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Abstract
In this White Paper I address the installation and use of the Revolution cell antenna kit (Shown in Figure 1), including some of the questions customers have about antenna selection for a specific Revolution antenna site. PMI’s antenna kit is a package of different gain omnidirectional antennas with different elevation patterns, allowing the user to maximize the cell signal strength, and thus its reliability. In this paper I will cover the different antennas that are currently included in PMI’s antenna kit, and how to choose the best antenna for a given location and how to properly install the antenna.
Figure 1. Cell Revolution antenna kit (From left to right: “Beaver tail” antenna, “Rubber duck” antenna, antenna mount, 13.88” antenna, and Phantom antenna)
Antenna Setup
In the Cell Revolution Kit, PMI offer 4 antennas for uses at different locations. PMI’s Cell Kit spec sheet gives the antenna gain in dB comparing to an isotropic radiator, dBi. An “isotropic radiator” is defined as a theoretical point source which radiates equally well in all directions. To compare this with a dBd value, dB dipole reference, simply subtract 2.14 from the dBi value. The two higher gain antennas come with a 3” diameter NMO (new Motorola mount), and a magnetic mount with 12’ of RG-58/U coax. The coax is terminated into a SMA male connector which mates perfectly with the Revolution.
When installing the antenna or coax to the Revolution’s SMA connector (Figure 2), it is important to realize that slightly snug is best. DO NOT OVERTIGHTEN. The center-pin of the coax and antenna engages into the SMA connector after only a couple of clockwise turns and actually is designed to make a good RF connection with just being finger tight. If the SMA connector itself turns when you are tightening the cable, stop or the small internal coax cable will be damaged. Do not use a wrench to tighten this connector, as light finger tightening is best. Tightening a tiny RF connector is not the same as tightening a lug on a power connection, where more pressure is required to reduce the resistance; just remember hand tight is right! More force will cause damage to the Revolution.
Figure 2. Cell Revolution with SMA connector exposed
After the connector is properly finger tightened, it is essential to use heat shrink tubing to cover the connection for safety reasons. The heat shrink also helps keep the RF connector from working loose and help to prevent water ingression into the connector. Heat shrink of the proper diameter and type is included in the antenna kit, so please use it, or other equivalent that your own safety procedures may call for when installing or removing the coax, make sure the Revolution is not energized, or near hazardous live conductors – the time for antenna kit installation or de-installation is in the office or meter shop, not in the field. When removing the antenna, carefully remove the heat shrink or other insulation. If a knife is used, carefully cut the heat shrink off without penetrating into the coax cable and again only after the power has been removed from the unit.
When installing a Revolution with an external antenna, always insure that the antenna or coax cannot come in contact with energized conductors. Although the coax is insulated, it is not meant for direct contact with high energy hazardous conductors. The antenna itself must also not come in contact with energized conductors. If coax is used to locate the antenna outside a metal enclosure, insure that there is no risk of conducting live voltage from conductors inside the enclosure to the outside via the antenna or coax. Always put safety first, and follow any internal safety procedures.
Antenna Specifications
In this paper, antenna plots are shown that represent the typical response of the antennas in our kit under ideal conditions. It is important to note that the antenna’s environment can and will change the patterns. The antenna patterns are supplied as a guide to help the installer, but should not be taken as an absolute due to the unknown variables introduced in each antenna site.
The smallest slender 3.2” rubber duck antenna, with 0 dBi gain on the US PCS band and slightly less on the US Cellular band, would be an ideal antenna to use at the base of a cell tower. It also has a knuckle hinged close to the connector. This allows for aiming at an ideal angle to avoid the null in the antenna pattern coinciding with the cell tower’s antenna output. See Figure 3 where the E-Plane or elevation antenna plot shows the null in the antenna pattern in reference to the antenna. In almost every monopole design, there is a null in the pattern located directly over head of the antenna. If the Revolution’s location happens to be directly under the cell tower’s antenna, then tilting this antenna can sometimes make a big difference. Also, another counterintuitive misconception that many people have is that the signal strength directly under a cell tower is the strongest. It may be the least amount of distance to the antenna; however, cell tower antennas also have gain thus usually have a null toward the base of the tower. The signals received at the base of the tower often reflections from buildings, objects and terrain or sometimes from another nearby cell tower. As you start moving away from the base of the tower, you will gradually start to get into the pattern of the cell tower’s antenna, and the signal strength will increase until the inverse square losses start to out weight the antenna gain.
