Mobile Antennas for
Cell - Cellular Modems


(1) Roof - Mounted Antennas

Ideally, the place for a mobile antenna is on the roof of your car. First, this is the highest point on the vehicle so the signals emitted from and coming into the antenna are less likely to be obstructed by nearby objects. Second, to function best, an antenna needs a ground plane, a surface that actually works as a part of the antenna system to " launch" radio waves, even through there is no electrical connection between it and the antenna.

A ground plane should have a radius equal to at least one- quarter of the wavelength of the radio wave. Since the frequencies used by cellular phones wavelengths are quite short a little under a foot-very little in the way of a ground plane is needed. The roof of a car-provided it's made a metal, of course, and not some material such as fiberglass that is transparent to radio waves-provides more than enough surface area.

Roof-mounted antennas are not intended for trunk mounting. They are designed differently, and their performance suffers if they are used in a situation for which they are not intended. This holds true for both permanent and temporary antennas.

There are several reasons, however, why most antennas are not roof mounted. The first is because it's inconvenient to lead the antenna cable into the car from an antenna mounted on top. It used to be that most cars had dome lights in their ceiling, and these provided a means for concealing the entrance of the cable (which came into the car through the bottom of the antenna, which, itself, was mounted in a hole cut in the roof). The cable could then be routed inside the ceiling liner toward the trunk or front of the car, where it had a relatively short exposed run to the radio equipment.

Today's cars, if they have dome lights at all, usually have them mounted toward the front of the roof, which means that if an antenna were mounted over them, the ground plane would be asymmetrical and probably not extend for enough forward. For an antenna to radiate an omni directional signal-one that radiates out in all directions from the antenna-it should be mounted as close to the center of the ground plane area as possible.

Further more, since a roof-mounted antenna extended at least a foot or so above the top of the car, it can easily be damage by low garage doors and low headroom passages such as the inside if a car wash.


(2) Trunk Mounted Antennas

The next best location for an antenna is on a car's trunk lid since, after the roof, it offers the largest surface area for a ground plane. Although you sacrifice a little height by mounting an antenna there rather than in the roof, trunk mounting can offer benefits that outweighs the losses.

Trunk mounting is much easier and more convenient than putting an antenna on the roof. Putting a hole in the trunk of your car is not nearly as traumatic as putting one through the roof, and it's much simpler since not as much attention has to be paid to cosmetic detail - it is also possible to mount the antenna with a clip that attaches it to the edge of the trunk and to lead the antenna cable into the trunk through the space between the trunk and the body of the car. Some ground plane efficiency is sacrificed this way, but this method is still practical.

One antenna frequency used for trunk mounting is the elevated-feed antenna, where the signal is injected into the antenna at a point above its base. This serves two purposes. First, it makes the performance of the antenna resemble that of a roof-mounted one, and, second, it eliminates the need to rely on the car trunk to provide a ground plane.

For several reasons-appearance and security among them- a cellular phone's transceiver/logic unit is frequently mounted in the trunk of a car and connected to the rest of the system by cables. This makes it very convenient to connect the transceiver to a trunk- mounted antenna, which is right about it.



(3) Through - Glass Antennas

One problem with a permanently mounted antenna of the type just described is that if you decide to remove it when you sell your car or transfer your phone, you have a hole that has to be field. Probably the least expensive solution to this problem is to leave the old antenna where it is and buy a new one for the new installation. But an increasingly popular type of antenna-and one that requires no holes to be drilled-is the  through-glass type, which is mounted on the car's rear window.

The antenna is in two parts. The antenna portion, which is attached to the outside of the window, consists of the antenna rod, base, and a mounting plate. The mounting plate is cemented to the window with a special weatherproof adhesive, and the base( which may also act as a mounting plate) contains a circuit that substitutes for the metal ground plane that would otherwise be provided by the roof or trunk lid of the car.

The other part of the antenna is glued to the inside of the window, directly opposite the outside portion. The signal to and from the antenna is coupled through the glass by capacitive action, which allows a current to develop between the two sections of the antenna system without any physical connection between them. A coaxial cable that disappears into the car's rear deck connects the inside half of the antenna to the transceiver/logic unit, which, as usual, is probably located in the trunk.

Although through-glass antennas operate without ground planes, their efficiency can be affected by the presence of anything that acts a ground plane, such as the missing elements that may be a in the presence of these elements, if might not work as well.

The inside and the outside of the through-the-glass antenna must be aligned extremely carefully. lf they are not, the efficiency of the antennas will be greatly reduced.



(4) Flex Antennas

Flex, or "rubber Duckie" antennas that look like stubby black breadsticks are frequently used with handheld and transportable phones. They are manufactured by packing a wire and a flexible core inside a protective rubber or synthetic coating.

Flex antennas are not particularly efficient, but their small size and unobtrusiveness makes them popular with units that are to be carried. lf you are using a transportable phone with a flex antenna in a marginal signal situation, try placing the phone with the antenna) upright on a large metal object such as the roof of a car. The metal will act as a ground plane and improve the antenna's transmission and reception characteristics.

(5) Magnetic-Mount Antennas 

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Magnetic-mount, or mag-mount, antennas are intended for easy installation and removal on the roof or trunk lid of a vehicle in its heavy base, a mag-mount antenna contains a strong permanent magnet, that holes the unit firmly to the metal surface. The cable is usually let into the car, or into 'the trunk of the car
if that's where the phone is located, through the doorframe or edge of the trunk lid. There is usually sufficient clearance to prevent it from being pinched, and the insulation around the door or trunk lid keeps water and wind from getting in.

If you use or store your car in an area where thefts are common, a removable mag-mount antenna can make your cellular phone installation less noticeable.


(6) Feed-lines

Radio signals are transported to and from an antenna by a type of feed-line cabled coaxial cable, or "coax" for short. This cable consists of a center conductor surrounded by an electrical insulating layer and then by a shield of (usually) very thin braided copper wire. The whole is then encased in an outer layer of material to insulate it electrically and from the elements.

The most commonly used type of coax is designed RG-58, which is usually imprinted on the outer layer of insulation. RG-58 coax is similar to the RG-59 variety used in cable TV and master antenna installation.

Although this type of cable is thin (about one-quarter inch in diameter) and flexible, making it well suited for use in mobile installations, it has one major short coming. Like all coaxial cables using a braided shield, it is lossy at high frequencies that is, the higher the frequency of operation (and the 800 MHz range of cellular phones is considered pretty high) the more radio frequency energy leaks out, meaning that less of it gets to ( or from) the antenna . In long cable runs this loss can become significant. Antenna coax is usually terminated in a plug-in connector with a screw on shell to hold it in its shock. While most cellular phones and antennas use the same type of connector, if you do your own cellular installation make sure that the antenna connector on the transceiver and plug on the end of the antenna cable are compatible.


(7) The Phasing Coil

The most visible characteristic of many cellular antennas, and one which sets them apart from most other type, is the little "pigtail" squiggle partway up its length. This is more than just decoration. It's called a phasing coil and serves to divide what looks like a single antenna into two antennas, one above it and the other below. Such an arrangement increasing the efficiency of the antenna by about 50 percent.

The purpose of the phasing coil is to maintain the proper phase relationship of the radio waves so that each half of the antenna gets the portion of the signal intended for it.


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