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Gain of an Antenna


Antenna Gain might seem an odd concept. Many of us think of gain as something that needs amplifiers and batteries or an electrical wall outlet with ac current. But antenna gain is really a relative measurement between different types of antennas. An antenna that has “less signal loss” than another is said to have more antenna gain. So gain for antennas, means in reality, “less loss”.

How do we measure antenna gain? Well, we measure gain relative to something called an isotropic antenna – an antenna that claims to be able to radiate the same amount of power equally in all directions. There is some question about whether or not someone could make such an antenna in real life, but we use the concept to determine the gain of more normal antennas.

The reason why isotropic antennas do not exist, is that just about any antenna in real life is at least partially a directional antenna. By that we mean that the power radiating from the antenna is higher in some directions than others. That is often a good thing by the way. Note that cellular never needs an isotropic antenna but always could use some form of a directional antenna. For one reason, there are no people in the sky, so antennas need not radiate there, and there are no people under ground. Therefore at the very least, cellular antennas should concentrate on the surface of the earth and building height above and below that surface.

Antenna Gain is defined as the ratio of the radiation intensity of an antenna in a given direction, to the intensity of the same antenna as it radiates in all directions (isotropically). Since the radiation intensity of an isotropically radiated power is equal to the power into the antenna divided by 4п (360 degrees) we can express the following equation:

Gain = 4pileft(frac{mbox{Radiatio n Intensity}}{mbox{Antenna Input Power}}right)

Gain = 4pileft(frac{mbox{U}left (theta,phiright)}{mbox{Pin }}right) qquadqquad mbox{Dimensionless Units}.

Although the gain of an antenna is directly related to its directivity, it is important to note that the antenna gain is a measure that takes into account the efficiency of the antenna as well as its directional capabilities. In contrast, directivity is defined as a measure that takes into account only the directional properties of the antenna and therefore it is only influenced by the antenna pattern. However, if we assumed an ideal antenna without losses then Antenna Gain will equal directivity as the antenna efficiency factor equals 1 (100% efficiency). In practice, the gain of an antenna is always less than its directivity.

D(theta,phi) = 4pileft(frac{mbox{U}left (theta,phiright)}{mbox{Pra d}}right)

D(theta,phi) = epsilon_{cd}left (4pifrac{mbox{U}left (theta,phiright)}{mbox{Pra d}}right)

D(theta,phi) = epsilon_{cd}left (D(theta,phi)right)

The formulas above show the relationship between antenna gain and directivity, where εcd is the antenna efficiency factor, D the directivity of the antenna and G the antenna gain. In the antenna world, we usually deal with a “relative gain” which is defined as the power gain ratio in a specific direction of the antenna, to the power gain ratio of a reference antenna in the same direction. The input power must be the same for both antennas while performing this type of measurement. The reference antenna is usually a dipole, horn or any other type of antenna whose power gain is already calculated or known.

Gain = mbox{G(ref ant)}left(frac{mbox{Pmax(AU T)}}{mbox{Pmax(ref ant)}}right)

The ratio of the power required at the input of a loss-free reference antenna to the power supplied to the input of the given antenna to produce, in a given direction, the same field strength at the same distance. Note 1: Antenna gain is usually expressed in dB. Note 2: Unless otherwise specified, the gain refers to the direction of maximum radiation. The gain may be considered for a specified polarization. Depending on the choice of the reference antenna, a distinction is made between:

* absolute or isotropic gain (Gi), when the reference antenna is an isotropic antenna isolated in space;

* gain relative to a half-wave dipole (Gd) when the reference antenna is a half-wave dipole isolated in space and with an equatorial plane that contains the given direction;

* gain relative to a short vertical antenna (Gr), when the reference antenna is a linear conductor, much shorter than one quarter of the wavelength, normal to the surface of a perfectly conducting plane which contains the given direction. [RR] (188) Synonyms gain of an antenna, power gain of an antenna.
antenna design, gain is the logarithm of the ratio of the intensity of an antenna’s radiation pattern in the direction of strongest radiation to that of a reference antenna. If the reference antenna is an isotropic antenna, the gain is often expressed in units of dBi (decibels over isotropic). For example, a dipole antenna has a gain of 2.14 dBi [1]. Sometimes, the dipole antenna is used as the reference (since a perfect isotropic reference is impossible to produce), in which case the gain of the antenna in question is measured in dBd (decibels over dipole).

