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comparison between AM and FM

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forte

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Hi. I am new here and I want to ask some questions related to AM and FM:
1. why is AM is cheaper than FM?

2. why AM can travel further compared to FM?
** for this question, I got several answers such that AM can use ground wave to travel and easier to be reflected by ionosphere. Another questions occur in my mind after reading the answer:
---- why AM can use ground wave while FM can't? I guess the answer is related to wavelength but I don't know for sure
---- why FM is harder to be reflected by ionosphere?

3. why FM is less susceptible to noise rather than AM?
** I know that noise can alter the amplitude of information signal transmitted, but why can't the noise alter the frequency as well?

Thanks for the help
 

When thinking about AM/FM in terms of actual radio transmission, don't forget that FM is generally used on much higher frequencies than AM. That's mostly because to carry information at a certain rate, needs a certain amount of frequency modulation. Putting that amount on a high frequency carrier is a smaller percentage change (therefore easier) than putting it on a low frequency carrier. So, FM is much easier to use at high frequencies than at low frequencies.

Now, back to your question...

1. Because it is much simpler electronically to encode and decode AM than FM. AM is so easy to detect that it's often done accidentally in amplifiers (like the one I made quickly a couple of days ago which instantly picked up a local transmitter!)

2. I don't think that FM CANNOT do those two things... it's about the frequency, not the modulation type. Low frequencies do those things better than high frequencies and FM (as above) is generally high frequency. (Your guess is correct. Long wavelength = low frequency. That's why radio amateurs interested in long range use long wavelengths, not VHF)

3. Atmospheric noise is usually very low frequency, so has practically no effect on the frequencies used with FM. The effect it has the amplitude (of both FM and AM) does not effect FM reception because the amplitude does not carry the information (other than needing to be strong enough for reception) whereas in AM the amplitude is the information, so AM is much more damaged by low frequency noise than FM.
 
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FoxyRick said:
1. Because it is much simpler electronically to encode and decode AM than FM. AM is so easy to detect that it's often done accidentally in amplifiers (like the one I made quickly a couple of days ago which instantly picked up a local transmitter!)
Click to expand...

What factors that make AM is simpler than FM?

2. I don't think that FM CANNOT do those two things... it's about the frequency, not the modulation type. Low frequencies do those things better than high frequencies and FM (as above) is generally high frequency. (Your guess is correct. Long wavelength = low frequency. That's why radio amateurs interested in long range use long wavelengths, not VHF)
Click to expand...

Why low frequency does those things better than the high one?
 

Facts:

1. The cost is entirely in the components used and there is no reason why one should be more expensive then the other. In many (most?) cases FM is cheaper.
2. AM signals and FM signals, under the same conditions, travel exactly the same distance. The kind of modulation does not change the distance it travels,
3. It isn't. Assuming you mean interference when you say noise, by the nature of it's creation, most is a change in the amplitude domain. Because FM is not reliant on amplitude changes, it is possible for the receiver to clip the signal level and remove the spikes that AM interprets as interference.

Brian.
 
betwixt said:
Facts:

1. The cost is entirely in the components used and there is no reason why one should be more expensive then the other. In many (most?) cases FM is cheaper.
Click to expand...

Then saying AM is cheaper than FM is incorrect? But my book wrote so.

2. AM signals and FM signals, under the same conditions, travel exactly the same distance. The kind of modulation does not change the distance it travels,
Click to expand...

My book also wrote that AM travels further than FM. Maybe you can explain why the book wrote so?
Is it correct to state that AM is easier to be reflected by ionosphere rather than FM? If yes, what is the explanation?

Thank you for the help
 

AM and FM receivers and transmitters are similar in complexity so their costs are never going to be much different from each other. There are some very simple AM receivers (Crystal radio for example) but their performance is usually poor. Good performance receivers only differ in the kind of detector they use and that is only a low cost part of the system. AM transmitters are generally more complicated than FM ones and the need for linear amplification after the modulation stage makes them less efficient and more complicated to design than FM types.

As far as distance is concerned, it's the strength of the signal that decides how far away it can be received, not the type of modulation. Consider this situation: the transmitter microphone is unplugged so only a silent carrier is transmitted. Because it has no modulation, it's frequency and amplitude would be constant, so why should it make any difference what kind of transmitter sent it?

I think your book is in error, it confuses AM with low frequency broadcasts and FM with VHF broadcasts. It is the frequency that decides the propagation not the modulation method. In truth, there is nothing stopping FM being used at low frequencies or AM being used on VHF and above. There are several FM broadcasts at frequencies below 100KHz which can be received everywhere in the World, there are also AM broadcasts on microwave frequencies with very short range.