Figure 3. Hinged “Rubber duck” antenna and polar graph radiation patterns
The 6.3”flat appearance HEPTA Band antenna with a SMA direct connect that some of our customers refer to as the “beaver tail” antenna (Figure 4), has 2 dBi gain on the US PCS band. This is a good choice for a Revolution installation close to but not directly under a cell tower. The advantage of the direct connector antennas is that there are no losses due to the coax. Any loss between the antenna and the first active stage affects the system’s noise figure and also reduces the amount of power reaching the antenna. This antenna is the best choice without using coax, and is the easiest of the kit antennas to use inside an enclosure where it’s difficult or impossible to mount an external antenna.
Figure 4. HEPTA band “beaver tail” antenna and polar graph radiation patterns
Previously, we discussed how coax loss hurts the output power getting to the antenna and hurts the sensitivity of the receiver due to an increase in noise figure. The coax loss for the 12 foot of RG58/U is 1.7 dB for the 850 MHz US Cellular band and is 2.6 dB for the 1900 MHz US PCS band. In many cases the advantage of locating the antenna in the clear away from electrically generated noise sources and away from metal obstructions can offset the coax loss and results in a much more reliable link. Antenna placement can be critical in cases where optimum performance is needed, which will be discussed later in this paper.
The 2.3” Phantom antenna (as shown in Figure 5) is a unique design that differs from the other antennas in our kit. All of the other antennas have only a vertical polarization. The Phantom has a field diversity design allowing it to be both vertical and horizontal polarization. Also the Phantom antenna has more gain than the SMA direct connect antenna and the rubber duck type antenna, allowing it to make up for some losses due to the coax. In our literature we list the Phantom antenna with up to 4.4 dBi gain at 1900MHz. According to Laird, the Phantom in most cases will outperform a 3 dBi monopole. Laird has elected to rate the Phantom gain in M.E.G. which stands for Mean Effective Gain in terms of the spherical vector wave expansion of the electromagnetic field. Since there are two polarities involved, it is difficult to make a direct comparison with the other antennas. This is antenna is a good choice where a low profile, unobtrusive antenna is needed.
Figure 5. Phantom antenna with polar graph radiation patterns
The 13.88” antenna (Figure 6), which is the largest in our kit, has the most gain and thus under ideal conditions should provide the most communication range. However, there are caveats to that. Antenna gain is always a type of compromises. The antenna gain in a particular direction comes from a loss of gain in another direction. Typically a vertically polarized monopole antenna achieves gain toward the horizon by having a lower angle of radiation, thus sacrificing gain directly above and at higher radiation angles. Most of the time the height of a properly design vertical monopole with a sufficient ground plane determines the amount of gain an antenna has toward the horizon. Usually, monopole antennas that are longer than 5/8 of a wavelength at their operating frequency use an element stacking technique referred to as a collinear array, where the elements are arranged so the RF phase relationships add, providing for more overall antenna gain. As the length of a collinear antenna increases, if the antenna is designed properly, so does its gain at the expense of a lower radiations angle. If the cell tower is a long distance away, due to the curvature of the earth and simple geometry, the associated antenna located normally close to the top of the tower becomes very low to the horizon in reference to the distant antenna view angle. For this application, a longer vertical antenna with more gain close to the horizon is most desirable for maximum range. For long distances from the cell tower, the best antenna to use in our kit is the longer 13.88” antenna. This longer antenna has 4.9dBi gain in the US Cellular band, 806 to 894 MHz and 5.9dBi in the US PCS band between 1850 and 1990 MHz.
