FM Antenna Configuration vs Performance
Harris Broadcast Communications Division
sales@bryony.net

Number of Bays In the discussions of the expected coverage for a 3-bay vs a 4-bay FM antenna from 100m HAAT... for a given set of conditions [same antenna type, same ERP, same radiation center elevation, same tower site, and same mounting configuration (that is, the h- and v-pol antenna+tower azimuth plane radiation patterns are the same for the two antennas)], then there would be no significant change in coverage area, or in field strength values at any radius or azimuth beyond the immediate vicinity of the tower, where the elevation plane pattern minima are found.

When either "good" OR "bad" changes are observed in coverage after changing antennas while using the same ERP, transmitting site and HAAT, it is probably due to some combination of the following:

1) The net radiation pattern of the antenna+tower in the azimuth plane is different between the two antennas.

2) Propagation conditions were different during the times of comparison.

3) Changes were expected after the antenna was changed.

Most of the coverage of an antenna is provided by the portion of the elevation plane pattern lying from about zero degrees (the horizontal) to about 2.5 degrees below the horizontal. Antenna relative field over this sector does not vary greatly for all FM antennas of practical physical size. In this example, a 3-bay, full-wave spaced antenna with no beam tilt or null fill has a relative field of about 0.975 at -2.5 degrees, and for a 4-bay for the same conditions it is about 0.950. Starting from the same ERP directed at the horizontal by the two antennas, the difference in ERP they produce at -2.5 degrees is negligible -- 0.21dB in this case, and that is at a point on the ground only 1.4 miles away from the tower. Even at 6kW ERP, field strength is above 100mV/m for both antennas, and the difference in field strengths is progressively less than 0.21dB from that 1.4 miles on out to the radio horizon.

For some perspective on antenna gain or number of bays, consider that most UHF television transmitting antennas have very high gain when compared to FM transmitting antennas (UHF elevation gains of up to 45X compared to FM elevation gains of less than 16X). Yet UHF antennas have no problems in producing the signal strengths predicted by the FCC propagation curves for UHF, and one seldom, if ever hears concerns about "overshooting my market," "penetrating power" etc for them. And (analog) television transmission is much more susceptible to multipath problems, where time-displaced reflections 20dB below the main signal are visible as "ghosts" in a television picture, but the same -20dB reflection from an FM antenna has little or zero affect on FM receivers.

Beam Tilt For the reasons stated earlier, adding beam tilt to an FM antenna yields only fairly minor changes in ERP in the sector of the elevation pattern from the horizontal to -2.5 degrees. Beam tilt typically adds only a few tenths of a decibel in field strength over the great majority of the station’s coverage area, and actually has much more affect quite close to the tower, where field strength without beam tilt is already very high (30mV/m or more). Usually there is no need to make it higher in this region.

Azimuth Pattern Optimization Improving the azimuth pattern of the antenna can have much more of an affect on coverage in some directions than adding beam tilt, by reducing the depths of the nulls in the net antenna+tower azimuth pattern. This is done on a calibrated test range using one or two bays of the antenna mounted on a tower section similar to the customer’s. The azimuth pattern response is measured while the entire assembly is rotated on its vertical axis. Additional "parasitic" radiators are fitted to the antenna/tower assembly to smooth out the azimuth plane pattern, usually of the v-pol component only.

The goal of optimization of an omnidirectional antenna is to avoid any great reduction of ERP at any azimuth in the horizontal plane, and NOT to provide lobes of higher gain so as to increase coverage beyond the contours expected by the station’s FCC license. Doing so would create a directional FM antenna, which may be used only when and as authorized by the station’s construction permit or license, and would be a violation of FCC Rules and Regulations.

Null Fill The nulls in the elevation pattern may need to be filled if they land in a populated area. In the case discussed here, and assuming full-wave vertical spacing of the antenna bays, the first null in the elevation pattern for these antennas, and the point at which the null will "hit the ground" for a 100m radiation center above level terrain is (approximately):

The locations of the nulls in the cases shown in the table are probably too close to the tower to be a problem in most installations. Higher order nulls (2^nd, 3^rd, etc) will fall closer to the base of the tower than the 1^st null on any given radial.

Elevation pattern nulls rarely reach their theoretical value due to re-radiation from the tower and reflections from nearby terrain, but FM receivers may have greater multipath distortion problems if located in roughly circular zones around the tower centered on each "null."

"Low RFR" Antennas The FCC has mandated that each transmitting site must meet guidelines to reduce radiation hazards to station personnel, and to the general public that might have access to areas close to the antenna. Several antenna designs will reduce the amplitude and/or number of off-axis sidelobes from the antenna, which in turn reduce the amount of energy radiated toward the ground near the antenna.

The most popular way to do this with standard, non-panel FM antennas is to use ½ wave vertical spacing of the antenna elements. However, for a given ERP and number of bays, antenna gain is reduced and higher transmitter power is required. Adding more ½-wave spaced bays can recover the gain, but the antenna becomes more expensive, and the weight and windload on the tower increases. The wider main lobe of these ½-wave spaced antennas for a given number of bays means that power density in some regions close to the tower can actually be higher than if a full-wave spaced antenna is used. But usually some number of bays can be found that reduces power density sufficiently, while not overloading the tower or calling for very high transmitter power.

Recently an FM antenna manufacturer has advertised an antenna line with special power distribution among the bays to reduce the amplitude of the sidelobes. Again, gain is reduced over the standard antenna. An additional claim for these antennas is that reducing the reflections from the ground and other surfaces or objects near the tower at ground level gives improved antenna performance toward and at the radio horizon. However, even when TRYING to affect coverage out near the radio horizon, typically it takes several tuned dipole parasitic radiators positioned within feet or inches of each bay to have much affect on the net azimuth pattern toward the horizon. A reflection from the ground, or even a metal roof on a shed some few hundreds or thousands of feet away from the base of the tower is probably negligible at any distant location due to its low amplitude and unfavorable path geometry. The special coverage capabilities claimed for these antennas has yet to be conclusively proven.

Other techniques for reducing sidelobe radiation levels are possible and can be quoted by most antenna suppliers.

Affect of Antenna Configuration on Coverage Looking at a rectilinear plot of the relative field of the elevation pattern of an antenna from the horizontal to -90º sometimes causes fears that the relatively narrow-appearing main lobe might not be able to produce adequate field strengths over the station’s coverage area. But for most situations this is not an issue. Remember that most of the coverage area is served by the sector of the main beam lying between zero and -2.5 degrees, where relative field is high. This point is easy to see in the following plot of field strength versus distance for low to "high" gain antennas, all with the same ERP and HAAT. The plot shows that most of the affect of the different numbers of bays occurs quite close to the tower, where field strength outside of the "nulls" is very high for all of the antennas compared.

Although number of bays may be less important than commonly believed when specifying an FM antenna, still there are good reasons to select one antenna type or configuration over another, and/or to specify certain pattern measuring options that can help predict or shape the azimuth radiation pattern. If best performance is desired, it is always advisable to use the services of an experienced consulting engineer when choosing or changing any antenna and/or transmitting site.