Antenna Polarization--was (Re: XETV San Diego and KSBY San Luis Obispo)
w9wi said:
Garrett said:
Also, don't same frequency stations usually broadcaset differently (and not exactly on the same frequency), like XETV transmitting horizontally and at 5.9, with a differet 6 going Vertical and at 6.1?
All TV stations in the U.S. and Canada are horizontally polarized. (I'm pretty sure this is the case in Mexico too, but not 100% certain) A small number of stations also have a vertically-polarized component.
Sorry W9... but your reference to horizontal polarization is not accurate.
Many US stations transmit with circularly polarized emissions, to help reduce "ghosting" and problems that occur when the viewed station's signal hits the antenna from multiple paths (multi-path).
Check the FCC database at
http://fcc.gov/fcc-bin/audio/tvq.html for television or
http://fcc.gov/fcc-bin/audio/fmq.html for FM stations to see what polarization a station uses.
Horizontal polarization was the original polarization used for VHF, which includes North American TV channels 2-13 and the FM band, which falls between TV channels 6-7.
As more technical and "real world" experience developed over the last 60 years, new concepts were sought to improve reception for the end-user and to extend the coverage for broadcasters, which improved the revenue possibilities of stations, particularly where reception was a problem.
Some history on CP as it relates to FM is found here:
http://www.acmi.net.au/AIC/RADIO_HIST_WHITAKER.html
"Circular polarization of the transmitted signal was another major step for FM radio. One of the early proponents was KPEN-FM, Atherton (on the San Francisco Peninsula), which later would become KIOI (better known as K-101), San Francisco. KPEN received a special temporary authorization from the FCC in the later part of 1963 to start testing the effects of adding a vertical component to the existing horizontal signal.
A second Western Electric 10kW transmitter was purchased and modified to provide the needed power. Separate vertical dipoles were manufactured and installed on the station's tower. With this setup, engineers were able to vary the phase relationship and amplitude so that the station could switch from a horizontally polarized signal to a circular pattern. Monitoring points were established in rugged areas of San Francisco to observe the results. It was found that as the vertical component of the transmitted signal was increased, reception of stereo signals improved significantly.
At the same time, Lew Wetzel of WFIL-FM was proving to the commission that the vertical component of a circularly polarized transmission did not extend the 1mV contour. With these two reports, the FCC decided that it would indeed be in the public interest for FM stations to transmit with circular polarization."
Details on antenna polarization can be found in Chapter 8 of the "Television Engineering Handbook" by K. Blair Benson, McGraw Hill Publishing.
Here's a pretty good description of what happens at the receiver end, as written by Brian Beezley in a construction guide for an FM antenna using rabbit ears.
http://users.tns.net/~bb/rabbit.htm
The FM band's between Channels 6 and 7, so you might have some fun with this concept for TV.
"When a circularly polarized signal reflects from a surface, the circularity reverses. Thus right-circular signals produce left-circular reflections. If you orient a circularly polarized antenna so that reflections arrive at opposite-circularity nulls, you can reduce multipath distortion on FM signals. Unlike the simple pattern nulls characteristic of linearly polarized antennas, opposite-circularity nulls of circularly polarized antennas occur within the desired-circularity main lobes. Thus you can maximize the desired signal while rejecting multipath reflections from the same general direction. With the bidirectional nulls of rabbit ears, you can simultaneously reject reflections from the front and rear.
Rabbit ears reduce opposite-circularity reflections by at least 10 dB over nearly 40 degrees in azimuth. This wide range allows the antenna to reject diffuse forward diffraction components that may accompany a signal scattered over a range of hills or mountains. For a single specular reflection, orient the antenna to place a null at the reflection angle. The broad main lobe permits good direct-path pickup even when nulling a wide-angle reflection."
Ted.
