I'm amazed at the people who do NOT take advantage of GOOGLE. There are over 120,000 articles about "polarization, like this one.
YOU'RE WELCOME!
From IBE Systems, E Norwich NY;
The question of the optimum choice of polarization for FM has historically been troublesome and confusing. The original plans for FM broadcasts in the U.S., made prior to World War II, called for horizontal polarization. Shortly after WWII, it was recognized that horizontal polarization was not suitable for mobile, omnidirectional reception of FM broadcasts in automobiles. This is due to the fact that the ‘whip’ antenna, used for the omnidirectional reception of AM and FM broadcasts in automobiles, is primarily a vertically polarized antenna and has a weak response to a horizontally polarized signal, (see Fig. 1). Mounting a separate omnidirectional horizontally polarized antenna on automobiles for FM reception is not a practical option. A similar condition exists for portable FM receivers having telescoping ‘whip’ antennas. In 1946 in order to overcome the deficiencies of horizontal polarization, the FCC authorized the use of ..... supplemental vertically polarized effective radiated power..." [Ref. 3]. In practice, this has come to mean "circular" or "dual" polarization in which presumably equal amounts of horizontally and vertically polarized energy are radiated. From an engineering rationale, the polarization used for transmission should be chosen so as to best match the polarization of the receive antennas used for car and home stereos. One may therefore ask " How many car or home FM receivers have a circularly polarized receive antenna? ". The answer is – zero to none. If one asks " How many have a horizontally polarized receive antenna ? "; the answer is - a tiny percentage. If one asks " How many have a vertically polarized receive antenna? "; the answer is - most cars, the bulk of today’s market, have a vertical whip receive antenna. Most home FM receivers have a wire hanging out the back. The conclusion to be drawn from this is that: for optimum match between the polarization of transmitted and received signals in order to provide optimum area coverage, vertical polarization is best.
This conclusion is supported by the highly successful results achieved in practice using vertical polarization. These results showed increases in signal levels that exceeded 5 dB as compared to the levels with equivalent CP antennas in both urban and rural areas. Besides increased signal levels, there are other important advantages to be gained by the use of vertical polarization: a) a vertical dipole has a null in the tower base direction, thereby minimizing radiation hazard in accordance with OSHA regulations. Also RF interference to nearby studio equipment and telephone lines is minimized, b) vertical polarization provides an extra 10-20 dB interference rejection between Low VHF (Channel 6) and FM, and c) interference by power lines and telephone lines is reduced. 2.3 Broadband Operation: The FM band is 88 to 108 MHz, and the FM channel width is 200 KHz. It is desirable that the FM transmitter antenna have multi-channel, broadband impedance performance (i.e. low SWR) in order to minimize frequency sensitive performance degradation and to allow multiplexed operation. Ideally, the antenna should cover the entire 88-108 MHz band.
