-watt transmitter feeding an antenna with a gain of 4 (6 dBi) will have the same signal strength in the direction of its main lobe, and thus the same ERP and EIRP, as a 4,000-watt transmitter feeding an antenna with a gain of 1 (0 dBi). So ERP and EIRP are measures of radiated power that can compare different combinations of transmitters and antennas on an equal basis.
The difference between ERP and EIRP is that antenna gain has traditionally been measured in two different units, comparing the antenna to two different standard antennas; an isotropic antenna and a half-wave dipole antenna:
- Isotropic gain is the ratio of the power density (signal strength in watts per square meter) received at a point far from the antenna (in the far field) in the direction of its maximum radiation (main lobe), to the power received at the same point from a hypothetical lossless isotropic antenna, which radiates equal power in all directions
- Gain is often expressed in logarithmic units of decibels (dB). The decibel gain relative to an isotropic antenna (dBi) is given by
- Dipole gain is the ratio of the power density received from the antenna in the direction of its maximum radiation to the power density received from a lossless half-wave dipole antenna in the direction of its maximum radiation
- The decibel gain relative to a dipole (dBd) is given by
In contrast to an isotropic antenna, the dipole has a 'donut-shaped' radiation pattern, its radiated power is maximum in directions perpendicular to the antenna, declining to zero on the antenna axis. Since the radiation of the dipole is concentrated in horizontal directions, the gain of a half-wave dipole is greater than that of an isotropic antenna. The isotropic gain of a half-wave dipole is 1.64, or in decibels 10 log 1.64 = 2.15 dBi, so
In decibels
The two measures EIRP and ERP are based on the two different standard antennas above:[1][3][2][4]
- EIRP is defined as the RMS power input in watts required to a lossless isotropic antenna to give the same maximum power density far from the antenna as the actual transmitter. It is equal to the power input to the transmitter's antenna multiplied by the isotropic antenna gain
- The ERP and EIRP are also often expressed in decibels (dB). The input power in decibels is usually calculated with comparison to a reference level of one watt (W): . Since multiplication of two factors is equivalent to addition of their decibel values
- ERP is defined as the RMS power input in watts required to a lossless half-wave dipole antenna to give the same maximum power density far from the antenna as the actual transmitter. It is equal to the power input to the transmitter's antenna multiplied by the antenna gain relative to a half-wave dipole
- In decibels
Since the two definitions of gain only differ by a constant factor, so do ERP and EIRP
In decibels
Relation to transmitter output power[edit]
The transmitter is usually connected to the antenna through a transmission line. Since the transmission line may have significant losses , the power applied to the antenna is usually less than the output power of the transmitter . The relation of ERP and EIRP to transmitter output power is
Losses in the antenna itself are included in the gain.
Dipole vs. isotropic radiators[edit]
Because ERP is calculated as antenna gain (in a given direction) as compared with the maximum directivity of a half-wave dipole antenna, it creates a mathematically virtual effective dipole antenna oriented in the direction of the receiver. In other words, a notional receiver in a given direction from the transmitter would receive the same power if the source were replaced with an ideal dipole oriented with maximum directivity and matched polarization towards the receiver and with an antenna input power equal to the ERP. The receiver would not be able to determine a difference. Maximum directivity of an ideal half-wave dipole is a constant, i.e., 0 dBd = 2.15 dBi. Therefore, ERP is always 2.15 dB less than EIRP. The ideal dipole antenna could be further replaced by an isotropic radiator (a purely mathematical device which cannot exist in the real world), and the receiver cannot know the difference so long as the input power is increased by 2.15 dB.
Unfortunately, the distinction between dBd and dBi is often left unstated and the reader is sometimes forced to infer which was used. For example, a Yagi-Uda antenna is constructed from several dipoles arranged at precise intervals to create better energy focusing (directivity) than a simple dipole. Since it is constructed from dipoles, often its antenna gain is expressed in dBd, but listed only as dB. Obviously this ambiguity is undesirable with respect to engineering specifications. A Yagi-Uda antenna's maximum directivity is 8.77 dBd = 10.92 dBi. Its gain necessarily must be less than this by the factor η, which must be negative in units of dB. Neither ERP nor EIRP can be calculated without knowledge of the power accepted by the antenna, i.e., it is not correct to use units of dBd or dBi with ERP and EIRP. Let us assume a 100-watt (20 dBW) transmitter with losses of 6 dB prior to the antenna. ERP < 22.77dBW and EIRP < 24.92dBW, both less than ideal by η in dB. Assuming that the receiver is in the first side-lobe of the transmitting antenna, and each value is further reduced by 7.2 dB, which is the decrease in directivity from the main to side-lobe of a Yagi-Uda. Therefore, anywhere along the side-lobe direction from this transmitter, a blind receiver could not tell the difference if a Yagi-Uda was replaced with either an ideal dipole (oriented towards the receiver) or an isotropic radiator with antenna input power increased by 1.57 dB.[5]
Polarization[edit]
Polarization has not been taken into account so far, but it must be properly clarified. When considering the dipole radiator previously we assumed that it was perfectly aligned with the receiver. Now assume, however, that the receiving antenna is circularly polarized, and there will be a minimum 3 dB polarization loss regardless of antenna orientation. If the receiver is also a dipole, it is possible to align it orthogonally to the transmitter such that theoretically zero energy is received. However, this polarization loss is not accounted for in the calculation of ERP or EIRP. Rather, the receiving system designer must account for this loss as appropriate. For example, a cellular telephone tower has a fixed linear polarization, but the mobile handset must function well at any arbitrary orientation. Therefore, a handset design might provide dual polarization receive on the handset so that captured energy is maximized regardless of orientation, or the designer might use a circularly polarized antenna and account for the extra 3 dB of loss with amplification.
