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Asaf Pe'er

Physics for all

Can radio telescopes get FM signals, i.e. can I listen to Red FM in space?

In principle, the answer is yes.

FM (Frequency modulation) is a technique of producing high quality radio signals. A radio signal is an electromagnetic wave, which is characterize by long wavelength, or low frequencies (well, low is a relative term...). FM radio stations use the VHF ("very high frequencies") part of the radio spectrum, which typically refer to 87.5 - 108 MHz frequencies. The typical wavelength is about 3 meters, which explains the size of radio antennas - which, for best performance need to be half that size.

The question of hearing the radio signal from space depends on two issues: the first is transmittance through the atmosphere. Although the atmosphere is transparent to visible light, it absorbs some other wavelength, while it easily transmits other wavebands very well. The reason is that the atmosphere is not empty, but contains various types of molecules, such as water vapour, etc, which absorb some frequencies. However, under most atmospheric conditions, the atmosphere is transparent to most radio wavelength, including VHF. This is why the radio band is in general important for astronomy - many huge radio telescopes use this property to observe the sky at the radio band, learning about radio emission from astronomical objects. Furthermore, connection with astronauts in space is done via radio transmission.

However, it can happen that the atmospheric conditions do not enable transmission of the radio waves. For example, if a temperature inversion occurs, with upper air warmer than lower air, VHF and UHF radio waves can be refracted over the Earth's surface instead of following a straight-line path into space or into the ground.

The second issue is of course that of the power: as you get further away from the transmitter, the power of the electromagnetic signal gets weaker. Far in space, it would simply be very difficult to detect the signal, as it will be too weak!.

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