Polarization and Multipath, page 3

CHRONOLOGY OF POLARIZATION RESEARCH

"By radiating the vertical power in addition to the horizontal component, some receivers were able to receive stereo signals that could not with horizontal alone..."
Most FM chiefs have a heavy stag-horn fern planter or rusting doughnut ring on the roof to remind us that for a long time FM broadcasting was nearly universally horizontally-polarized. What brought about the change to circular (or more properly "elliptical") polarization? According to some, it was a 1965 report by Peter Onnigian of Jampro that prompted the conversion. Onnigian speculated that "perhaps due to a lack of technical knowledge about its usefulness" vertical polarization had received limited application in the United States despite being permitted by the FCC since 1946.

The study used a power divider to split the transmitter output power between a horizontally polarized antenna and a vertical antenna. By radiating the vertical power in addition to the horizontal component, some receivers were able to receive stereo signals that could not with horizontal alone (when the vertical power feed was fed to a dummy load). It is of little surprise that, when radiating twice as much power, more receivers would be triggered into stereo capture. This was precisely the point Onnigian set out to illustrate since existing FCC rules permitted the addition of the vertical component without requiring a reduction in the horizontal ERP.

Additionally, Onnigian demonstrated that mobile reception with a vertical whip resulted in much higher field intensity with the vertical signal being radiated in addition to the horizontal power. However, the issue of mounting current concern in the broadcast industry, stereo reception quality, was considered in determining differences in stereo separation at only four fixed receiving locations. The separation in the four tests average 21 dB between left and right channels and resulted in little discernible improvement, despite the addition of the vertical power.

No mention was made of the type of receiving antennas involved or geographic characteristics of the receiving sites. Sacramento, where the tests were conducted, is essentially smooth terrain. The study surmised, "While the number of [stereo separation] tests were limited, we feel it is indicative that vertical polarization does not in itself significantly improve stereo separation, nor does it degrade it." Onnigian not only concluded that adding vertical power to the horizontal power would improve signal strength in all receivers but summarily added that vertical power alone would be sufficient for automobiles, "for good limiting and elimination of picketing."

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