Figure 6. 13.88” antenna and polar graph radiation patterns
Factors in Cell Reception
If the Revolution cell site is very remote and the larger 13.88” antenna will not establish contact, but the direction of the nearest cell tower is known, it may be possible to use a higher gain directional yagi type antenna that has a beam pattern, and mount it as high as possible and aimed in the direction of the cell tower. This can help by providing additional antenna gain in only the direction of the cell tower. This can also reduce interference from directions other than the direction of the cell tower. If this still is not enough, a cell phone booster could be used allowing about 6 more dB of boost which would raise the output power of the cell phone booster driven by a Revolution to slightly under 1 watt. This is 6 dBs, or 4 times the standard power, and could double the range under ideal conditions. Most companies that produce cell phone boosters limit the output power of their boosters to less than 1 watt, or 30dBm, to stay below the FCC output requirements.
Another major factor when trying to establish a communication link, (especially when the signal path is not line of sight), is multipath. Multipath is a wave phenomenon that occurs when there are two or more propagation paths between the transmitting and receiving antenna, in our case, the Cell tower’s antenna and the Revolution’s cell antenna. This becomes most problematic when the two signals arrive at the receive antenna out of phase with a similar signal strength. The worst case is if the signals are 180 degrees out of phase and are the same signal strength. In this scenario, signals can completely cancel out one another. This multipath signal cancellation is referred to as destructive interference. If a monopole cell antenna is in a location where multipath is occurring, moving the antenna slightly can in some cases change the signal strength by as much as 30 dB which is a power factor of 1000. This can be the difference of not being able to establish a communication link, to a solid communication link.
By moving the antenna in the correct direction, it may be possible to find a hot spot where the two signals are in phase, thus improving the signal strength, which is call constructive interference. As the frequency increases, their wavelengths decrease, so the distance between the signal’s peaks and valleys becomes smaller. This suggests that at US PCS frequencies in the 1900 MHz band, sometimes inches can make a huge difference. A wavelength at 1900 MHz is approximately 6” in which the EM wave travels through 360 degrees. A change of 1” in a given direction can change the phase by 60 degrees, and 3” by 180 degrees, so this goes to show very minor changes in antenna placement can make large differences when conditions are correct for multipath to take place. In most cases the multipath comes in at different levels and phases other than 180 degrees, so it is rare to have cases of total cancelation, but it is very common to have some cancelation and on an already weak signal, this can sometimes be enough to breakdown the connection.
Overall, the antenna choices can be summarized as follows:
• 3.2” “rubber duck” antenna – Basic antenna, best for smallest enclosures, or near base of cell tower
• “Beaver tail” – Best non-coax antenna, where space allows inside enclosure or outside
• 2.3” Phantom antenna – Low profile, unobtrusive - not as visible to the public, good performance
• 13.88” antenna – Highest gain for poor coverage areas
Conclusion
PMI includes 4 antennas of different gains and radiation patterns in their current antenna package. This offers our customers a number of options to optimize their communication link to the Cell Revolution platform. In this paper I covered basic radiation pattern of each antenna in the kit and a little background knowledge to the help the user pick, place, and adjust each antenna for a solid communication link. Also covered is how connect the coax or antenna to the Revolution to prevent damage and to ensure a reliable and safe installation.
The antenna radiation patterns in this paper are based on factual plots under ideal conditions, however some artistic liberties were taken to simplify their presentation. The purpose of these plots is for a guide line in order to help with antenna selection and placement not, for their absolute accuracy. The actually placement and objects surrounding the antennas will affect their patterns, so at each location the patterns will vary one from another even with the same antenna.
Again I will stress, safety first, always have the power removed when installing and removing the antenna from the Revolution. Only tighten the RF connector by hand until it is snug, and always keep the antennas away from any high voltages sources. Make sure that all exposed metal on the RF antenna connection is covered with heat shrink tubing before applying power to the Revolution.
Cowles Andrus III
Communications Specialist
candrus@powermonitors.com
http://www.powermonitors.com
(800) 296-4120
