(N/A) Difference between AM and FM radio?

RyanB

RyanB

Super Administrator
Management Council Member
PoA Supporter
Joined
Jul 21, 2010
Messages
16,524
Location
Chattanooga, TN
Display Name
Display name:
Ryan
I know I could sort through a laundry list of information on the web, but I figured it would be easier to ask here.

How does AM radio work compared to FM? And why is FM so much clearer?

Reason I bring this up is during my drive home I tuned to 650 AM WSM Nashville and could pick up the Grand Ole Opry. Of course it had a little static, but otherwise clear as a bell. We can’t pick up Nashville’s FM stations down here in Chattanooga, but I could hear it on AM.

I know...I should know this already, but frankly, I don’t.

<throws the grenade> <takes cover>

Thanks in advance

Ryan
 
In short? Modulation. Amplitude or Frequency.

for a long, and I mean long answer, you need @denverpilot. Nate can no doubt give a long explanation. I think @weirdjim can also provide much info on this.
 
Ravioli said:
In short? Modulation. Amplitude or Frequency.

for a long, and I mean long answer, you need @denverpilot. Nate can no doubt give a long explanation. I think @weirdjim can also provide much info on this.
Click to expand...
Thanks. @weirdjim was actually the one I was targeting with this question, as I know he has an extensive background with this type of subject.
 
A little music while you’re waiting for a definitive answer...
The girls don't seem to care what's on
As long as they play till dawn
Nothin' but blues and Elvis
And somebody elses' favorite song

Give her some funked up music
She treats you nice
Feed her some hungry reggae
She'll love you twice
The girls don't seem to care tonight
As long as the mood is right

No static at all
(no static, no static at all)
F.M.
(no static at all)
Click to expand...
 
Ryanb said:
Thanks. @weirdjim was actually the one I was targeting with this question, as I know he has an extensive background with this type of subject.
Click to expand...
Can I ask that you also pile on QAM, QPSK and OFDM? :)
 
AM vs. FM is just how the audio is modulated on the carrier. It doesn't have anything to do with how, or how far, the signal propagates. That difference is due to the characteristics of the frequency band of the carrier.

Commercial AM radio is 525kHz - 1705Khz. (525,000 - 1,705,000)
Commercial FM radio is 87.5MHz - 108.0MHz. (87,500,000 - 108,000,000)

Lower frequencies propagate farther and can skip off the upper atmosphere to reach beyond the horizon. Higher frequencies are more line-of-sight.
 
Static occurs because atmospheric (and other) interference gets superimposed on the audio signal in Amplitude Modulation. In Frequency Modulation, the deviation of frequency of the signal determines the audio you hear. A spike from lightning or nearby interference doesn't affect that. As an FM signal gets weaker, what you get is an increase in the background noise, until the thing craps out entirely.

As pointed out, the distance propagation has little to do with the modulation, the AM broadcast band is a much lower frequency and will travel beyond line of sight as it gets reflected by the ionosphere, especially at night.
FM broadcast is on the VHF frequencies (much like your Amplitude Modulated aviation radio) and fairly limited to "line of sight." There are some exceptions (tropospheric ducting) that can open up long range VHF communications, but it's not as easily predictable as at the HF frequencies.

And to answer the "Why is aviation radio AM?" question: tradition more than anything else. Same reason your aviation mic plug circuit still emulates an old telephone transmitter carbon button.
 
FM change frequency to transmit info, keep amplitude constant.
AM change amplitude to transmit info, keep frequency constant,
PM change phase to transmit info

XM receive info from satellites
SM stream info over Internet
LV listen to live music
LP listen to vinyl
 
AM change amplitude to transmit info, keep frequency constant,
Click to expand...
Actually the frequency of the sidebands change with the frequency of the modulating signal.
 
You must log in or register to reply here.
Back
Top