GSM Antenna Basics
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Antenna Basics An antenna is a device which converts an electric
wave guided by a conductor into a free-space, unguided electromagnetic wave, and vice versa. Isotropic Antenna (dBi) is a theoretical antenna radiating energy equally in all direction of space. It is typically used as a reference standard. The halfwave dipole (dBd) is an antenna which is center fed as to have equal current distribution in both halves.
The requirements on the Antennas Strictly defined radiation patterns for a most
accurate network planning. Growing concern for the level of intermodulation due to the radiation of many HF-carriers via one antenna. Dual polarization Electrical down-tilting of the vertical diagram. Unobtrusive design.
Factors affecting performance An antennas radiation pattern and gain are
perhaps the two antenna characteristics that most affect system coverage and performance. Radiation pattern simply describes how an antenna focuses or directs the energy it radiates or receives. The antenna does not act as an absolute power amplifier, rather, it acts as a directional amplifier, transmitting or receiving energy in one specific region of space more so than others.
Antenna Gain : This is often referred to as "power gain" and is the ratio of the maximum radiation in a given direction to that of a reference antenna in the same direction for equal power input. Usually this gain is referenced to either an isotropic antenna or a half wave dipole in free space at 0 degrees elevation. When used as a gain reference, the isotropic antenna has a power of 0 dBi. The halfwave dipole (dBd) when used as a theoretical reference antenna it has a power gain of 0 dBd, which equates to a 2.14 dB difference compared to an Isotropic antenna. Maximum Antenna Gain = 10 log[41,253 / (UA . UB)] dB Where , UA is the half-power beamwidth in the azimuth plane and UB is the half-power beamwidth in the elevation plane. There is an inverse relationship between antenna gain and antenna beamwidth. Additionally, there is a proportional relationship between antenna gain and the antenna size.
dBd Vs. dBi dBi = dBd + 2.14
dBd = dBi - 2.14
Antenna Gain VS . . . Vertical Beamwidth: Greater the gain of the antenna, the
narrower the vertical beamwidth.
Physical Size: The size of an antenna will generally be
greater as an antenna gain increases. This is due to the greater number of dipole array and electrical elements required to reach the desired gain.
Height of Antenna: Doubling the antenna height causes a
gain increase of 6 dB. But height increase also increases the transmission line losses & possible creation of nulls near the site.
Other Important Parameters Antenna Beamwidth :Antenna beamwidth is measured in
degrees between the half power points (3 dB) of the major lobe of the antenna, Beamwidth can be expressed in terms of azimuth (horizontal or H-plane) and elevation (vertical or E-plane). Power Rating :The Power Rating of an antenna is the
maximum power, normally expressed in Watts that the antenna will pass without degraded performance. Typical values for the power rating of an antenna are between 300 and 500 Watts.
Front to Back Ratio: The front to back ratio of an antenna is an important measure of performance. It is the ratio of the power radiated from the main ray beam forward to that radiated from the back lobe behind the antenna. Front to back ratio is normally expressed in terms of dB. The front to back ratio for a typical GSM antenna should be in the region of 25 dB. Side Lobes & Back Lobes: Side and Back lobes are those undesirable directions where the chosen "directional" antenna may present gain.
Antenna Diversity Antenna(Spatial) diversity is implemented through the use
of two receive antennas at the base station. Receive antenna diversity is employed at the base site to
improve the uplink by approximately 3 to 5 dB. Horizontal Antenna Diversity Vertical Antenna Diversity Polarization Diversity
Recommended Separation for Horizontal Antenna Diversity The recommendation for standard cellular implementation has generally been
accepted as 10 times the frequency wavelength (lambda).
For example, If the Frequency is 1,850 MHz then the Wavelength is 16 cms. Therefore, Diversity Rx antenna Separation = (Wavelength x 10)=1.6 m (5.3 ft.) For greater accuracy we may use the Lees equation.
Lees Equation: d = 77.27*h/f Where d = Rx antenna separation, h = Rx antenna height (ft.), f = frequency (MHz) Example (1,850 MHz @ 100 ft.) d = 77.27*100/1,850 .Therefore d = 4.2 ft.
Vertical Antenna Diversity is not preferred . . . The vertical separation of antennas provides poor
diversity performance. This is due to a higher degree of correlation for a given distance compared to horizontal separation. In other words, the vertical separation distance required between two base site antennas is much larger than the horizontal separation required to gain the same correlation coefficient of two received branches. a base site is horizontal diversity.
The preferred method of implementing diversity at
Polarization Diversity A mobile telephone is never held exactly upright which
means that all polarizations between vertical and horizontal are possible. Furthermore the reflections which take place within urban areas are not all of the same polarization, ie. Horizontal components also exist. As Space diversity uses 2 vertically polarized antennas as
reception antennas and compares the signal level. Polarization diversity uses 2 orthogonally polarized antennas and compares the resulting signals.
Some Antenna Types
1/2 Wave Dipole Yagi Horn Leaky Coax Helices Yagi- Uda Frequency Independent Log-Periodic Loops Slot Antennas Printed Circuit Antennas Antenna Arrays