Antenna gain

The antenna gain directivity and combines the efficiency of an antenna together. It is the ratio of the votes in the main direction radiation power density, compared with a loss-free reference antenna same supply power, which by definition has a gain of 0 dB. The reference antenna is a dipole antenna or a hypothetical isotropic radiator is usually chosen with equal strength in all directions beam.

High performance antennas are always strong direction sensitive, in other than the principal direction of the radiation or reception is strongly suppressed. With them you can reach more distant transmitter or receiver. Typical directional antennas are the parabolic antenna, colloquially referred to as " satellite dish " and the Yagi -Uda antenna, as used for example for terrestrial television.

Calculation

The antenna gain G, the product of the directivity factor D, and the efficiency of the antenna:

With

• Directivity
• Efficiency

Strictly speaking, each must be made ​​between sending and receiving:

Since the efficiencies for sending and receiving are defined differently (see below).

Often, however, both efficiencies are set equal to 1 in the first approximation:

From which it follows:

In the profit losses of the connecting cable and the contact to the connector of the antenna are not included.

Directivity

The directivity D of an antenna represents the ratio of the square of the maximum produced by their electric field strength Emax in the main beam direction (or equivalent of the magnetic field strength Hmax ) to the square of the field strength Ek of an isotropic radiator adopted in the far field, with the same power supplied and the same distance:

Sk is the radiation density of an isotropic radiator at the same distance. It is equal to the square of the field strengths produced because it is a far field.

Antenna efficiency

The antenna efficiency η denotes the electrical losses of the antenna, for example, by ohmic line resistance in the antenna.

Since the current distribution in the antenna in the transmission case is different in the case of reception (which can be seen from the fact that the near field of a receiving antenna differs from the near field of a transmitting antenna ), must be distinguished in efficiency between transmit and receive mode:

• Ps0: electric power supplied ( = radiation power in main, secondary and back lobe )

• Receiving: Pe0: to the consumer electric power output
• Pe: the electromagnetic radiation field taken from electrical power; this is determined from the antenna effective area, which is proportional to the gain and to the square of the wavelength of the electromagnetic field.

Application and Benefits

• Since both the transmitting antenna and the receiving antenna can be directed to the transmitting antenna combining their radiation power in the direction of the receiving antenna, the range of a radio link can be considerably increased.
• Alternatively can be saved while maintaining the same range of transmission power, since the desired spatial direction is illuminated with higher efficiency.
• The simultaneous reduction of the opening angle stations are off the desired direction less disturbed.
• This means that a frequency of several radio links can be used as long as they are not located in the same path.
• Since the received power depends on the correct alignment of the receiver antenna, the direction in which the transmitter is located, identified, so this will be targeted. This will, for example, applied for the detection of tracking devices or navigation using non-directional radio beacons.

Unit

The antenna gain is usually specified in the auxiliary decibels (dB). Because dB is a relative ( logarithmic ) amount with respect to a reference antenna, it will be calculated on the basis of the reference antenna:

The reference antenna must be specified:

• Usually, the antenna gain is given in relation to the isotropic radiator, then to write a unit dBi ( isotropic).
• While a value of in terms of an antenna of the type λ/2-Dipol to write dBd (dipole ). For example, the antenna gain of 0 dBd is λ/2-Dipols (because self-referential and log10 (1) = 0) and about 2.15 dBi.

Antenna design and profit

An antenna with a higher gain will inevitably lead to a reduction in its half-width, as " focused " on a narrower range of the available energy is thus only redistributed. Following approximation illustrates this relationship.

Another approximation delivers a statement on the profit by the relationship between antenna size and wavelength. Applicable this is eg parabolic antennas, but not with Yagi antennas.

The following table shows the antenna gain is specified for some antennas: