Selecting antennas for PCS and DCS systems

Selecting antennas for a PCS or DCS system may be the most important decision an RF engineer makes. There are several points, including electrical issues, for the engineer to consider, and attention to detail is key.

Many personal communications service (PCS) and digital cellular service (DCS) systems are being designed and optimized around the world. With system performance on the line, selecting antennas for a PCS or DCS system can be the most important decision an RF engineer makes. The antennas may represent only 1% to 3% of the total cost of a typical site, but they can have a big effect on performance. In fact, using a “dollar/performance” criterion, antennas can achieve the “most bang for the buck” of any system component. With this in mind, it is important to consider several points when selecting antennas for PCS and DCS systems.

Several electrical issues can significantly affect overall system performance. Horizontal and vertical beamwidths define antenna gain. Horizontal beamwidths typically used on PCS and DCS systems are 338, 658, 908 and 1208. Each has its proper application. Horizontal beamwidths at 1208 are typically used by engineers who subscribe to the design theory developed during the early days of cellular. The conventional wisdom currently accepted is that 908 horizontal beamwidth antennas perform better in digital systems.

By using 908, or even narrower antennas, excessive overlap is avoided as shown in Figure 1 below. This excessive overlap can cause higher bit-error rates (BERs) in digital systems and capacity loss because of excess soft hand-off zones in code-division, multiple-access (CDMA) systems. By using 908 or narrower horizontal beamwidth sector antennas, operators can also help reduce the possibility of capturing mobiles from adjacent sectors, thus optimizing hand-offs. For PCS and DCS systems, most operators use 908 horizontal beamwidth antennas in rural and suburban sites.

For core urban sites, some operators use 658 horizontal beamwidth antennas. The 338 beamwidth antennas can be used for special bidirectional “corridor” applications. By mounting two high-gain, 338 horizontal beamwidth antennas back-to-back and then feeding them through a two-way coaxial power divider, a high-gain, bidirectional pattern is created. This pattern is ideal for covering long interstate highway corridors.

When the antennas are combined with a power divider, however, the energy is defocused, thus decreasing the gain by 3dB. To maintain maximum gain, the antennas should be fed separately, with the RF channels split between the two. An example of a bidirectional array for corridor coverage is shown in Figure 2 below. The front-to-back ratio (F/B) is also an important aspect of the horizontal beamwidth. The F/B typically varies between 20dB and 45dB, depending on the model and manufacturer. Log periodic dipole array antennas usually have a higher F/B than panel antennas. The highest F/B available on the market today is 45dB, which is useful for rejecting co-channel and adjacent channel interference. Not only should you look at the front-to-back specification, but the overall reduction of signal toward the back of the antenna. The horizontal pattern of an antenna with a F/B of 45dB is shown in Figure 3 above.

Once a horizontal beamwidth is chosen, the next step is to determine the proper vertical beamwidth. Vertical beamwidth, horizontal beamwidth and antenna efficiency define the overall gain. PCS and DCS antennas are typically available in lengths from one foot to six feet. A six-foot-tall PCS-DCS antenna has a vertical beamwidth of about 48. This is about as narrow a vertical beamwidth as should be used in a PCS or DCS system. A PCS antenna with a 658 horizontal beamwidth and a 78 vertical beamwidth should have a gain of about 16dBd. An antenna with these characteristics is shown in Photo 1 on page 12. Two PCS-DCS antennas with the same beamwidths should have about the same gain, provided they have reasonable antenna efficiency. Unfortunately, this is not always the case. Most operators use antennas between four feet and six feet in length for rural and suburban sites. For urban sites, antennas from one to four feet long are being used, depending on the particular application. The vertical pattern also defines the amount of null-fill available. Null-fill can be useful for improving coverage close to the site, particularly for low-power portables. An antenna with good null-fill can be seen in Figure 4 on page 12.