FM example[edit]
Four bay crossed-dipole antenna of an FM broadcasting station.
For example, an FMradio station which advertises that it has 100,000 watts of power actually has 100,000 watts ERP, and not an actual 100,000-watt transmitter. The transmitter power output (TPO) of such a station typically may be 10,000 to 20,000 watts, with a gain factor of 5 to 10 (5× to 10×, or 7 to 10 dB). In most antenna designs, gain is realized primarily by concentrating power toward the horizontal plane and suppressing it at upward and downward angles, through the use of phased arrays of antenna elements. The distribution of power versus elevation angle is known as the vertical pattern. When an antenna is also directional horizontally, gain and ERP will vary with azimuth (compass direction). Rather than the average power over all directions, it is the apparent power in the direction of the antenna's main lobe that is quoted as a station's ERP (this statement is just another way of stating the definition of ERP). This is particularly applicable to the huge ERPs reported for shortwave broadcasting stations, which use very narrow beam widths to get their signals across continents and oceans.
United States regulatory usage[edit]
ERP for FM radio in the United States is always relative to a theoretical reference half-wave dipole antenna. (That is, when calculating ERP, the most direct approach is to work with antenna gain in dBd). To deal with antenna polarization, the Federal Communications Commission (FCC) lists ERP in both the horizontal and vertical measurements for FM and TV. Horizontal is the standard for both, but if the vertical ERP is larger it will be used instead.
The maximum ERP for US FM broadcasting is usually 100,000 watts (FM Zone II) or 50,000 watts (in the generally more densely populated Zones I and I-A), though exact restrictions vary depending on the class of license and the antenna height above average terrain (HAAT).[6] Some stations have been grandfathered in or, very infrequently, been given a waiver, and can exceed normal restrictions.
Microwave band issues[edit]
For most microwave systems, a completely non-directional isotropic antenna (one which radiates equally and perfectly well in every direction – a physical impossibility) is used as a reference antenna, and then one speaks of EIRP (effective isotropic radiated power) rather than ERP. This includes satellitetransponders, radar, and other systems which use microwave dishes and reflectors rather than dipole-style antennas.
Lower-frequency issues[edit]
In the case of medium wave (AM) stations in the United States, power limits are set to the actual transmitter power output, and ERP is not used in normal calculations. Omnidirectional antennas used by a number of stations radiate the signal equally in all directions. Directional arrays are used to protect co- or adjacent channel stations, usually at night, but some run directionally 24 hours. While antenna efficiency and ground conductivity are taken into account when designing such an array, the FCC database shows the station's transmitter power output, not ERP.
Related terms[edit]
Effective monopole radiated power (EMRP) may be used in Europe, particularly in relation to mediumwave broadcasting antennas. This is the same as ERP, except that a short vertical antenna (i.e. a short monopole) is used as the reference antenna instead of a half-wave dipole.
Wireless Satellite Transmitter
HAAT[edit]
The height above average terrain for VHF and higher frequencies is extremely important when considering ERP, as the signal coverage (broadcast range) produced by a given ERP dramatically increases with antenna height. Because of this, it is possible for a station of only a few hundred watts ERP to cover more area than a station of a few thousand watts ERP, if its signal travels above obstructions on the ground.
Rf Satellite Transmitter Power Cord
See also[edit]
References[edit]
- ^ abJones, Graham A.; Layer, David H.; Osenkowsky, Thomas G. (2007). National Association of Broadcasters Engineering Handbook, 10th Ed. Elsevier. p. 1632. ISBN1136034102.
- ^ abHuang, Yi; Boyle, Kevin (2008). Antennas: From Theory to Practice. John Wiley and Sons. pp. 117–118. ISBN0470772921.
- ^ abSeybold, John S. (2005). Introduction to RF Propagation. John Wiley and Sons. p. 292. ISBN0471743682.
- ^ abWeik, Martin H. (2012). Communications Standard Dictionary. Springer Science and Business Media. p. 327. ISBN146156672X.
- ^Cheng, David K. (1992). Field and Wave Electromagnetics, 2nd Ed. Addison-Wesley. pp. 648–650.
- ^47 CFR 73.211
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