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Inspired by this question:

What is the basic difference between AM and FM radio?

I got to wondering: If I put my FM radio on some radio channel (Let's just say 100 MHz to pick some number), shouldn't it just pick up whatever is transmitted at 100 MHz? If you modulate the frequency you are transmitting with, why do I not only pick up, essentially, the equivalent of an AM signal at 100 MHz, instead of picking up on frequencies that are not 100 MHz (Since they've been Frequency Modulated, they're not 100 MHz, which is what my radio is on).

Now obviously FM radio WORKS, so I must be misunderstanding something - but what?

Is this question "why does it pick up a band (100MHz to eg. 100MHz+20kHz) rather than a single point frequency?" – pjc50 5 mins ago

Yes, that is a fair rephrasing. In my head, an antenna is built + filtered for one frequency. If it's picking up stuff at other frequencies, that's unpredictable and definitely not a way to transfer a signal - yet clearly I'm wrong. How?

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shieldfoss
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  • To play the devil's advocate: which AM radio station is broadcasting at 100MHz? :P – Polynomial Sep 03 '13 at 16:13
  • Is this question "why does it pick up a band (100MHz to eg. 100MHz+20kHz) rather than a single point frequency?" – pjc50 Sep 03 '13 at 16:23
  • Would it be too much to suggest you read up on the answer to the first question first, that may help with this... – John U Sep 03 '13 at 16:24
  • @Polynomial Pick a frequency; there's nothing in the math preventing FM at 5 Hz or AM at 2 GHz except how terrible the broadcasting would be. – shieldfoss Sep 03 '13 at 16:28
  • @pjc50 Yeah, that's probably a fair rephrasing. – shieldfoss Sep 03 '13 at 16:28
  • @JohnU I've read all the answers to that post, none of them answer my question. I know what the difference between AM and FM is, I just don't understand how FM works in the first place. – shieldfoss Sep 03 '13 at 16:29

2 Answers2

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Your FM radio works because it receives a band of frequencies around the frequency it shows or that you tune it at. As you say, the nature of FM means that the frequency will vary. However, the extent of these variations is well defined, and the radio is designed to "find" the carrier anywhere in this range.

This may surprise you, but AM also requires the receiver to pick up a range. I'm not going to go into Fourier analisys right now, but basically changing amplitude is adding frequencies. Put another way, a true pure single frequency can't ever change in amplitude and can't carry any information.

The way AM works, there is a frequency band on either side of the carrier that is the width of the highest frequency that can be transmitted. For example, if the signal being modulated onto the AM carrier can be up to 10 kHz, then there is a 10 kHz band of stuff on either side of the carrier. In fact, the carrier is constant and the actual information is in these side bands. Yes, I know that may be unintuitive, but at the level you are asking and what I have time to explain here you'll just have to trust me on this. Look up Fourier analisys if you want to learn more.

For example a AM radio station a 1 MHz carrying up to 10 kHz content will have a signal spread out over the range of 990 kHz to 1.01 MHz.

Olin Lathrop
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    What is the range for FM? If I am listening to an FM radio at 100 MHz, what range am I really listening to? – Koray Tugay Jan 25 '14 at 20:12
  • @Koray: I don't remember what the deviation is for commercial FM. However, it can't be more than 1/2 the distance between stations. I think that's 200 kHz, but that's just from memory without looking it up or a FM radio to look at right now. – Olin Lathrop Jan 26 '14 at 13:58
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    @KorayTugay The deviation is +/-75KHz but that includes SCA so for what you actually receive in a normal tuner it is 90% of that. The station spacing is 200kHz to allow a margin; 400kHz in the same city. – user207421 Jan 31 '14 at 12:56
  • What's the reason that a true pure single frequency can't ever change in amplitude? Is it because a single frequency changing in amplitude can be described as the single frequency plus the frequency of the amplitude change? – David Liu Dec 02 '14 at 18:57
  • @David: I go into more detail about AM modulation at http://electronics.stackexchange.com/a/131640/4512 – Olin Lathrop Dec 02 '14 at 20:22
  • @DavidLiu A pure single frequency is $Asin(\omega t + \phi)$ where $A$ and $\omega$ and $\phi$ are constants. That's the defintiion of a pure single frequency. – user253751 Apr 20 '18 at 06:00
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The key thing to understand about analog electronics is that frequency is a continuum. Any circuit's behaviour for frequency X will apply to $X \plusminus \epsilon$, to some extent.

All antennas will, suprisingly, pick up all frequencies. An antenna (in fact, any piece of unshielded wire) will have a frequency response, such that certain frequencies are recieved more strongly. The FM antenna will pick up signals most strongly in the FM band, signals nearby less strongly, and signals far away much less strongly.

The antenna is linked to an LC resonant circuit which is tunable and again has narrow (but not infinitely narrow!) frequency response: signals at or close to its resonant frequency get amplified by, effectively, adding to echoes of themselves.

Occasionally a very strong local signal will be picked up louder than a weak desired signal; people near airports sometimes hear the traffic control radio on their FM radios despite it being transmitted on a very different band.

As to "definitely not a way to transfer a signal": it's the only way it's possible.

pjc50
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