Abstract

This white paper describes a method of finding and installing the proper cell phone antenna for the Revolution Cell Modem product.

Installation

Proper antenna installation on any cell modem based product is very important for reliability, dependability and for a high data transfer rate.

The Revolution Cell modem model is sold with an external compact antenna that will work well in most situations in or close to cities or large highways where cell coverage is good. The surrounding environment effects the operation of any antenna, so for best results, the Revolution’s antenna needs to be placed as far away from any vertical metal objects as possible. Due to the nature of the Revolution product, this may not always be possible, so some compromises may need to be made or another antenna solution may need to be implemented for good performance.

It may be possible for the compact antenna on a Revolution to work in a metal enclosure when in a very strong cell coverage area. Installing a Revolution with the compact antenna in a metal enclosure is never recommended nor should it be implemented! The signal will be attenuated many dBs and will create strong RF reflections. The modem under these marginal conditions will increase the output power of the cell modem trying to compensate for the higher path losses.

If it is impossible or impractical to mount the Revolution outside of a metal enclosure, then it will be necessary to use an external cell antenna. PMI offers an antenna kit that contains 4 different antennas. With this kit and a little knowledge, the proper antenna can be selected for the appropriate installation.

It goes without saying; the external antenna needs to be mounted outside of the metal enclosure. If the metal enclosure is magnetic, it can be used as a ground plane for the external antenna.

Make sure when running the coax out of a metal enclosure that it does not come in contact with sharp edges that might cut into the coax or mash the coax changing its internal impedance. Running the coax through a rubber grommet is recommended for a safe and reliable installation. The extra coax should be coiled neatly with a 5 to 6 inch diameter bend and left inside the enclosure. The coax should never be bent tighter than a radius of 2 inches, or 10 times the diameter of the actual outside dimension of the coax. Bend radius tighter than 2 inches could cause non uniformed impedances in the coax and compromises the overall systems performance.

NOTE: Please follow and observe your company’s safety requirements that deal with running a wire or conductive cable outside of a high voltage enclosure!

Placement

Try to determine the least obstructed RF path from cell tower to the Revolution’s antenna site. If there is a metal pole, tower or building nearby, try to get to the side facing the cell tower. Also if the antenna is mounted to a metal roof or enclosure, try to center the antenna as much as possible. In some cases there is an advantage to have a larger amount of the ground plane in the direction of the cell site. It is always good to be a 1/4 wavelength or greater from the edge of the ground plane to the center of the antenna. For cell phone frequencies if you are at least 6″ from the edge of the ground plane, the antenna should perform well.

Some cell phone antenna designs require very little or no ground plane to operate. These antennas will function perfectly well with or without a ground plane. If you are in doubt, it is better to be safe and assume it does require a ground plane. It is impossible to cover every possible antenna and installation that could exist. The purpose of this paper is to give some guidelines to the most common issues that may occur in the antenna installation process.

If it is decided that an external antenna would be required to establish a reliable communication link, the next question is what type and size of antenna is needed?

When the cell phone antenna is mounted line of sight, high on a tower close to the Revolution’s installation, go with a small antenna, such as the 4″ antenna that is included in the PMI antenna kit. The 4″ antenna has the advantage of having a higher angle of radiation so it is much better than a larger antenna close in.

If the cell tower is a long distance away and is closer to the horizon, then the larger 24″ antenna should perform better. The 24″ antenna has a much higher antenna gain and lower angle of radiation. It should be noted that the larger the antenna, in most cases, the more sensitive the antenna is to metal objects close to the antenna. More clear space is required surrounding the larger antennas for it to operate as designed and to get the proper antenna pattern.

Antenna Kit Information

The antenna kit comes with four different size antennas. This allows the customer to pick the proper antenna for his cell site. The main difference between the antennas in this kit is the angle of radiation and pattern of each antenna.

NOTE: No matter what antenna is picked, it is very important to cover the antenna’s coax connector with a non conductive tape or heat shrink tubing before continuing with the installation!

As the antenna becomes longer due to vertical stacking of a collinear type array, the lower the vertical angle of radiation becomes, thus more gain is directed toward the horizon and less gain in the vertical direction. The four sizes of antennas in the kits are 4″, 12″, 24″ and 40″.

The approximate isotropic gain (shown in Figure 1) of each of the four size antenna is as follows for the 1900 MHz GSM Band:

• 4″ = 3dBi gain, Beam Width ~ 143 degrees
• 12″ = 5dBi gain, Beam Width ~ 114 degrees
• 24″ = 9dBi gain, Beam Width ~ 72 degrees
• 40″ = 11dBi gain, Beam Width ~ 57 degrees

Isotropic gain, dBi, refers to the gain of an isotropic antenna. An isotropic antenna is a theoretical antenna which radiates and receives radio waves equally well in all directions from a point source. Another antenna standard that is commonly used to compare antenna gain is a dipole antenna. A dipole antenna has approximately 2.15 dBi gain. A good rule of thumb derived from the inverse square law is for each 6 dBs of antenna gain the usable distance is doubled from the Cell Modem to the Cell Site. Thus the 24″ antenna should increase the usable range by 2 times over the 4″ rubber duck antenna.

Figure 1 below shows the relationship between antenna gain and the antenna’s radiation pattern.

Summary

The larger the antenna, the higher the antenna’s gain thus its horizontal range and the lower the antenna’s radiation angle. Use the smaller antennas when cell tower is close in and above Revolution’s antenna and use a larger antenna when cell tower is located farther away, closer to the horizon. In some cases when there is a lot of metal close in to the antenna, a medium size antenna will be effected less and will outperform a larger antenna.

• Never install the cell phone antenna inside a metal enclosure!
• When routing coax through a metal wall or enclosure use a rubber grommet. Never mash, pull or cut into the coax!
• Keep the bend radius of coax greater than ten times the coax’s diameter.
• Keep antenna and coax as far away from high voltage as possible.
• Mount the antenna on top of the metal enclosure or roof out of reach when possible.
• Mount the antenna as far from vertical conductors as possible.