Downtilt is another issue that RF engineers often discuss when designing and optimizing PCS and DCS antenna systems. Downtilt focuses below the horizon. (See Figure 5 at the left.) First, because PCS and DCS systems reuse frequencies, controlling the level of signal on the horizon is important. By tilting the main beam below the horizon, less co-channel interference occurs at the site that reuses the same frequency. Second, focusing the signal below the horizon can help to improve close-in coverage. When designing a system, the operator must be careful to consider both issues when choosing the best amount of downtilt at each specific site. Some operators use a fixed value at every site, say 108. This type of thinking can get the designer into trouble quickly. Every site has a specific height above average terrain (HAAT), desired coverage area and reduction of signal on the horizon. By using the equations in Figure 5, an RF engineer can more intelligently choose an initial value of downtilt for a particular site. Operators should be cautious of companies offering solutions with a selection of signal reduction on the horizon, such as 3dB, 6dB and 12dB down. This type of solution does not consider the necessary coverage area, nor does it offer the optimized solution for each site. Once the optimal value of downtilt has been chosen, the RF engineer must decide among mechanical tilt, electrical tilt, or a combination. Mechanical tilt is achieved with a mechanical downtilt bracket designed for the antenna. Electrical tilt is designed into the antenna, and the operator purchases the antenna with a fixed value of tilt. Figure 6 above shows the H-plane pattern on the horizon for an eight-element array with a 908 horizontal beamwidth mechanically tilted 28, 48, 68, 88 and 108. As can be seen, with 108 of tilt, the horizontal beamwidth on the horizon is considerably distorted by mechanical tilt. However, the patterns with 48 of mechanical tilt show that with a reasonable amount of tilt, say 48 for an eight-element array, there is no significant difference between mechanical and electrical tilt. The H-plane cut at the angle of mechanical tilt has the same pattern as the antenna without mechanical tilt. It is for this reason that mechanical tilt makes so much sense for reasonable values of downtilt. As long as no more than 108 of mechanical tilt on a four-element array, 48 to 58 on a eight-element array and 28 to 38 on a 12-element array are used, there is no significant distortion of the horizontal pattern, and the mechanical tilt allows for optimization of tilt in the field. If electrical tilt is required because of the amount of tilt needed, it may be wise to use a combination of electrical and mechanical tilt. The mechanical tilt bracket allows for easy adjustment of the tilt during the site optimization stage.

Last, but not least, among electrical issues to consider when choosing PCS and DCS antennas is voltage standing wave ratio (VSWR or return loss). Many operators choose antennas based on VSWR, but one must be careful when assuming that low VSWR means high quality. Back in the CB radio days, there was a company “X” that manufactured a high-quality 5/8-wave mobile antenna. This antenna had a VSWR of about 1.5:1. A new manufacturer, company “Y,” developed a 5/8-wave mobile antenna with a VSWR of 1.2:1. However, the low VSWR was achieved with a lossy coil. The antenna’s overall gain was lower than the antenna from company “X,” as can be seen in Figure 7 on page 16. Unfortunately, company “Y” took over the market with the antenna with less gain only because it had a better VSWR. Another way to view this situation is to imagine that every site in the system has an antenna VSWR of 1.5:1. The director of engineering comes to you on Friday afternoon and says, “Every site better have a 1.3:1 VSWR by Monday morning, or else.” One easy way out is to go to an electronics store and buy a couple hundred 3dB loss pads. Install one on the output of every antenna over the weekend and, voil , every antenna has a VSWR of about 1.22:1, and you never need to find out what “or else” means. Of course, there is one problem. Every antenna has its gain reduced by 3dB. The moral of the story is that lossy materials in an antenna will give the antenna a better VSWR, but they will also reduce gain and system performance.

When PC board antennas are used at 1.9GHz, a similar situation arises. If you measure a PC board antenna for VSWR, it looks great, but try to measure one on an antenna range for gain, and you will typically find that they do not have the gain you would expect for the vertical aperture of the array. When selecting antennas, it is wise to have the antennas measured on an antenna range for gain and patterns. This should preferably be done all on the same day and with one of your engineers as a witness. (It is amazing how a laptop can improve digitized patterns.) Many of the PCS-DCS antennas on the market do not meet advertised gain specifications. Although a half dB is not worth quibbling over, some of these products are overrated by 2dB, or even 3dB. The only way to protect yourself is to verify advertised antenna gains on an antenna test range.

