收音機 AM 與 FM 頻道的分別 (20分)

They are of different frequency ranges.
AM broadcasting is of frequency range from 535~1705 kHz. Because of its low frequency, hence long wavelength(180~560 m), AM broadcasting waves undergo strong diffraction. Therefore, it is practically not blocked by any building.

Since the nature of long wavelength, there requires only one AM broadcasting antenna to serve the whole territory of Hong Kong. It is at present located at Peng Chau. The long wavelength of AM radio waves can also travel long distances, by refraction at the ionosphere (an ionzed layer at the upper atmosphere), to areas outside Hong Kong.

The disadvantage of AM waves is again because of its low frequency. Low frequency waves are associated with narrow bandwidth for each channel. In this case, the quality of (sound) signals transmitted by AM waves is not as good as those transmitted using higher frequencies.

FM broadcasting uses frequency range from 88~108 MHz. Because of the high frequency, and hence short wavelength (~3 m), diffraction effect is less prominent. This is why for good reception of FM signals, a receiving antenna is sometimes needed. Unlike AM transmission which doesn't need much relay station, FM transmission needs a number of relay stations (or repeater stations) to cover the whole territory of Hong Kong.

The advantage FM broadcasting is because of its high frequency, hence each channel can have a wider bandwidth. The quality of sound and music transmitted by FM is of higher quality than using AM.

The following web-page gives a good presentation on AM and FM waves, you may wish to take a look on it:

http://www.cybercollege.com/frtv/frtv017.htm

收音機 AM 與 FM 頻道的分別係頻率調製模式，由於要將我們聲音或訊號的頻率調製到能夠經過發射的無綫電波。

AM 是調幅，是用音頻訊號來改變發射電波的幅度，也就是說發射電波的頻率是固定的，改變的只是幅度罷了，所以，發射電波會根據訊號的大小來改變強度，適合任何的發射頻率。

FM是調頻，是用音頻訊號來改變發射電波的頻率，也就是說發射電波的幅度是固定的，改變的只是少許頻率罷了，發射電波強度非常穩定，適合任何超過50Mhz以上的發射頻率。

還有 。 。 。 。 。