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ANTENNA
An antenna is a device to transmit and/or receive electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned (matched) to the same frequency band as the radio system to which it is connected, otherwise reception and/or transmission will be impaired.
WAVELENGTH
We often refer to antenna size relative to wavelength. For example: a 1/2 wave dipole is approximately half a wavelength long. Wavelength is the distance a radio wave travels during one cycle. The formula for wavelength is: λ = c f Where:
λ is the wavelength expressed in units of length, typically meters, feet or inches
c is the speed of light (11,802,877,050 inches/second) f is the frequency For example: wavelength in air at 825 MHz is 11.803 X 109 in./sec^ = 14.307 inches 825 x 106 cycles/sec. Note : The physical length of a half-wave dipole is slightly less than half a wavelength due to end effect. The speed of propagation in coaxial cable is slower than in air, so the wavelength in the cable is shorter. The velocity of propagation of electromagnetic waves in coax is usually given as a percentage of free space velocity, and is different for different types of coax.
IMPEDANCE MATCHING
For efficient transfer of energy, the impedance of the radio, the antenna and the transmission line connecting the radio to the antenna must be the same. Radios typically are designed for 50 Ohms impedance, and the coaxial cables (transmission lines) used with them also have 50 Ohms impedance. Efficient antenna configurations often have an impedance other than 50 Ohms. Some sort of impedance matching circuit is then required to transform the antenna impedance to 50 Ohms. Larsen antennas come with the necessary impedance matching circuitry as part of the antenna. We use low-loss components in our matching circuits to provide the maximum transfer of energy between the transmission line and the antenna.
VSWR AND REFLECTED POWER
Voltage Standing Wave Ratio (VSWR) is an indication of the quality of the impedance match. VSWR is often abbreviated as SWR. A high VSWR is an indication the signal is reflected prior to being radiated by the antenna. VSWR and reflected power are different ways of measuring and expressing the same thing. A VSWR of 2.0:1 or less is often considered acceptable. Most commercial antennas are specified to be 1.5:1 or less over some bandwidth. Based on a 100 watt radio, a 1.5:1 VSWR equates to a forward power of 96 watts and a reflected power of 4 watts, or the reflected power is 4.2% of the forward power.
BANDWIDTH
Bandwidth can be defined in terms of radiation patterns or VSWR/reflected power. The definition used is based on VSWR. Bandwidth is often expressed in terms of percent bandwidth, because the percent bandwidth is constant relative to frequency. If bandwidth is expressed in absolute units of frequency, for example MHz, the bandwidth is then different depending upon whether the frequencies in question are near 150 MHz, 450 MHz or 825 MHz. Percentage bandwidth is defined as: FH - FL FC BW = 100 FH is the highest frequency in the band FL is the lowest frequency in the band FC = FH + FL FC is the center frequency of the band 2 Example: If you need an antenna to operate in the 150 to 156 MHz band, you need an antenna covering at least a 156 - 150 153 3.9% bandwidth. 100 = 3.9% The problem might need to be worked in a different way. If the antenna is tuned to 460 MHz and provides a VSWR bandwidth of 5%, what are FL and FH? The equations above can be solved for FL and FH:
FH = FC (1 + BW ) and FL = FC (1 - BW )
Plugging the numbers into the equations, the answers are:
FH = 460 (1 + 5 ) = 471.5 MHz
FL = 460 (1 - 5 ) = 448.5 MHz
GAIN MEASUREMENT
One method of measuring gain is to compare the antenna under test against a known standard antenna. This is known as a gain transfer technique. At lower frequencies, it is convenient to use a 1/2-wave dipole as the standard. At higher frequencies, it is common to use a calibrated gain horn as a gain standard with gain typically expressed in dBi. Another method for measuring gain is the 3-antenna method. Transmitted and received powers at the antenna terminal are measured between three arbitrary antennas at a known fixed distance. The Friis transmission formula is used to develop three equations and three unknowns. The equations are solved to find the gain expressed in dBi of all three antennas. Pulse-Larsen uses both methods for measurement of gain. The method is selected based on antenna type, frequency and customer requirement. Use the following conversion factor to convert between dBd and dBi: 0 dBd = 2.15 dBi. Example: 3.6 dBd + 2.15 dB = 5.75 dBi
RADIATION PATTERNS
Radiation or antenna pattern describes the relative strength of the radiated field in various directions from the antenna at a constant distance. The radiation pattern is a “reception pattern” as well, since it also describes the receiving properties of the antenna. The radiation pattern is three-dimensional, but it is difficult to display the three-dimensional radiation pattern in a meaningful manner. It is also time-consuming to measure a three-dimensional radiation pattern. Often radiation patterns measured are a slice of the three-dimensional pattern, resulting in a two-dimensional radiation pattern which can be displayed easily on a screen or piece of paper. These pattern measurements are presented in either a rectangular or a polar format.
