Abstract

This white paper describes the advantages of Commercial and Residential Cell Phone Repeaters and how to properly install them to enhance signal quality to devices such as PMI’s Revolution and Boomerang products.

Cellular Repeaters

The Revolution and Boomerang Plug In model are sometimes installed inside a building where cell phone coverage is poor or in some cases a complete dead zone. Many buildings are constructed from materials such as metal or concrete that attenuate or eliminate cell phone signal making it impossible to use devices that use this infrastructure. Even the highest gain cell phone antenna will do little or no good in this type of situation. If only one device is needed, it is possible and usually more cost effective to simply install an external antenna to the outside of the building and pipe the signal in via a coax cable. If there are two or more devices, a cell phone repeater may be the best solution.

A cell phone repeater is a device used to extend useable range and allows cell phone coverage even in buildings where the construction materials attenuate the cell phone signals greatly, or in a fringe cell phone coverage areas where the signal is very weak. Cell phone repeaters can handle many signals simultaneously, allowing only one localized repeater to be required to boost the range of many Revolutions or Boomerangs in a particular building or room.

A cell phone repeater consists of three major parts, a bidirectional amplifier, an internal antenna, and an external antenna. Low loss coax is used to couple the antennas with the amplifier unit. In order to keep the bidirectional amplifier from oscillating, it is necessary to have high isolation between the external and internal antennas. This can be achieved by using the building’s natural attenuation and spacing the internal and external antenna as far apart as practical. Also it may be a good idea to use a directional antenna for the external antenna and make sure it is not aimed in the direction of the internal antenna. If possible, the ideal situation is to mount the external directional antenna so the internal antenna is directly behind it. This in most cases will allow the most isolation between the two antennas.

There are many companies that offer cell phone repeaters. Make sure that you pick a repeater that has been FCC approved and covers the correct frequency bands. Not all repeaters cover all cell phone bands, so if the wrong repeater is chosen, it will not increase the cell modem’s range.

There are two main cell phone formats that exist in the United States and are being used in PMI’s product line, the CDMA and the GSM format. CDMA stands for Code Division Multiple Access, and GSM stands for Global System for Mobile Communications. GSM is the world’s most popular cell phone format at this time. Both utilize some of the same popular US frequency bands, the 850 MHz Cellular Band and 1900 MHz PCS bands. The modem in our Revolution is currently using the CDMA 850 and CDMA 1900 MHz bands. The Boomerang’s modem is using the GSM 850 MHz and GSM 1900 MHz bands. Since both the Boomerang and Revolution are using the same frequency bands, a repeater that will cover the 850 MHz and 1900 MHz bands should work fine for either product.

If you already know what frequency band is being used in your area, you may be able to optimize the repeater system by using a repeater tailored for the band you need. In some cases money can be saved by purchasing a single band repeater with the appropriate antenna system.

Normally a yagi type antenna is used for the external antenna. Yagi antennas are frequency dependent devices, normally designed for one band only. Dual band yagi are made for the cell phone band. A dual band yagi does not offer as high a gain as a single band yagi of similar size, but will usually work for most installations. The dual band yagi is a compromise, however if both bands are used, and the correct band is not known, a dual band system may be necessary.

If a dual band repeater is being used, and one band is known to be active, in some cases the other band’s gain can be turned down at the bidirectional amplifier, thus reducing the potential of out-of-band interference. If it would happen that both bands are covered in your area, by using a dual band antenna or two single band yagis with a combiner may give redundancy in the event that the cell provider would happen to have an outage on one band or the other.

Some of the companies that sell Cell Phone Repeaters include:

• Wilson
• Wireless Coverage Solutions
• Cellphone Mate
• Wi-Ex Z-Boost

Installation

Try to find a location for the external antenna that has the strongest cell phone signal for the band you are operating on. A cell phone with the same service provider can be used as a poor man’s field strength meter. The more bars the better the signal! If the location of the cell tower that is providing service is known, the antenna can be mounted on the side that faces the cell tower. In some situations, the cell tower is visible; when it is not, a GPS with a bearing function that allows aiming the antenna toward a waypoint can be used. Find out the physical coordinates of the cell phone tower and enter it in the GPS as a waypoint. The GPS will then point you in the direction of the tower.

With the proper adapter cable, a cell phone with an external antenna input can be connected to a directional antenna to determine the direction with the highest signal strength. Some repeaters come with a built-in signal strength meter which can be very handy during installation. Others will have lights that indicate proper and improper operation. Due to the nature of digital cell phone signals and spread spectrum, it is very hard if not impossible to use a simple Field Strength Meter for antenna alignment.

When mounting the external antenna, try to get it up as high as possible, and at least 2 feet or more from anything metal including cords, guy wires, or roofing. Always install a grounding block and a low inductance ground wire or metal ribbon to a ground rod. Use a #6 copper ground wire between the ground block and ground rod. An 8-foot or longer copper clad ground rod is recommended if the system’s ground is not close by. Connect this ground rod to the structure’s ground system via a #6 copper wire. Be sure to include a 4″ or slightly larger diameter drip loop on the outside before passing the coax through the wall. Seal off the hole with Silicon RTV Sealant to weatherproof the installation. It is also a good idea to use an adhesive heat shrink tubing around any coax connections to keep water out of the coax. Some antennas come with weather proof rubber boots, remember to use them to ensure many years of trouble free operation.

NOTE: Do not use Silicon RTV Sealant that uses acetic acid as the curing agent on coax connections or any type of metal. This type of Silicon RTV has a smell similar to vinegar, and will corrode electrical contacts rapidly. Non-Acetic Silicon RTV sometimes called neutral curing RTV, is available. Examples include Permatex Black, Model number DXE-RTV598335 and RTV 120 Series made by Momentive Performance Materials. These Silicon RTV are safe for use on metals and electrical connections. If RTV is used to seal around an antenna connector that may come in contact with antenna elements and ground, be aware that the dielectric constant is different from air and may detune the antenna.

This paper covers general installation procedure and practices that should be observed for proper repeater operation. For more detailed procedures and instructions, please follow the manufacturer’s user’s guide, installation and set-up instructions.

Summary

A cell phone repeater is an excellent addition to improve the reliability and dependable operation of multiple cellular communication devices inside a building or structure that attenuates cell phone signals. Remember the following:

• Use a repeater that covers the correct bands. For Boomerangs and Revolutions the correct bands are the 850 MHz and 1900 MHz bands.
• To avoid RF oscillation, always observe the proper antenna spacing and never point the antennas toward each other.
• If a directional external antenna is used, aim the antenna in the direction of the service provider’s cell tower.
• Always install a ground block with a low impedance connection to ground to protect against lightning and static build.
• Weather-proof the coax connections and remember to install the drip loop and seal the coax where it enters the building. Do not use RTV Silicon sealant that releases acetic acid during the curing process on coax connections or any electrical connections. Only use neutral curing RTV.
• Read the operation manual that comes with the repeater and follow instructions.