Along with these electrical issues, it is just as important to consider mechanical issues. Catastrophic antenna system failures usually can be traced to poor mechanical antenna design. By looking for a few key features, you can help to protect yourself from these catastrophic failures. A key ingredient in successfully designing PCS and DCS antennas is to use the proper materials. Table 1 on page 18 shows compatible couples. Using this table, engineers can determine which metals can be placed in contact with other metals without causing galvanic corrosion. Galvanic corrosion can lead to intermod (IM) over time.

Another issue with the mechanical design of antennas is the large number of cables and solder joints used in certain designs. These soldered joints can be difficult to control and may break down over time, or may leave the factory in less-than-perfect condition. Although an antenna with numerous soldered joints may have no intermod when it is tested at the factory, operators have to be concerned with how it performs over the long term. One way to avoid this problem is to avoid cables and soldered joints completely. Although this is not always possible, sometimes a third construction technique can be used. By using a “monolithic” construction, the reliability of an antenna can be greatly increased. Photo 2 on page 22 shows an antenna constructed with this type of new technology. By eliminating cables and soldered joints, this monolithic technology improves the probability that an antenna will function reliably over the long term. If an antenna’s chance of failure is statistically related to the number of soldered joints, eliminating all of the joints should dramatically reduce the number of catastrophic failures in the field. The monolithic design also has an added benefit that the feed lines use air as a dielectric, which helps to reduce loss.

Another key mechanical aspect of an antenna is the connector. Most PCS and DCS operators are using 7/16 DIN connectors. They use these connectors on antennas for their good intermodulation specifications and durability during installation. Another aspect for RF engineers to consider when selecting antennas for PCS and DCS systems is environmental testing such as IEC (International Electro Technical) testing. Table 2 on page 22 shows the minimal IEC testing a manufacturer should perform on an antenna design. When discussing IEC testing with your antenna vendor, be sure to ask for a copy of the results. You should receive a description of each test along with final VSWR, patterns and IM test results after all the tests have been completed.

A new development in antennas is polarization diversity antennas. Cellular and PCS systems use a concept known as space diversity reception to improve uplink performance and to help balance the uplink and downlink paths. Space diversity works under the assumption that an antenna spaced 10l horizontally from another antenna will not manifest signal fading identical with that of the first antenna. With a diversity receiver combiner that can use both antennas, the average uplink signal is improved between 4dB and 8dB, depending on conditions (and whose textbook you read). Polarization diversity is based on the assumption that the signal arriving from a portable phone in a multipath environment will exhibit random and changing polarization at the base site. By using an antenna that has elements in +458 and 2458 polarizations, the signals can be fed into a diversity combiner, and the average signal can be improved. This polarization diversity technique has been shown to be basically equivalent to space diversity schemes in urban and heavy urban environments with a lot of multipath. An operator considering polarization diversity needs to keep two important points in mind: First, polarization diversity only works for portables in high-multipath environments. This technique may not work well in suburban and rural areas with little multipath. Second, polarization diversity generally uses two input antennas, which requires duplexing the transmitters with one of the receiver outputs. Many of the PCS-DCS polarization diversity antennas exhibit poor IM performance. When checking IM on some of these antennas, I have seen some third-order products reach 260dBm to 280dBm with two 20W inputs. I have even seen this level on some standard, vertically polarized PCS antennas. Even the fifth-order levels on these antennas would cause serious noise problems and call quality degradation once a system is loaded. Any operator considering polarization diversity antennas, or any antenna to be used in a duplexed system, should ensure that the antennas exhibit excellent IM performance. One method of testing the IM performance of a particular design is to obtain several samples and test them at an IM test facility. Because most PCS-DCS carriers will only see fifth-order hits, a third-order level of 2100dBm with two 20W inputs should be the minimum acceptable level.

An RF engineer can often ascertain a lot about an antenna design by looking at the little details. Do the downtilt clamps that came with your antenna look as though your child made them from an erector set? How will they hold up in a 100mph wind? What about a 125mph wind? Does the antenna have a drain hole? Has the antenna been opened to examine internal construction and workmanship? Have you performed the “jump” test on the antenna? Additionally, keep in mind that it can pay to spend a few more dollars and make sure you are purchasing high quality, reliable antennas for your PCS or DCS system.

The author would like to hear how your PCS-DCS antennas have been performing and about any experiments being conducted.