The gain of an antenna is a passive phenomenon—power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. If an antenna has a positive gain in some directions, it must have a negative gain in other directions as energy is conserved by the antenna. The gain that can be achieved by an antenna is therefore trade-off between the range of directions that must be covered and the gain of the antenna. For example, a dish antenna on a spacecraft has a very large gain, but only over a very small range of directions—it must be accurately pointed at Earth—but a broadcasting transmitter has a very small gain as it is required to radiate in all directions.

For dish-type antennas, gain is proportional to the aperture (reflective area) and surface accuracy of the dish, as well as the frequency being transmitted/received. In general, a larger aperture provides a higher gain. Also, the higher the frequency, the higher the gain, but surface inaccuracies lead to a larger degradation of gain at higher frequencies.

The antenna’s efficiency rating is the percentage of signal captured by the parabolic reflector that actually is received by the feedhorn. The feed-horn’s illumination of the outer portion of the dish is attenuated or tapered, which leads us to conclude that antenna gain is not as important a factor as it might first appear to be.

The ultimate figure of merit for all receiving antennas is the G/T (pronounced “G over T”); that is, the gain of the antenna (in dBi) minus the noise temperature of the receiving system (in kelvins, but converted to dBK before subtraction). A typical C-band system will have a G/T of around 20 dB/K, while most Ku-band digital direct to home systems have a G/T of 12.7 dB/K. The more powerful the satellite signal, the lower the G/T value that will be needed at the receiving system on the ground.

The noise value (T) primarily comes from two sources. The antenna noise is a function of the amount of noise that the feedhorn sees as it looks over the antenna rim towards the hot earth (which has a noise temperature of 290 K). Antenna noise generally ranges between 30 and 50 K.

The noise contribution of the LNB’s internal circuitry is the other major source of concern. C-band LNB performance now ranges as low as 20 K. If we add an antenna/feed noise of 40 K to LNB noise of 35 K = 75 K. Ten times the logarithm of 75 K equals a T of 18.8 dB. A typical 1.8 m diameter C-band antenna will produce a gain of 38 dB. Therefore the G/T of the system described above would be G 38 dB minus T 18.8 equals 19.2 dB/K.

This describes an antenna’s transmission power as a ratio of its output (send) signal strength to its input (receive) signal strength. It is usually expressed in dBi; a higher dBi means a stronger antenna.

Antenna Gain is defined as the ratio of the radiation intensity of an antenna in a given direction, to the intensity of the same antenna as it radiates in all directions (isotropically). Since the radiation intensity of an isotropically radiated power is equal to the power into the antenna divided by 4п (360 degrees)

Although the gain of an antenna is directly related to its directivity, it is important to note that the antenna gain is a measure that takes into account the efficiency of the antenna as well as its directional capabilities. In contrast, directivity is defined as a measure that takes into account only the directional properties of the antenna and therefore it is only influenced by the antenna pattern. However, if we assumed an ideal antenna without losses then Antenna Gain will equal directivity as the antenna efficiency factor equals 1 (100% efficiency). In practice, the gain of an antenna is always less than its directivity.
A better antenna just provides less interference by the larger antenna. Just to note, each Db is a measure of noise. The 16db antenna will help reduce ambient radio interference (microwaves, portable phones, etc) but it won’t necessarily increase your reception. The alternative is get a router that uses more wattage to transmit. 5 watts (about 100 yards) is the standard but signal repeaters and wireless access points with a more powerful transmitter will eliminate the need for a expensive antenna. 5 watts used to be the ‘standard’ but you can purchase more powerful ones now.





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