Brian.
 
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    forte

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I also read from some sources that signal with longer wavelength can travel further compared to signal with shorter wavelength. Is that correct?
Based on your post, the strength of the signal decides the distance. In my opinion, longer wavelength will have lower frequency so it has lower energy. Hence, shorter wavelength should be able to travel further.
 
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forte,
Good question.
AM waves echo back from ionosphere. That's the reason why the reach longer distances. Whereas the FM waves travel just like that. So the important point here is: The AM waves hit ionosphere and then get back to the receiver. So it covers larger area.
 

ElectroEnthusiast said:
forte,
Good question.
AM waves echo back from ionosphere. That's the reason why the reach longer distances. Whereas the FM waves travel just like that. So the important point here is: The AM waves hit ionosphere and then get back to the receiver. So it covers larger area.
Click to expand...

Ok. Then, why AM can echo back from ionosphere while FM just passing through ionosphere?

Thank you for the help
 

forte said:
Hi. I am new here and I want to ask some questions related to AM and FM:
1. why is AM is cheaper than FM?

2. why AM can travel further compared to FM?
** for this question, I got several answers such that AM can use ground wave to travel and easier to be reflected by ionosphere. Another questions occur in my mind after reading the answer:
---- why AM can use ground wave while FM can't? I guess the answer is related to wavelength but I don't know for sure
---- why FM is harder to be reflected by ionosphere?

3. why FM is less susceptible to noise rather than AM?
** I know that noise can alter the amplitude of information signal transmitted, but why can't the noise alter the frequency as well?

Thanks for the help
Click to expand...
About the 3rd Question,You have answered a part yourself, Noise alters the amplitude and in AM transmitter,information is superimposed over the carrier frequency,so any change in amplitude,will result in a error in AM receiver...

But in the case of FM modulation,the very very slight change in frequencies is detected by the FM receiver and reproduces,any change in amplitude will not affect the FM transmission...
hope i answer the last question

ANd for your first question, Decoding AM signals are easy,just with a Diode and few parts one can build AM receiver, but where as in FM ,it much complex and to get excellent reproduction audio from FM ,one has to use the superhetrodyne Receiver,which is complex and costly method...
 

Please click Helped, if you like the previews post.

It's just because ionosphere has the ability to absorb AM Waves.
And they use this effect to enhance the reach of AM waves.
 

Do not confuse modulation (AM DSB,AM SSB FM, PM,PCM and others ) with the carrier wave.
The frequency of the carrier will determine the propagation characteristics, not the modulation
 
klystron said:
If you FM modulate an HF carrier you will get ionospheric propagation. In the The Ionosphere and Radio Wave Propagation URL the AM is referring to a HF carriers.
Click to expand...
True, even i had a doubt abt AM, thanks for correcting it.
but again, i contradict you... FM = MHz ; AM = KHz. So FM is High Freq, yet AM can undergo ionosphere propagation.
 

ElectroEnthusiast said:
True, even i had a doubt abt AM, thanks for correcting it.
but again, i contradict you... FM = MHz ; AM = KHz. So FM is High Freq, yet AM can undergo ionosphere propagation.
Click to expand...

HF is a term used to mean frequencies up to 30MHz, so saying AM is usually used at HF is correct. Some FM is used in HF as well - like 27/28MHz bands, so it's not exclusively AM. And yes, 27MHz FM can travel just the same as 27MHz AM (old CB'er here!)

FM is generally used at VHF (and above) rather than AM. VHF=30MHz to 200MHz.
 

VHF aircraft radios are AM. Analog TV broadcasting (US) used FM for audio, AM for video.
 

I still stand with my original answers.

1. You CAN build simple, inexpensive, single transistor FM transmitters and receivers.
2. The distance a radio signal travels is totally unrelated to the kind of modulation, whether its AM, FM ,PM or any other kind of signal.


AM does not mean KHz and FM does not mean MHZ. It's a bit like saying red lights are round and blue lights are square, the two attributes are not in any way related.

LOW frequencies in some circumstances have their range extended by ionospheric reflection but it depends on other factors as well, the modulation type is not one of them.

Brian.
 

I think my first and third questions have been answered yet but I am still confused about the second question.

Why longer wavelength signal can be reflected by ionosphere better than shorter wavelength signal?


@ electro enthusiast: I don't understand your post
"It's just because ionosphere has the ability to absorb AM Waves. And they use this effect to enhance the reach of AM waves."

My interpretation of your post is that AM waves can be absorbed by ionosphere, means that they are not reflected but vanished because they have been absorbed. Can you pleaase explain it more?


Thank you for the help
 

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