File:Helical UHF TV broadcast antenna.jpg
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Dimensioni di questa anteprima: 186 × 597 pixel. Altre risoluzioni: 74 × 240 pixel | 441 × 1 416 pixel.
File originale (441 × 1 416 pixel, dimensione del file: 56 KB, tipo MIME: image/jpeg)
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| Descrizione |
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 |
| Data | |
| Fonte | 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 |
| Autore | Sconosciuto |
| Licenza (Riusare questo 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. |
Licenza
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Quest'opera è nel pubblico dominio perché pubblicata negli Stati Uniti fra il 1929 e il 1977, inclusi, senza un avviso di copyright. A meno che l'autore non sia morto da molti anni, non è nel pubblico dominio nei Paesi o nelle aree che non applicano la regola della durata più breve per le opere statunitensi, come Canada (50 pma), Cina (50 pma, ma non Hong Kong o Macao), Germania (70 pma), Messico (100 pma), Svizzera (70 pma), e altri Paesi con trattati individuali. Vedi questa pagina per ulteriori informazioni.
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| attuale | 03:56, 31 mag 2015 | | 441 × 1 416 (56 KB) | Chetvorno | User created page with UploadWizard |
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