Question

The frequency which is not a part of AM broadcast is:
(A) $100kHz$
(B) $700kHz$
(C) $600kHz$
(D) $1500kHz$

Answer 1

Before discussing the frequencies of AM broadcast, we should know what AM stands for. AM or amplitude modulation is a modulation technique, used for transmitting messages with a radio carrier wave. The signal strength depends upon the amplitude and hence the carrier wave can be modified according to the information signal.

Complete step by step answer:
AM wave signals follow the curvature of the earth, using ground wave propagation but can also bounce off the ionosphere at night, resulting in skywave propagation.
AM broadcasting is a radio broadcasting technology, which employs AM transmissions. AM radio broadcasting has assigned channels ranging from $540kHz$ to $1700kHz$ and spaced at $10kHz$ intervals.
Now we can check the options for whether they do or not lie within this specified range of AM broadcast. Only option (A), which says $100kHz$ , falls out of the specified range and hence it is the required answer. The correct answer is (A) $100kHz$ .

Additional Information:
AM broadcasting was the first method developed for making audio radio transmissions and is still used worldwide, primarily for medium wave transmissions, but also on the longwave and shortwave radio bands. As mentioned above, the channels for AM transmission are spaced at $10kHz$ intervals. So if your desired station is at $750kHz$ , adjacent channels lie $10kHz$ above and below at $760kHz$ and $740kHz$ respectively. Each $10kHz$ channel contains an upper sideband (USB) and a lower sideband (LSB).

Note: The reason why Amplitude modulation transmissions are still used is that its transmitters are less complex, its receivers are very cheap as no specialized component is needed. AM waves can travel a longer distance and have less bandwidth. AM forms the basics of almost all the major forms of modulation that are still being widely used.