ANTENNA BASIC CONCEPTS
λ = c
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 in.
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.
transmission line and the antenna.
VSWR AND REFLECTED POWER
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.
dB = 10 log Power A
“A´ might be the power applied to the connector on an antenna, the input terminal of an amplifier or one end of a transmission line. “B´ might be the power arriving at the opposite end of the transmission line, the amplifier output or the peak power in the main lobe of radiated energy from an antenna. If “A´ is larger than “B´, the result will be a positive number or gain. If “A´ is smaller than “B´, the result will be a negative number or loss.
At 1700 MHz, one fourth of the power applied to one end of a coax cable arrives at the other end. What is the cable loss in dB?
In the above case, taking the log of 1/4 (0.25) automatically results in a minus sign, which signifies negative gain or loss.
It is convenient to remember these simple dB values which are handy when approximating gain and loss:
Power Gain Power Loss
3 dB = 2X power – 3 dB = 1/2 power
6 dB = 4X power – 6 dB = 1/4 power
10 dB = 10X power -10 dB = 1/10 power
20 dB = 100X power -20 dB = 1/100 power
In the case of antennas, passive structures cannot generate power. dB is used to describe the ability of these structures to focus energy in a part of space.
DIRECTIVITY AND GAIN
G = η • D
For example an antenna with 3 dB of directivity and 50% of efficiency will have a gain of 0 dB.
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
ANTENNA PATTERN TYPES
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 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.
For example, for a 0 dB gain antenna, 3 db beamwidth is the area where the gain is higher than –3 dB.
NEAR-FIELD AND FAR-FIELD PATTERNS
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:
With linear polarization the electric field vector stays in the same plane. In circular polarization the electric field vector appears to be rotating with circular motion about the direction of propagation, making one full turn for each RF cycle. The rotation may be right-hand or left-hand.
Choice of polarization is one of the design choices available to the RF system designer. For example, low frequency (<1 MHz) vertically polarized radio waves propagate much more successfully near the earth than horizontally polarized radio waves because horizontally polarized waves will be cancelled out by reflections from the earth. Mobile radio system waves generally are vertically polarized. TV broadcasting has adopted horizontal polarization as a standard. This choice was made to maximize signal-to-noise ratios. At frequencies above 1 GHz, there is little basis for a choice of horizontal or vertical polarization, although in specific applications there may be some possible advantage in one or the other. Circular polarization has also been found to be of advantage in satellite applications such as GPS. Circular polarization can also be used to reduce multipath.
DETERMINING WHIP LENGTH
DETERMINING GROUND PLANE SIZE
– Electrical tilt: The maximum energy is not radiated in the expected direction.
– Beamwidth Increased: The aperture of the radiating element is modified, and the gain of the antenna is
In conclusion, we could say the bigger the ground plane, the better the control of the electrical performance of the antenna.
BASIC ANTENNA TYPES
A single radiating element approximately 1/4 wavelength long. Directivity 2.2 dBi, 0 dBd.
LOADED 1/4 WAVE
The loaded 1/4 wave antenna looks electrically like a 1/4 wave antenna but the loading allows the antenna to be physically smaller than a 1/4 wave antenna. Quite often this is implemented by placing a loading coil at the base of the antenna. Gain depends upon the amount of loading used. Directivity 2.2 dBi, 0 dBd.
A single radiating element 1/2 wavelength long. Directivity 3.8 dBi, 1.6 dBd. A special design is the end fed 1/2 wave.
A single radiating element 5/8 wavelength long. Directivity 5.2 dBi, 3.0 dBd.
Two or three radiating elements separated by phasing coils for increased gain. Four common styles are:
1) 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.
2) 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.
3) 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.
4) 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.
The vertical portion of the antenna assembly acting as the radiator of the radio frequency
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.
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 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).
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.
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.
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
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