Antenna Systems

The Basics

The antenna gain is a measure of how well the antenna will send or pick up a radio signal. This "gain" is measured in decibels-isotropic (dBi) or in decibel-dipole (dBd). The decibel is a unit of comparison to a reference. The letter following the "dB" indicates the reference used. The dBi is a unit measuring how much better the antenna is compared to an isotropic radiator. An isotropic radiator is an antenna which sends signals equally in all directions (including up and down). An antenna which does this has an 0dBi gain. The higher the decibel figure the higher the gain. For instance, a 6dBi gain antenna will receive a signal better than a 3dBi antenna. A dBd unit is a measurement of how much better an antenna performs against a dipole antenna. As a result a dipole antenna has a 0dBd gain. However a dipole antenna typically has a 2.4dBi gain as dipole antenna are better than isotropic radiators. Any dBi figure may be converted to dBd by adding 2.4. The radio antenna industry tends to use dBi to specify gain of whip antenna and dBd for all others. However this is not a firm rule and care must be taken when reading manufacturer specifications which give gain in "dB" without specifying the reference as either a dipole or isotropic radiator. All antenna have identical transmit and receive characteristics. That is, if it is a poor transmitter it will be equally poor at receiving.

 

The radiation pattern of an antenna has a near field and a far field. The near field is composed of electric and magnetic force fields. This holds true to about 4 wavelengths from the radiator, and then the predominate radiation is the electromagnetic field.

There are basically two categories of antenna, omnidirectional and unidirectional. An omnidirectional antenna is one which radiates in all directions. A unidirectional antenna radiates in one direction only. An omnidirectional antenna should not be confused with an isotropic antenna for while an isotropic antenna will radiate in all 3 dimensional directions an omnidirectional antenna may not radiate vertically (up or down).

 Directionality is closely related to antenna gain. Antenna obtain more gain by concentrating their energies in a direction. Omnidirectional antenna (non-directional) try to concentrate their energies into the horizontal plane. Radio energy sent up or down is usually a waste. These antenna increase their gain by sending (or concentrating) this energy out horizontally rather than vertically. The tighter the concentration into the horizontal the higher the gain.

The antenna has a larger capture area than you would assume by the cross section alone. The capture area in square meters is given in the formula below compared to an isotropic radiator.

Adding multiple antenna to one transceiver requires the use of special splitter units. The energies to and from the radio are split between the antenna. This means that adding such antenna will reduce the effective gain of each individual antenna. The polarization of an antenna is very important and in many countries is specified by the governing authority on the license issued. Most antenna may be installed horizontally or vertically.

Generally an antenna which has its elements vertical is vertically polarized. A good example are car radio antenna which are vertical. If the elements are horizontal the antenna is horizontally polarized. It is important that all antenna in a system be either horizontal or vertical. Attempting to communicate between a station with a horizontal antenna and one with a vertical antenna will result in a weak signal.

Generally all systems which require the central site to communicate in all directions use vertical polarization. Most governments prefer that fixed point systems use horizontal polarization in order to provide isolation from more common mobile (car) systems which are vertically polarized. Often government regulations may limit the power of the radio transmitter as well as the effective radiated power. The effective radiated power is the power sent out by the antenna in the direction of maximum gain. As the antenna concentrates power in one direction this has the same effect of increasing the transmitter power, i.e. effective power. As a result it may not be possible to utilize antenna over a certain gain as this could result in excessive effective radiated power. Selecting an antenna may be a complex task in attempting to balance the need to communicate to several stations in different directions without tall masts and within government power restrictions.

When selecting an antenna the following needs to be considered:

As a general guide the following needs to be considered when installing an antenna:

Many different types of whip antenna are available and these are discussed below:

Whip (Vertical) Antenna

Generally whip antenna appear as a small vertical rod. Whip antenna are commonly used in automotive applications and are generally not recommended for fixed point telemetry systems. They are used in applications that do not require high gain, where lightning risk is not high, and low cost is required. Whip antenna usually require mounting at the highest point of a mast and are therefore the most likely item to be struck by lightning. They are also not generally tolerant of metallic objects alongside. Whip antennas may only be used for vertically polarized systems. There are many types of whip antenna and each have specific mounting and tuning requirements. Generally these antenna require on-site tuning. Some must be mounted on a metallic surface to provide a "ground plane", others do not. Consult the manufacturer's data for further information.

