Chapter 3

How Antennas Work: Their Radiation Patterns, Their Affect on Range and their Limitations

External Antenna vs Phone antenna:

External antennas for cellular phones are usually beneficial for two reasons. Firstly, a well designed external antenna is usually a better radiator than the phone’s own antenna. Phone antennas have limitations because of size limitations and they must comply with federal electromagnetic radiation exposure requirements; as such there is a limit to the amount of power a phone/antenna can transmit.

Secondly, an external antenna benefits by usually being in a better RF environment than the phone’s antenna. Cellular signal on the outside of a vehicle or structure is better than on the inside of the vehicle or structure.

Thirdly, all radio frequency signals benefit when the antenna is placed on a metal surface. An antenna’s signal benefits when it is placed on a metal surface that has a diameter greater than one wavelength (of the desired frequency). This effect is called “groundplane” and the beneficial effect increases with the increase of the area of the groundplane. An antenna can be designed to have its own groundplane if a metal surface is not available to place it on.

Benefit from RF Environment (antenna location):

Cellular signal is always better on the outside of a vehicle than it is on the inside a vehicle. The signal that the phone transmits and receives is attenuated (made quieter) by the sheet metal and steel frame of the vehicle. RF (radio frequency) signal is electromagnetic energy that is measured in watts and usually expressed in db (decibels). Measured cellular signal is usually 8 to 10 db better outside of a vehicle than it is when measured inside of a vehicle. In relative terms, this means that the signal on the outside of the vehicle is about 20 % better than it is on the inside. In absolute terms, this means that when the signal is low, a call from inside the vehicle will fail before a call from outside of the vehicle.

Factors Effecting Antennas:

Gain: Type of Antenna and Radiation Pattern:

In absolute terms there is no such thing as one antenna that is better than another because antennas do not create energy, however, antennas are most often rated in “gain” and as such, most people believe that a gain antenna is better or more powerful than another antenna with a lower number, for example a 3db gain antenna versus a 0db gain antenna.

In reality, there is no energy increase with a gain antenna. An antenna without gain (0db) radiates energy in all directions and antenna with gain re-directs or concentrates energy in a certain direction. In very simplistic terms it can be said that a low gain cellular antenna will perform better in geographic areas where signal is bouncing, reflecting or is generally located vertically from the antenna mast (cities, mountains, areas with obstructions). A vertically mounted high gain antenna will generally concentrate energy parallel to the earth and will generally perform better when the tower is unobstructed and located on the horizon (oceans, lakes, deserts.)

It is advisable for people who travel to remote areas of poor cellular signal to have both types of antennas because of this phenomenon.

Factors That Effect Antenna Performance:

Cables and Connectors :

Regardless of the gain rating of the antenna there are several factors that are critical to signal improvement when an external antenna is connected to a cellular phone. The most important component effecting antenna performance is the co-axial cable that connects the antenna to the phone. Co-ax cables and the connectors that are used to connect them have losses commonly termed “line loss” (cable) and insertion loss (connector).

All cables and connectors have loss. The amount of loss is measured in db and cable loss increases when cable length is increased. Losses from cables and connectors also increase when the frequency of the signal increases. For example: a 15 foot long cable that has 4 db of loss in the 800 mhz frequency of the cellular band will have a loss of 8db or more in the 1900 mhz frequency of the PCS band. If the cable is of low quality, it is possible to have a small improvement when the phone is in the lower frequency cellular band (usually in rural areas) and have a substantial degradation in signal when in the high frequency PCS band (usually in urban areas).

Note: most RG58 size cellular antenna cables with a connector at each end have losses of 3.8 dB loss at 850 Mhz and 7.4 db loss at 1900 Mhz or greater for every 15 ft. of cable. Smoothtalker RG58 cable has 1.8 dB loss at 850 Mhz and 3.25 dB loss at 1900 Mhz.

There are several popular locations to mount cellular antennas. The best location is the center of a vehicle roof where it is unobstructed and has a large metal surface known as a groundplane under it.

A second choice would be a mirror mount or fender mount using a “thru-hole” type antenna that passes through a metal mounting bracket. When this type of mounting is used it is recommended that an antenna with a built-in groundplane is used.

A third and very popular mounting type is known as “on-glass”. This type of antenna mounting configuration is the least efficient of all. This type of antenna does not have the benefit of groundplane and transfers the signal by an electrical phenomenon that is called “capacitive coupling”. There is signal loss between the antenna and the coupling box on the inside of the glass hence this type of antenna configuration will have inferior performance when compared to the same antenna with same antenna cable that is connected directly to the co-ax cable.