Why can AM radio waves travel over mountains while FM signals cannot?

[Haftred]

I'm curious why can AM radio waves be heard on the other side of a mountain from where they are propogated and FM not?

Thanks

 

[ceptimus]

The 'mountain' phenomenon is not really an AM/FM thing. It's just that AM tends to be transmitted on lower frequencies, that are better at going around objects, and happen to be (sometimes) reflected back to Earth by an upper layer of the atmosphere (the ionosphere).

FM is more usually transmitted on higher frequencies, and these are not so good at penetrating or going aroung objects, and (usually) pass straight through the ionosphere without reflection.

 

[Billc]

Is the reflection off the ionosphere why AM signals do not come in as well at night as during the day? If so can the stations somehow change the signal to compensate.

 

[sophiecentaur]

Is the reflection off the ionosphere why AM signals do not come in as well at night as during the day? If so can the stations somehow change the signal to compensate.

The reflection off the ionosphere is a real problem for which there's no good solution.
It allows (co-channel) transmissions from distant stations to get into the service area at night so that reception is only reasonable at short distances from the transmitter. In addition, the sky wave from a transmitter (reflected off the ionosphere) can bounce back and interfere with the direct, ground wave where the signal strengths are near equal and give severe 'fading' and distortion.
The only possible way to deal with multipath interference like that is to use a network of really low power transmitters, whose service areas are so small that the difference in level between ground wave and sky wave is high enough to reduce the interference. The sky wave has to make a total trip of several hundred km so the interfering signal will be very low compared with the ground wave signal at a distance of a few tens of km. The same strategy can help with co-channel interference but such a system needs a lot of different channels to be available so that a network can be set up which is similar to that for TV and VHF radio. There are not enough frequency slots for this in the MF bands (with only about 1Mz of spectrum space to play with ) to allow good coverage- certainly not in a densely populated country like the UK.

It is not helped by the fact that it's pretty well impossible (v. expensive at least) to design transmitting and receiving antennas that can 'point low enough' to suppress transmission and reception of the sky wave.

It must be true to say that an appropriate system of digital transmissions, using the same frequency slot, would work a lot better but that would require an impossible amount of international agreement and re-engineering. The fact is, though, that the available spectrum is very small and with such low capacity, it's hardly worth the bother of re-engineering the mf bands. Compare that available 1mHz of spectrum space at MF with the 10Mb data streams on many Broadband connections to get an idea of how useful it would be. Of course, when the bomb drops, we may be restricted to using all the old fashioned comms frequencies again.

OH, yes - and the ionosphere is much lower during the day (Sun's radiation) and the sky waves are absorbed in the more dense ionised layers. This is why it's not a problem in the day. Soon after sunset, the ions in the lower layers re-combine because the molecules are closer together and only the higher less lossy layers are available.

 

[Billc]

The reflection off the ionosphere is a real problem for which there's no good solution.
It allows (co-channel) transmissions from distant stations to get into the service area at night so that reception is only reasonable at short distances from the transmitter. In addition, the sky wave from a transmitter (reflected off the ionosphere) can bounce back and interfere with the direct, ground wave where the signal strengths are near equal and give severe 'fading' and distortion.
The only possible way to deal with multipath interference like that is to use a network of really low power transmitters, whose service areas are so small that the difference in level between ground wave and sky wave is high enough to reduce the interference. The sky wave has to make a total trip of several hundred km so the interfering signal will be very low compared with the ground wave signal at a distance of a few tens of km. The same strategy can help with co-channel interference but such a system needs a lot of different channels to be available so that a network can be set up which is similar to that for TV and VHF radio. There are not enough frequency slots for this in the MF bands (with only about 1Mz of spectrum space to play with ) to allow good coverage- certainly not in a densely populated country like the UK.

It is not helped by the fact that it's pretty well impossible (v. expensive at least) to design transmitting and receiving antennas that can 'point low enough' to suppress transmission and reception of the sky wave.

It must be true to say that an appropriate system of digital transmissions, using the same frequency slot, would work a lot better but that would require an impossible amount of international agreement and re-engineering. The fact is, though, that the available spectrum is very small and with such low capacity, it's hardly worth the bother of re-engineering the mf bands. Compare that available 1mHz of spectrum space at MF with the 10Mb data streams on many Broadband connections to get an idea of how useful it would be. Of course, when the bomb drops, we may be restricted to using all the old fashioned comms frequencies again.

OH, yes - and the ionosphere is much lower during the day (Sun's radiation) and the sky waves are absorbed in the more dense ionised layers. This is why it's not a problem in the day. Soon after sunset, the ions in the lower layers re-combine because the molecules are closer together and only the higher less lossy layers are available.

Thanks Sophie for the quick and information filled response.