The author Gertrude Stein once said “a rose is a rose, is a rose is a rose.” Maybe “an antenna is an antenna, is an antenna” also applies, but an AM antenna is not an FM antenna. Their requirements are very different.
AM antenna requirements are almost the opposite of FM requirements. AM coverage is based on groundwave propagation. Hence, ground conductivity is far more important to an AM antenna than actual height above ground. On the other hand, successful FM transmission is dependent on antenna height above ground, or line-of-sight. The antenna height above the average elevation of the ground surrounding the antenna is of particular importance.
Most AM transmissions rely on groundwave signals to maintain coverage. Only a relatively few clear channel stations produce a useful skywave signal that is protected by the FCC's Rules. There is a daily change in AM signals caused by reduction of power at night to avoid interference and interference caused by skywaves that return to earth only at night. But FM signals do not normally suffer from similar diurnal changes, and once in place, an FM signal is usually constant within its service contour. Nevertheless, FM signals can also be affected by unusual weather conditions such as ducting and sporadic e-layer conditions.
Radio waves are similar to light waves in that they cannot be bent around a radius. However, they can be caused to navigate a bend or made to change direction by refraction in the atmosphere, or even by a phenomenon called knife edge refraction. The latter can cause unexpected interference and unusual long distance reception.
As a result of these radio-wave characteristics, FM relies on line-of-sight transmission for station coverage. Service contour location calculations (0.5mV/m or 1mV/m) are based on the height above average terrain of the transmitting antenna and the height of the receiving antenna above ground. FCC Rules assume a height of 30 feet for the receiving antenna. This is a figure originally taken from the height of an average two-story house with an outdoor antenna. Soil conductivity and vegetation can have considerable effect on FM reception. The result of lowering the receiving antenna to a few feet above ground is a loss of around 6 or 7dB, some of which is caused by soil effect.
In AM operation, the signal is vertically polarized (Vpol). FM signals were originally horizontally polarized (Hpol), but now, circular polarization (Cpol) is used almost exclusively. Sometimes, the signal separation provided by vertical versus horizontal polarization is used to prevent interference between NCE stations. This is especially useful in the educational FM band when interference is often experienced on TV channel 6, and the interfering NCE station can obtain an extra 6dB of interference reduction by using vertical polarization.
Vpol does not necessarily improve or increase coverage, but it generally tends to produce better FM reception on automobile radios. Vertical antennas and windshield antennas take advantage of the vertical component; although the polarization is generally quite mixed by the time the signal has traversed many different reflecting surfaces. As a matter of fact, elliptical polarization is often preferred to Cpol.
There are two schools of thought regarding the efficacy of Cpol. It was originally tried in the western mountainous country, and the deep canyons of Chicago. First reports were quite glowing — provided that Cpol receiving antennas were used; now it seems that opinions are varied. Unfortunately, Cpol receiving antennas are seldom used. The vertical signal portion of Cpol is sometimes blamed for excessive multipath interference. The term picket fencing is sometimes applied to this phenomenon.
There are several other problems associated with finding a good FM transmitter antenna location. These include fresnel zone clearance, in which mid-path clearance can affect reception. In flat regions, the midpath beam radius can have considerable effect on the received signal. Ground reflections and grazing angle effects can produce varying Vpol and Hpol signal attenuation. Signal depolarization is caused by scattering, refracting and reflecting surfaces. Additionally, vegetation losses increase attenuation. At times, it seems a miracle that any signal gets anywhere.
The whole of a massive AM tower is used as the radiator. The FM antenna is a comparatively small device mounted on the top or side of a tall tower. Provided that proper isolation is used, this tower may also be used as an AM radiator if desired, while supporting the FM antenna.
Apart from the radiation requirements of FM, it is the difference in frequency that makes the FM antenna so convenient to build, install and adjust to a desired pattern. In AM, a single wavelength is 984 feet for a 1MHz signal. In FM, one wavelength is about 12 feet. Thus, the popular quarter-wave antenna often used for AM might be 246 feet high, and the FM antenna elements would be only around three or six feet long, depending on the antenna design.
Directional AM stations require large ground areas to mount two or more antennas to produce the desired directional pattern. On the other hand, FM antenna systems require a single tower to mount any reasonable number of FM radiators. Generally the desired radiation pattern can be obtained using one FM antenna unit, and additional similar units are stacked to obtain antenna gain.
Remember that, as the antenna gain figure is increased by adding bays, the radiated beam is narrowed considerably, and with very high gain antennas, it is possible to beam the signal over closer-in receivers. To overcome this, either beam tilt or null fill can be used. Beam tilt can be produced electrically or mechanically.
Electrical beam tilt is accomplished by advancing current in the upper bays and reducing it in the lower levels. Mechanical beam tilt produces a problem in which the beam on the side opposite to the downward tilt is projected upwards as the antenna is moved from the vertical. Still more listeners are lost on that side. It is not often used intentionally. Null fill is produced by introducing sufficient power to fill the elevation nulls produced by high-gain multiple bays.
It is important to realize that the average FM transmitting antenna is basically non-directional. It is the method of installing the antenna on a support that distorts the pattern and can make it quasi-directional. Examination of the vertical and horizontal radiation patterns of an antenna mounted on a tower generally shows an appreciable difference between the Hpol and Vpol coverage. Engineers usually take advantage of this distortion by mounting the antenna to obtain the best coverage of desired areas and to dedicate Vpol coverage of heavily traveled roads serving auto receivers.
These differences in radiation patterns are produced by the presence of the tower itself. These effects vary depending on where the antenna is mounted — on the leg or the face of a tower, as well as the size of the tower and the actual construction and spacing between the antenna and tower. In cases of severe Vpol distortion, vertical parasitic elements are installed to correct the pattern and restore it to better circularity. In my opinion, the best basic circularity is obtained from a pole-mounted antenna on top of a tower or from panel antennas mounted on all sides of the tower.
When a directional pattern is required, the effects of antenna location on the tower, the mounting method and the tower iteself are considered in the pattern design. The final pattern is measured on an antenna range. The manufacturer's report then forms the proof that the FCC requires with the license application. This is very different from an AM DA, where the consulting engineer performs the proof and submits the data.