File:Helical UHF TV broadcast antenna.jpg
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Summary
| Description |
English: A normal-mode helical antenna for UHF television broadcasting, from an advertisement in a 1954 magazine. It consists of a helix of wire around a supporting mast, supported on standoff insulators. This type of antenna was widely used in the first television stations broadcasting in the UHF band, from 470 to 890 MHz, in the 1950s, and is still used. The antenna is divided into vertical "bays"; five are visible in the photo. Since the feed voltage is progressively delayed in phase as it progresses up the helical wire, without correction the radio waves radiated by each portion of the antenna would be out of phase with other portions, reducing the gain. So the antenna must have "phase shifters" at intervals up the pole (located at the joints between the bays) which correct the signal to each bay so it is in phase with the other bays. These consist of metal contact rings encircling the pole; to change the phase of a bay the helical radiator is rotated. Wire stubs can be seen sticking out from the antenna at top. These are quarter-wave directional stubs which are used to modify the omnidirectional radiation pattern of the basic antenna to give it more gain in directions where more audience coverage is needed. The vertical line of posts seen sticking out of the center of the mast are foot brackets for climbing the pole. Each bay has a gain of approximately 5 (7 dB). Information from NAB Engineering Handbook, 6th Ed., National Association of Broadcasters, 1975, p. 354, 370 |
| Date | |
| Source | Retrieved May 30, 2015 from Tele-Tech magazine, Caldwell-Clements Inc., New York, Vol. 13, No. 2, February 1954, p. 23 on http://www.americanradiohistory.com |
| Author | Unknown author |
| Permission (Reusing this file) |
This image is from an advertisement by General Electric without a copyright notice published in a 1954 magazine. In the United States, advertisements published in collective works (magazines and newspapers) are not covered by the copyright notice for the entire collective work. (See U.S. Copyright Office Circular 3, "Copyright Notice", page 3, "Contributions to Collective Works".) Since the advertisement was published before 1978 without a copyright notice, it falls into the public domain. |
Licensing
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This work is in the public domain in the United States because it was published in the United States between 1929 and 1977, inclusive, without a copyright notice. For further explanation, see Commons:Hirtle chart as well as a detailed definition of "publication" for public art. Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.
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| Date/Time | Thumbnail | Dimensions | User | Comment | |
|---|---|---|---|---|---|
| current | 01:56, 31 May 2015 | | 441 × 1,416 (56 KB) | Chetvorno | User created page with UploadWizard |
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