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Selecting an Antenna

Click:373Date:2013-10-22Information Source:

Virtual Technologies' telemetry and communications products operate in the cellular telephone band (835 MHz-880 MHz).  Virtual Technologies offers a wide variety of antennas for these applications.  All antennas that we offer can be classified into the following basic groups: quarter wave antenna, half-wave antenna, co-linear antenna, Yagi antenna, and patch antenna.

Each antenna type is detailed below.  Each of the sections discusses the attributes, strengths and weaknesses of each antenna type.  Along with information about specific antennas, keep the following points in mind as you choose an antenna:

1. Antenna's fields are distorted by nearby objects (particularly metal ones). Avoid placing the antenna near a distorting object, to insure proper performance.

2. Because cell site antennas are vertically polarized, antennas that are mounted vertically will perform better than ones that are mounted horizontally.

3. Keep the antenna as far away as practical from sensitive circuitry such as A/D converters, clock signal traces, and other noise generators. Otherwise you may find that the signals couple onto the antenna coaxial cable or CRFM board. If this happens, these signals will be conducted into the receiver, making it less sensitive to the desired control channel signals.

4. Use of connectors and interconnecting cable should be minimized.  Each of these items has some RF loss associated with it. For cable runs longer than 5 feet, we recommend Belden RF cable type 9913 or better.  Loss in the transmit chain allows less of the transmitter's power to get to the antenna to be radiated.  Loss in the receive chain causes less of the received power to get from the antenna to the receiver circuitry for decoding.  In fringe areas these additional tenths of dB may mean the difference between working and not working.

5. Do not paint the hole through which your antenna connects. Paint near the antenna hole or antenna connector hole will prevent good contact between the connector and the grounded case.

Quarter-Wave Antenna
The quarter wave antenna has an omnidirectional pattern; it radiates equally well in all directions.  This type of radiation pattern is preferred in mobile applications as the bearing from the device to the cellular tower is unknown or randomly changing.  The quarter-wave antenna may also be used in fixed applications having a good ground plane and adequate signal strength.

Of the different types of antennas, the quarter-wave has the least amount of gain.  The quarter-wave does nothing to boost the received signal and should be used where the signal strength is sufficiently strong.

The quarter-wave requires a ground plane to work properly.  A ground plane is a flat surface designed to limit the downward radiation of the antenna. Generally a large grounded piece of metal is used. The ground plane should extend a minimum of one wavelength (but preferably more) in each direction from the antenna location. The wavelength at 850 MHz is over twelve inches.

If a quarter-wave antenna is used without a sufficient ground plane, two things will happen:

a. Radiated power will tend to go out equally in all directions. Up and down as well as towards the horizon.  Because there are no radio towers up and down, energy is wasted which would be better used transmitting toward the towers. 

b. An improperly installed quarter-wave antenna does not establish a good reference plane for the radiating field.  When this happens, the antenna is said to be mismatched and the transmitter will see a high SWR (Standing Wave Ratio).  Much of the power that the transmitter generates will then bounce back into the transmitter and be dissipated as heat.  This reduces the signal that the cellular tower receives and may cause improper decoding in fringe areas, along with spurious emissions.

Half-Wave Antenna
The 0dB half-wave antenna is omnidirectional and is well suited to both fixed and mobile applications. It usually provides the same gain as the quarter-wave, and can be used in installations that won't support use of a quarter-wave antenna (i.e. an installation without a ground plane).  Though it has similar gain properties to a quarter-wave, but it does not require a large metal ground plane.

As it operates without a ground plane, the half-wave antenna's impact on installation is minimal, but it is taller (more than twice as tall, exceeding 9") than a quarter-wave antenna cut for the same frequency. As the antenna is omnidirectional, device placement during installation is straightforward.

The half-wave antenna with gain becomes increasingly bi-directional as gain increases.  It is best suited to fixed applications.  It can provide much more gain (typically +7dB) than the quarter-wave, and can be used in installations that won't support use of a quarter-wave antenna.  Like the 0db half wave, it does not require a large metal ground plane.

The half-wave antenna is slightly more costly than the quarter-wave but, the elimination of the quarter-wave's large ground plane offsets this cost.

Co-Linear Antenna
The co-linear antenna is omnidirectional and is well suited to fixed and mobile applications.  It provides higher gain than either the 0dB half-wave or the quarter-wave, and can be used in areas that won't support the use of either of those antennas.  It does this by stacking half-wave antennas on top of one another.  Depending on how many half-wave sections are internally combined, additional gain can be realized.  Typical gains are on the order of 3.0 - 4.5 dB. This is equivalent to increasing the transmitter power from its nominal 3 Watts to 6 or up to as much as 8.5 Watts (equivalent power radiated by a quarter-wave). This radiated energy is concentrated in an omnidirectional pattern, along the horizontal plane, and is more focused towards the horizon.

This type of antenna would most likely be installed in a far fringe area, where no clear indication of the nearest cell site direction was available, or in a fringe mobile application.

Yagi Antenna- Ideal for Fixed Remote Sites
The Yagi antenna is a highly directional antenna consisting of an array of single-wire dipole antennas and a series of reflectors.  It looks much like a UHF television antenna (though the pegs or pins are oriented vertically instead of horizontally).

It must be aimed at a cellular site, and for this reason it is only appropriate for fixed installations.  Of the four types of antennas discussed above, the Yagi has the highest gain (+11dB or greater).  It is the ideal antenna for fixed installation in very remote areas where the direction of the nearest cell tower is known.

Its proper installation requires the use of a signal strength indicator (available on all Virtual Technologies Virtu-Well and Water Watcher products).  External measurement equipment may also be used.  The installer must aim the antenna at the time of installation.  If a new cell site is installed by the cellular carrier, the installer may have to revisit the site to reorient the antenna to take advantage of the stronger, closer tower.

Patch Antenna
A patch antenna is generally flat or has very little thickness.  It has a directional characteristic and tends to radiate (or receive) best in the direction normal to the large flat surface.  You might choose a patch when you want to hide the antenna.  Depending on the size, they can have a varying amount of gain, from about the same as a dipole to as much or more than a Yagi.  Because of their directional nature, they are most effective in a fixed installation, though a low-gain patch pointed up may suffice in very strong signal areas. Unlike the whip type antennas mentioned above, it has an inherent built-in Radome that protects it from curious fingers.

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