ANTENNA PATTERN TYPES
Omnidirectional Antennas
For mobile, portable and some base station applications the type of antenna needed has an omnidirectional radiation pattern. Omnidirectional antennas radiate and receive equally well in all horizontal directions. The gain of an omnidirectional antenna can be increased by narrowing the beamwidth in the vertical or elevation plane. The net effect is to focus the antenna’s energy toward the horizon. Selecting the right antenna gain for the application is the subject of much analysis and investigation. Gain is achieved at the expense of beamwidth. Higher-gain antennas feature narrow beamwidths while the opposite is also true. Omnidirectional antennas with different gains are used to improve reception and transmission in certain types of terrain. A 0 dBd gain antenna radiates more energy higher in the vertical plane to reach radio communication sites located in higher places. Therefore they are more useful in mountainous and metropolitan areas with tall buildings. A 3 dBd gain antenna is a good compromise for use in suburban and general settings. A 5 dBd gain antenna radiates more energy toward the horizon compared to the 0 and 3 dBd antennas. This allows the signal to reach radio communication sites further apart and less obstructed. Therefore they are best used in deserts, plains, flatlands and open farm areas.
Directional Antennas
Directional antennas focus energy in a particular direction. Directional antennas are used in some base station applications where coverage over a sector by separate antennas is desired. Point-to-point links also benefit from directional antennas. Yagi and panel antennas are directional antennas.
BEAMWIDTH
Beamwidth describes the angular aperture where the most important part of the power is radiated. In general, we talk about the 3 db beamwidth which represents the aperture (in degrees) where more than 90% of the energy is radiated. For example, for a 0 dB gain antenna, 3 db beamwidth is the area where the gain is higher than –3 dB.
Value of θ in degree = 3 dB beamwidth.
NEAR-FIELD AND FAR-FIELD PATTERNS
The radiation pattern in the region close to the antenna is not exactly the same as the pattern at large distances. The term “near-field” refers to the field pattern existing close to the antenna. The term “far-field” refers to the field pattern at large distances. The far-field is also called the radiation field, and is what is most commonly of interest. The near- field is called the induction field (although it also has a radiation component). Ordinarily, it is the radiated power that is of interest so antenna patterns are usually measured in the far-field region. For pattern measurement, it is important to choose a distance sufficiently large to be in the far-field, well out of the near-field. The minimum permissible distance depends on the dimensions of the antenna in relation to the wavelength. The accepted formula for this distance is: rmin = 2D^2
Where: rmin is the minimum distance from the antenna D is the largest dimension of the antenna
λ is the wavelength
θ
Collinear
Two or three radiating elements separated by phasing coils for increased gain. Four common styles are:
- 5/8 over 1/4: The top element is 5/8 wave and the bottom element is 1/4 wave. Directivity 5.4 dBi, 3.2 dBd.
- 5/8 over 1/2: The top element is 5/8 wave and the bottom is 1/2 wave. Directivity 5.6 dBi, 3.4 dBd.
- 5/8 over 5/8 over 1/4: The top 2 elements are 5/8 wave and the bottom element is 1/4 wave. Directivity 7.2 dBi, 5.0 dBd.
