Solving ISM Band Coverage Problems
Using Antenna Diversity

DCB is often called upon to assist customers having unusual problems. One of those we commonly run into is the application of ISM band radio equipment in unique situations. Our engineers have a reputation of making wireless LAN equipment and license-free serial radios work where others have failed. While not a detailed explanation of our techniques and methods, this paper provides an introduction to some of the methodology and illustrates some of our past installations. It was written as an introduction, and we recommend that you call DCB and discuss your application with our engineers if you have an unusual or seemingly impossible communications problem to solve. While we can't break the laws of physics, we often bend them to fit the need.

ISM band (900 Mhz, 2.4 Ghz, and 5.8 Ghz) radios systems are used in many applications where maximum range is required, but the physical environment requires compromises in antenna placement. In some cases, these problems may be mitigated by innovative antenna mounting arrangements and multiple antennas.

When dealing with a point-to-point system, it is common to use a directional yagi or dish/reflector antenna at each location to raise system gain significantly. This process is well understood and often used.

Lower-gain omnidirectional antennas such as the typical vertical 3 dB "rod" radiate the signal in all horizontal directions equally. The vertical pattern is more restricted. This is visualized by the use of a donut shaped gain pattern. Other higher gain antennas "create" that gain by shaping the radiation pattern in a more useful manner and directing more signal in the direction where it is needed.

This is a typical omni antenna pattern as seen looking horizontally at the radiation pattern (that vertical line in the middle represents the antenna). You can see that the maximum radiation direction is horizontally out from the antenna (in all directions), and there is less radiation vertically above and below the antenna. The signal strength then increases as you get further from the antenna along the ground, until a maximum just where the shaded area reaches it's lowest spot.

The problem arises when higher system gain is required on an omnidirectional antenna system. Most antennas that radiate equally in all 360 degrees horizontally flatten that imaginary donut somewhat. This, in effect, pushes a larger portion of the signal out on a more horizontal plane while lowering the signal levels in areas that are higher or lower (vertical measurement from the earth surface) than the antenna. This creates "shadows" of low signal in areas that are nearer the antenna. The higher the gain, the more dramatic the effect.

This illustration shows the flatten lobes of a typical omni gain antenna. The diagram is viewed "straight-on" and the top of the circle is directly overhead of the antenna just as in the prior diagram

Note that the area below the flattened donut, especially nearer the antenna has low signal levels. This is because the antenna is designed to move most of the signal out to the horizontal, thus increasing the distance that it can cover.

There are several methods available to deal with this problem. The first, a more efficient antenna design, would require an antenna to be designed with the radiation pattern needed at a specific installation. Although numerous antennas are available with different radiation patterns, none have been found that are suitable for the applications we face daily.

Our most successful method has been to apply multiple antennas and power dividers. By selecting antennas with a pattern that covers only a portion of the needed area, we can combine several antennas to gain full coverage. Several installations of this type and the use of innovative mounting will be covered in this paper.

First, the power divider. Power Dividers match multiple antennas to a single feed line. They are readily available with 2, 3, or 4 antenna connectors and one connector for the radio. They may or may not pass the DC current required for in-line amplifiers, and it is often impossible to get detailed information from manufacturers about DC pass-through. Since amplifiers are often used in these systems, we will often choose to place the power divider at or near the radio, with the DC injection between the power divider and the antenna. In other cases, the amplifier might be placed between the divider and the radio, with the divider located near the antenna mounting. These topology decisions are based on the physical constraints at each installation.


Commonly Used Power Dividers (Combiners)

Example 1: Segmentation with three antennas for 360 degree coverage.

A three way power divider may be used with three 120 degree corner reflector antennas mounted equidistant around a ship mast. Most corner reflector type antennas have a fairly good vertical pattern and cover areas close-by the ship as well as the gain to reach out to the horizontal. In some cases, patch type antennas with 90 degree half-power beam width work well in trios. Depending upon the gain required, lower-gain yagi antennas sometimes work well in this application.

Example 2: Shadow Fill-in with Two Antennas

Often a single omni antenna will work well other than in a relatively small "shadow" area. For example, mounting a vertical omni on the side of a mast may cause a shadow on the opposite side of the mast. This shadow effect may sometimes be cancelled by mounting the antenna exactly a ½ wavelength away on a stand-off bracket. In other cases, the mast is so wide and electrically conductive that an additional antenna should be used along with a two port power divider. By carefully mounting the antennas and analyzing their radiation patterns, one can manipulate the coverage pattern to advantage. It may be that mounting both antennas on a standoff bracket on the same side of the bast works well, in other cases, placing them on opposite sides of the mast is preferred.

Experience has shown that other structures can cause a radiation shadow, especially at 2.4Ghz and higher frequencies. We have observed installations where handrail and weather protection steel on walkways blocks the signal in certain directions. In these cases, a small 3 element yagi may be used for "fill-in" in that direction by mounting it slightly higher or lower than the main antenna.

Example 3: Radiation Angle Fill-in

In some cases, a high gain omni antenna is required to obtain the desired distance coverage. This "flattened donut" radiation pattern leaves a weak coverage area "under" the pattern closer to the antenna. We've seen this on installations in which the system works well from 5 to 20 miles, but between 1 and 5 miles, coverage is non-existent. In some cases, we've solved this problem by mounting a second vertical antenna (having lower gain… say 3 to 6 dB) near the main antenna, but upside down. By analyzing at the vertical radiation pattern, we found an antenna with a pattern that worked well for this application. Simply mount both antennas on the same mast stand-off bracket, with one upside down from the other. Remember to weatherproof the upside-down one quite well, as it's not designed for that mounting arrangement.

Example 4: Radiation Angle Problem

Another problem occurred when a customer installed a high-gain omni antenna on a weather buoy. The buoy would swing about 45 degrees from vertical with waves and the wind. The high-gain antenna had only a 30 degree usable vertical beam width. Replacing the antenna with a LOWER gain antenna having a wider vertical beam width solved the problem.

Example 5: Higher Gain Required in All Directions

When higher gain is required than an omni will provide, multiple high-gain yagi antennas may be combined by sectoring the 360 degrees with as many individual yagi antennas as you need.

In actual practice, the most commonly used approach is for a two-way power divider… with one high gain omni vertical and one lower gain vertical placed in a way to fill in the weak areas. Next, the three antenna, segmented 360 degree circle method , followed by the four antenna segmented circle.

In all cases, the engineer must analyze the antenna radiation pattern for the antennas being considered, both in the e-plane and the h-plane. Unfortunately, most designers only look at the E-plane radiation characteristics.

Also, insure that the power output of the system is within government regulations. One should analyze over all system gain, output power, and consider the use of amplifiers carefully. Human RF exposure is now a concern in some cases.

These are common problems. Experienced engineers have been down this road before. It's the unique, "outside the box" analysis and problem solving along with quality products that make the difference between a quality installation and one that is marginal.

For further reading, some of the best, easily understood antenna information is from the Amateur Radio Community. The EasyNEC modeling program, and many handbooks and tutorials available from the ARRL (http://www.arrl.com) are great starting points for detailed antenna analysis. There is an inexpensive antenna design and construction available from there as well as an antenna modeling course.

Another quick reference web site with great antenna modeling information, and pattern analysis is provided by L.E. Cebik, W4RNL at http://www.cebik.com/ .

For more detailed information or a consultation, call our engineers. We sell communications solutions that work.

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