 

Directional Effects of Vertical Height

As the height of whip (vertical) antenna vary, the radiation pattern is modified. Generally more height yields more vertical range, up to , then the pattern becomes more horizontal. See the table below.

 

Dipole Antenna

The dipole antenna appears as a metal rectangle on the end of a short mounting beam. The loop is the antenna itself often referred to as the driven element. It is probably the basic antenna for industrial fixed point communications and is mounted off the side of a structure. Its electrical construction and that it may be mounted well down a mast means that it has good lightning avoidance characteristics. It is also physically robust.

Dipole antenna are omnidirectional (non-directional) when vertically polarized and have a relatively low gain. They are not commonly used in horizontally polarized systems as the higher gain yagi antenna are of similar cost. For applications in corrosive environments (such as near the ocean) stainless steel dipole antenna are available and should be used.

The distance from the dipole to the structure is important. If mounted for vertical polarization ensure that the drain hole (if present) on one end of the dipole is at the bottom. If mounting for horizontal polarization it is normal for the insulated portion of the dipole to be the highest point for proper moisture drainage. This may vary dependent on method of manufacture. Failure to do so may result in eventual damage to the antenna.

Usually the length is cut to and the antenna current and voltage distribution are antiphase to each other. The physical length is made 0.48 wavelengths due to the end effect. Recall the operation of a water hose. If you let the water flow from the hose orifice, the water flows in a stream slightly past the edge of the outlet. In an antenna the same thing happens to the current, it flows just past the ends of the antenna but is approaching zero. The voltage distribution has it's highest value just past the ends of the antenna.

 

Yagi Antenna

 

A yagi antenna is essentially a dipole with directors and reflectors added before and behind the dipole. These reflectors and directors are simply metal rods of similar size to the dipole. They concentrate the energies into a "beam" in order to increase the gain. There is usually only one reflector and one or more directors. A yagi antenna is rated by its total number of elements. A yagi antenna may be considered as a directional form of a dipole antenna and the comments on the dipole antenna are applicable.

If mounted for vertical polarization, ensure that the condensation drain hole (if present) on one end of the dipole is at the bottom. Failure to do so may result in eventual damage to the antenna. If mounted for horizontal polarization, ensure that the antenna feeder cable entry to the dipole element is at the bottom to allow condensation to drain.

Large yagis are best center mounted rather than end mounted. Mounting a vertically polarized antenna centrally is not recommended as the mast is also vertical and will affect the antenna performance. If end mounting a large antenna it is best to use a bracket recommended by the manufacturer to avoid possible degradation of the antenna's characteristics.

Miscellaneous

Turnstile antennas take two dipoles and mount them 90 degrees to each other. It is a poor attempt to get an omnidirectional antenna. Many hobby electronics stores sell these for FM antennas, and they only work because the FM signal in the city areas is usually quite strong. The full wave loop is similarly used as a cheep antenna for TV, but it can be a good antenna. Direction finders many times use the loop antenna. The folded dipole has more gain than it's single element brother. It is also a full wavelength antenna folded up. The helical antenna is like a wound spring coil and may be clockwise or counterclockwise polarized. They can be good for general communications since they have both horizontal and vertical components. Space telemetry uses helical array antennas many times. Parabolic and horn antennas are used to obtain high gain and directionality.

 

Antenna Chart

Name

Shape

Gain (over isotropic)

Beamwidth -3 dB

Radiation Pattern

Isotropic

0 dB

360

Dipole

2.14 dB

55

Folded Dipole

Cylindrical

 

5.64 dB

45

Turnstile

-0.86 dB

50

due to cusps

 

Full wave loop

3.14 dB

200

Yagi

7.14 dB

25

Helical

10.1 dB

30

Parabolic Dipole

 

14.7 dB

20

Horn

15 dB

15

Biconical Horn

14 dB

360x200

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© Copyright 1998 Dennis J. Ramsey
Updated 1998.12.12