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Onda Longa / LW
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Carlos Gonçalves



Joined: 04 Jan 2007
Posts: 705
Location: Lisboa

PostPosted: Sun Feb 17, 2008 12:41 pm    Post subject: Radiofaróis em LF... e não só. Reply with quote

"Cylon":

Não há total contrasenso, nem a história é recente, bem pelo contrário.

De resto, há radiofaróis até dentro do que se chama onda média (!) (e como se a onda média fosse apenas o espectro usado pela radiodifusão entre os 530 e 1700 kHz).

Nos 270 kHz, não há só esse farol das Flores: cf. uma lista de 2000, havia, ou ainda há, 9.

Tb. cf. a mesma lista:

Em LF: freqs. dos 150, 160, 187, ..., 267, 267,5, 268, 270, 271, 299,5 kHz. Em MF: 300, 300,5,... 529,532, .., 1290, 1600... 1734 kHz.

Como vê, só entre 1290 e 1600 kHz há um espaço livre deles.

Repare, os sinais dos radiofaróis são fracos, é certo, mas são em telegrafia, modulada (ouve-se o tal "silvo") ou não (neste caso, é criado um, artificialmente, pelo rx, mediante um oscilador de batimento). Como tal, a largura de faixa necessária p/ escutá-los é muito pequena comparativamente c/ a largª p/ o sinal de radiodifusão modulado em amplitude, vulgo AM.

Ao estreitar-se a faixa de uns 10 ou 6-8 kHz p/ menos de 1 kHz, pode crer que pouco ou nada passa da radiodifusora, mas capta-se o outro.
De resto, os rxs a bordo de aeronaves e navios já vêm preparados p/ isso.

Do outro lado, do lado da as radiodifusoras, os sinais são suficientemente fortes na zona de cobertura p/ que sejam molestados pelos radiofaróis.

Espero que tenha conseguido fazer-me explicar.

73.
Carlos Gonçalves.
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Aljezur



Joined: 19 Feb 2008
Posts: 5
Location: Aljezur (Algarve)

PostPosted: Tue Feb 19, 2008 11:03 pm    Post subject: Efeito Luxemburgo Reply with quote

Ainda sobre o Efeito Luxemburgo, vale a pena ler o seguinte artigo britânico, que explica o que ocorre na Grã-Bretanha:

Quote:


Its the time of year when AM radio can be interesting as the changes from night to day, and the change from summer to winter reception conditions, are in a state of flux for a few weeks affecting the reception of signals. It is nowhere more evident than on Long Wave. A generation of radio listeners used to the clear reception of music on FM and DAB miss out on the fun and games going on trying to receive radio stations on the AM bands.

Listeners to RTE Radio 1 on 252 long wave will, at sunrise and sunset and through the night, hear another radio station playing music in the background. This is Medi 1, a long wave station based in Algeria, which has a very powerful 2,000 kw transmitter. Medi 1 is a commercial 24 hour pop station which broadcasts in French and Arabic to an audience of some 22 to 25 million people in France, Spain, Italy, Algeria, Morocco and Libya since 1980. Medi 1 is the primary user of the frequency 252 long wave and has priority use of the frequency, so to prevent interfering with it at night, RTE has to reduce its power from 500 to around 200 kw during the hours of darkness. There is a reciprocal arrangement, too; Medi 1 reduce their power at night to prevent interference to RTE Radio 1 in Ireland at night.

At the moment that RTE power cut is taking place at 5.03 pm UK time. The cut in signal strength is sudden and it suggests to your radio that it needs to compensate, so the automatic gain control inside your radio turns up the receiver’s wick. This has the effect of making the background station louder. During dark, especially here in south Essex where RTE Radio 1’s signal is weak anyway, the two stations become almost equal in reception.

Long distance reception of signals which are normally inaudible happens at night when the Heaviside layer, a layer of particles in the ionosphere, cools down and lowers closer to the earth’s surface and reflects the sky wave component of signals transmitted. At the same time, the ground wave component of signals transmitted which you hear during the day does not travel as far at night and shrinks back to the transmitters primary coverage area. So the switch from day to night reception conditions occurs as the direct ground wave disappears and your radio starts picking up the erratic reflected sky wave signals from the Heaviside layer instead.

Once this reception of two equal strength stations is under way, at this time of year there is another interesting natural phenomenon going on which is affecting reception, as well. This is known as the Luxembourg Effect because it was first noticed on the transmissions of Radio Luxembourg on 208 metres medium wave back in the 1930s. Its proper name is ionospheric cross modulation but the Luxembourg Effect is a much nicer name. In the case of 252 metres long wave, here in South Essex the strong signals from the BBC Radio 4 long wave service at Droitwich, using their 600 kw transmitter, are being received by the ionosphere, where the Heaviside Layer is broadcasting a distorted version of Radio 4 on 252. At around 6pm UK time if you listen to the RTE broadcast you will hear also in the background the BBC weather forecast and the Pips or Big Ben as appropriate before the News starts. That is the Luxembourg Effect.



in http://followthatmouse.wordpress.com/category/radio/
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Carlos Gonçalves



Joined: 04 Jan 2007
Posts: 705
Location: Lisboa

PostPosted: Tue Feb 19, 2008 11:15 pm    Post subject: Erros. Reply with quote

"Aljezur":

Esse artigo contém uma imprecisão: a emissora árabe (mais correctamente, franco-árabe) Médi 1 emite de Nador, NE de Marrocos, junto a Melilla, e sai nos 171 kHz 2000 kW e 9575 kHz 250 kW.

A estação (árabe) nos 252 kHz é a R.Algérienne, prgrs em árabe e francês; noutro tempo (e é possível que volte a fazê-lo), tb. o serviço internacional.

As 3, RTÉ 252, RA 252 e Médi 1 171, captam-se bem por aí.

73.
Carlos Gonçalves.
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