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AM radio signals use carrier waves which are quite longer than in FM case. We know that in free space both high and low freq. RF signals should take the same distance in same period of time.

There must be a reason why in literature the longer EM waves(as in AM broadcasting case) can travel longer distances comparing to shorter waves(as in FM broadcasting case).

Does that mean "when we use the same power" in a real world application AM carrier signals can travel longer distances? Would FM carrier waves propagate longer distances as AM if much more power is used? Or is that only because AM waves can be reflected at the ionosphere and therefore FM broadcast can never be sent as far as AM? Or is that because short waves are effected by diffraction a lot more?

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    \$\begingroup\$ It is due to the Ionosphere. Consider that optical wavelengths and shorter can propagate astronomical distances ... if there is line of sight. Ditto VHF where FM is broadcast. But the ionosphere extends "line of sight" by reflecting the AM and short wave bands. You may notice AM reception range is longest at night when solar radiation isn't disturbing the ionosphere. \$\endgroup\$
    – user16324
    Commented Jan 16, 2016 at 12:36
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    \$\begingroup\$ do you know in which range of EM wave spectrum the waves can be reflected at the ionosphere? \$\endgroup\$
    – user16307
    Commented Jan 16, 2016 at 12:49
  • \$\begingroup\$ No, sorry. I do know it's variable and depends on solar activity and weather conditions. Before the Internet came along it was quite a serious hobby to map out this capability and exploit it for communication, usually between 1 and 30 MHz. I've listened to music from Hanoi on 21 metres (about 15 MHz) while restoring a radio from 1947. \$\endgroup\$
    – user16324
    Commented Jan 16, 2016 at 14:25
  • \$\begingroup\$ @Brian: Ionospheric bounce is real, and "short wave" stations in the 10-15 MHz range depend on it to get distant audiences. However, while you can sometimes pick up a distant 1 MHz station well outside its usual broadcast area due to ionospheric bounce, this isn't the main reason the usual broadcast area is larger in the first place. When picking up radio bounced off the ionosphere, you get constant fading in and out over a span of a few seconds. You don't get that from a 1 MHz station within its usual broadcast area. \$\endgroup\$
    – Olin Lathrop
    Commented Jan 16, 2016 at 15:16
  • \$\begingroup\$ @Olin : Yes. the constant fading is from different reflections cancelling or reinforcing each other (and actually you CAN get fading within the normal reception area when strong reflections cancel the ground wave) it's part of the variability that limits its usefulness. What I was aiming at in that comment is that (IMO) if the OP is looking for a map of what spectrum is reliable and when, he's probably out ef luck. \$\endgroup\$
    – user16324
    Commented Jan 16, 2016 at 15:26

2 Answers 2

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The answer is diffraction, and the fact that it takes larger objects to block the longer wavelengths.

1 MHz, which is in the middle of the commercial AM band, has a wavelength of 300 m. In contrast, 100 MHz, which is in the middle of the commercial FM band, has a wavelength of only 3 m.

300 m is large enough so that the waves can diffract around something the size of a typical house, for example. However, the house is much larger than 3 m, so it will largely block 100 MHz signal, assuming it is made of a material that blocks such frequencies. 1 MHz is much more able to "fill in" around house-size objects. At 100 MHz you get a lot more local dead zones and hot spots.

The real difference in propagation distance on the surface of the earth is due to the curvature and roughness of the earth. 300 m waves are able to refract around hills and the general curvature, whereas 3 m waves aren't. The smaller waves are more "line of sight" than the larger ones.

Of course there is still a huge difference between waves used for sight, around 500 nm, than those for commercial FM, around 3 m. The term "line of sight" for 3 m is therefore a bit misleading, the but the effect relative to 300 m waves is quite real. You can still pick up a 100 MHz station even with the antenna being a bit below the horizon while the visible beacon on the top of the antenna is completely blocked. But 3 m waves will get attenuated more quickly than 300 m waves as the transmitter gets further below the horizon.

Bouncing off the ionosphere is NOT the issue in most cases. It is true that 300 m waves can bounce off the ionosphere under the right conditions. This does allow picking up these stations significantly outside their usual broadcast area. However, ionospheric bounce is not why the usual broadcast area is larger in the first place.

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Lower frequency waves travel further as surface waves. They tend to follow the earth's curvature due diffraction and attenuation. Diffraction causes bending and is inversely proportional to the wave's frequency. Attenuation, along with energy loss, also makes the waveform to tilt downwards. This essentially helps in following the earth's curvature. Attenuation is directly proportional to the wave's frequency. enter image description hereenter image description hereenter image description here

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    \$\begingroup\$ Welcome to EE.SE! Here it is important to cite images that aren't yours. Also, adding some more context to the images greatly improves the answer quality. In your answer you should reference the figures rather than having a word block followed by an image block. \$\endgroup\$
    – Yet Another Michael
    Commented Oct 8, 2020 at 19:56

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