Why cant FM be transmitted over long distances like AM?

Answer 1

Bestonne has a good answer, while I really do have to wonder about Yoho. Yoho used jargon that just doesn't make a whole lot of sense. The description of modulation is strange, at best.
Anyway, the distances for either AM or FM really do depend upon the frequency in use. If a frequency in the standard AM range were FM modulated, it would have the same kind of range that an AM signal would. Operating frequency is the deciding factor, coupled with the effects of the Ionosphere on the signal. The higher the frequency, the greater the likelihood that it will just go straight out into space and not be deflected back to earth. For a quick point, some of most radio waves are reflected back to earth, regardless of their frequency. Microwave communications relied on a thing called Troposphere scattering for long range communications. You just are not going to see that in common consumer radio use.

Answer 2

It can if you use a different frequency.

The reason that AM transmit over long distances at night is because it is based in the MF (medium frequency) band while FM is based in the VHF (very high frequency) band.

MF in the day time is absorbed by the D region of the ionosphere so that MF signals don't travel very far but at night there is no D region so the MF signals can propagate all the way up to the F region where they can be reflected back to Earth and end up a long way away (which is why a lot of AM stations have to shut down for the night so they don't interfere with stations on the other side of the continent) whereas VHF just goes right through the ionosphere and out into space without being reflected.

Answer 3

The frequency of AM radio is from 0.55 MHz up to 1.60 MHz. The frequency of FM radio is from 88 MHz up to 108 MHz. The two different frequency bands propagate over the Earth in different ways. AM radio travels by 'ground wave'; the radio waves hug the ground and follow the curvature of the Earth over the horizon, so an AM receiver can hear a signal from an AM transmitter that is located relatively far away. In addtion, at night, the ionosphere is thicker and stronger, and this high altitude atmospheric layer acts like a mirror to radio waves in the AM band. So, AM stations can be heard over especially large distances at night. FM signals, at the much higher frequencies of around 100 MHz, propagate only by "line of sight", which means if the transmitter is not visible directly by the receiver, the signal cannot be picked up. This also is the case for broadcast TV signals. That's why FM and TV transmitters tend to be put on hilltops or mountain peaks, or in flat lands, on tall towers or buildings. This gives greater line of sight distance for receivers for these signals. So it has nothing to do with the modulation method (amplitude or frequency modulation). It has to do with the physics of radio propagation over the ground and through the sky.

Answer 4

There are two different ways of transmitting radio signals.

Amplitude modulation is the method of combining the carrier wave and electric wave such that the amplitude of the carrier waves varies to match the change in the electric waves. Frequency modulation is the method of combining the carrier and electric wave so the frequency of the carrier waves changes to match that of the electric waves.

An antenna sends out two kinds of AM radio waves – ground waves and sky waves. Ground waves travel directly to a person’s home via the earth, while sky waves travel up into the sky, hit the ionosphere, and are reflected back to the earth. This enables AM broadcasts to be received at great distances from the place of broadcast.

An FM radio antenna sends out waves that travel the same direction as AM radio waves, but these waves are not reflected. Therefore, FM transmissions cannot reach as far as AM transmissions. The advantages of FM broadcasts are that they are not as affected by static as much, and that they produce a truer reproduction of the original sound.

Answer 5

because FM has shoter wavelength than AM....and it does not bounce of in ionosphere by itself due to high frequency.it requires some repeating station to transmit it to a long distance just like a cellphone that requires a cellsite