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American research into the tradeoffs between vertical,
horizontal and circular polarization is perhaps as important to public radio as it is
sorely lacking. The vagaries of transmission anomalies are in many respects so whimsical
that Americans might readily dismiss our lack of expertise as but a sign that we are all
mere propagation savages when it comes to fully understanding and quantifying the effects
of mixed versus pure RF polarization. Globally, engineers are well behind the sophistication Carl Sagan envisioned when contemplating "polarization modulation" in the novel Contact. In America, however, we are also lamentably behind the basic FM polarization studies conducted abroad. Although we are the undisputed leaders in deployment of local radio service and homogeneity of relatively high quality audio, little effort has been made until recently to reexamine our fifty-year-old assumptions on transmitting polarization. There is a growing body of knowledge, developed in other nations, that suggests accepted practice in American FM transmission methods may be less effective than it could be for many listeners. |
MULTIPATH: THE MODERN MENACE?
| Pat Hawker of England's Independent Broadcast authority restated an obvious observation for many engineers: "Multipath propagation is probably the most serious cause of the degradation of quality when FM stereo broadcasts are reproduced ." In this crucial area, the great weight of the evidence to date suggests that vertical polarization creates significantly greater reflections in areas of rugged terrain and high-rise urban structures than does horizontal polarization. This factor in and of itself is of little moment for many communications services, but where complex waveforms such as television and FM stereo multiplexing are involved, every engineer is imbued with the obligation to consider factors affecting multipath echoes within his or her control. More especially where extension of public radio service under the TV-6 rules is at stake, the question is increasingly important. |
| Where multipath is the norm, the plight of one NPR member station may be instructive on a simple method for combating the multipath menace. This station is located in a community that is a multipath magician's dream: rolling hills with a backdrop of 4,500 foot mountains, homes and freeways built in finger canyons, and densely populated coastal cliffs twenty miles from the FM transmitter. The multipath was horrendous. |
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The former chief engineer had considered relocating the
transmitter to a more central location, he had installed a coastal translator, and even
considered the idea of a booster in the central valley (Yeech!) [see why this might be a
bad idea in our FM Booster reprint -ed.] But the
most obvious tool that would help—going to mono on speech programs was an alternative
he avoided to keep from alienating the listeners who use the stereo pilot as a
"tuning" indicator. He simply didn't realize that by switching to mono he could
eliminate most of the audible effects of multipath reflections whose primary-secondary
path difference was less than 15 miles. See Table One.
Going to mono during speech programming is an effective aid but the underlying question remains: for stereo FM is one polarization more or less susceptible to multipath than another? The answer appears to largely depend on the physical characteristics of the receiving area. Next page | Return to Continuing Education
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