- 5/8 over 5/8 over 1/2: The top 2 elements are 5/8 wave and the bottom element is 1/2 wave. Directivity 7.6 dBi, 5.4 dBd. Using more than three radiating elements in a base-fed collinear configuration does not significantly increase gain. The majority of the energy is radiated by the elements close to the feed point of the collinear antenna so there is only a small amount of energy left to be radiated by the elements which are farther away from the feed point. Please note the directivity is given above for common antenna configurations. Gain depends upon the electrical efficiency of the antenna. Here is where the real difference between antenna manufacturers is seen. If you cut corners in building an antenna, the gain may be significantly lower than the directivity. Larsen uses low-loss materials to minimize the difference between the gain and the directivity in our antennas.
Whip
The vertical portion of the antenna assembly acting as the radiator of the radio frequency
GPS
Active GPS antennas include an amplifier circuit in order to provide better reception of the satellite signal. This active stage generally includes a low noise amplifier and a power amplifier. Combi GPS/Cellular structures include several antennas in one radome to allow reception and transmission in different frequency bands.
Dipole
An antenna - usually 1/2 wavelength long - split at the exact center for connection to a feed line. Dipoles are the most common wire antenna. Length is equal to 1/2 of the wavelength for the frequency of operation. Fed by coaxial cable. Sleeve Dipoles are realized by the addition of a metallic tube on a coaxial structure. Printed Dipoles have a radiation structure supported by a printed circuit.
Embedded Omni
Embedded omni antennas are generally integrated on a base for applications such as access points. This structure could be externally mounted (ex: sleeve dipole) or directly integrated on the PC board of the system (ex: printed dipole).
Yagi
A directional, gain antenna utilizing one or more parasitic elements. A yagi consists of a boom supporting a series of elements which are typically aluminum rods.
Panel
Single Patch describes an elementary source obtained by means of a metallic strip printed on a microwave substrate. These antennas are included in the radiating slot category. Patch Arrays are a combination of several elementary patches. By adjusting the phase and magnitude of the power provided to each element, numerous forms of beamwidth (electric tilt, sectoral, directional.. .) can be obtained. Sectoral antennas can be depicted like a directive antenna with a beamwidth greater than 45°. A 1 dB beamwidth is generally defined for this kind of radiating structure.
Omni Ceiling Mount
Omni ceiling mount antennas are used for the propagation of data in an in-building environment. In order to provide good coverage, these antennas are vertically polarized and present an omnidirectional pattern in the horizontal plane and a dipolar pattern in the vertical plane.
Parabolic
An antenna consisting of a parabolic reflector and a radiating or receiving element at or near its focus. Solid Parabolics utilize a dish-like reflector to focus radio energy of a specific range of frequencies on a tuned element. Grid Parabolics employ an open-frame grid as a reflector, rather than a solid one. The grid spacing is sufficiently small to ensure waves of the desired frequency cannot pass through, and are hence reflected back toward the driven element.
PULSE-LARSEN ANTENNA TYPES
Mobile: Collinear, Whip, Low Profile, Active GPS, Combi GPS/Cellular Portable: Whip, Helical, End Fed Half Wave, Sleeve, Half Wave Dipole, Embedded Omni, Printed Dipole Base Station: Whip, Collinear, Yagi, Panel, In-building Sectoral, Omni-ceiling Mount
MOBILE ANTENNA PLACEMENT
Correct antenna placement is critical to the performance of an antenna. An antenna mounted on the roof of a car will function better than the same antenna installed on the hood or trunk. Knowledge of the vehicle may also be an important factor in determining what type of antenna to use. Do not install a glass mount antenna on the rear window of a vehicle in which metal has been used to reduce ultraviolet light. The metal tinting will work as a shield and not allow signals to pass through the glass. Magnetic Mount Trunk Corner -3.4 dB ● ● ● ● ●
Magnetic Mount Corner of Hood -2.4 dB Magnetic Mount Corner of Roof -0.02 dB Magnetic Mount Center of Roof -0.02 dB (Reference Antenna) Permanent Mount Center of Roof 0.0 dB Trunk Lip Mount Trunk Center -2.8 dB On Glass (Lower) Center -3.0 dB Permanent Trunk Mount -2.8 dB Magnetic Mount Trunk Center -2.1 dB On Glass (Middle) Center -1.2 dB On Glass (Upper) Center -0.5 dB
