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A later report by the Institut fur Rundfunketechnik (IRT) in Hamburg in 1966 concluded that in non-line-of-sight locations in hilly or mountainous terrain, especially those that were covered with woods, "picture degradation was definitely worse with vertical" polarization. The reflections were not as severe in rough terrain that was not forested. As the study stated, "it would appear, that there exists in nature a considerable number of vertically oriented reflectors" During the late 1960s and throughout the 1970s most FM stations converted to circular polarization to take advantage of the higher power obtainable over horizontal polarization alone. The added belief that vertical car antennas would greatly benefit from the vertically radiated component added zest to the pace of conversion. |
In 1980 two reports, one Norwegian, one from the IRT in Munich, independently concluded that there were multipath problems resulting with the use of vertical polarization in mountainous or hilly terrain. Of primary importance for noncommercial FMs in the U.S. under the soon- - be-adopted TV-6 rules was the conclusion of the IRT study that Any reduction in the horizontal component, made to allow the introduction of a vertical component, will bring about inevitable losses in the coverage for domestic reception. The IRT based this finding on the phase rotation at diffraction crests and near reflective bodies that increased the nonlinear distortion and crosstalk, even when the receiver input signal voltage was higher with the circularly polarized transmission. These tests involved a "powerful transmitter" (ERP of 10 kW) and measurements taken at 212 locations throughout Nordhelle, a community described as at an altitude of about 2,000 feet above sea level in a moderately hilly region where there were urban areas and moderately sized industrial zones. The Norwegian report, however, concluded that for mobile reception in the semicircular fjords and changing height of the sea-surface, initial results from one FM translator showed an improvement in mobile and fixed reception by adding twice the ERP and circular polarization. No empirical data was presented to support this conclusion and no mention was made concerning stereo distortion. |
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