Why does AM radio suck so bad???

[sao123]

I read a little about AM radio but i fail to understand...

Why does power wires cause so much interference? Particularly in the Even frequency ranges... 600hz, 1000Hz, 1200Hz

This is especially troublesome since AM is the preferred band for emergency broadcast situation. I learned in history class...way back when... that in case of a nuclear attack to have an AM radio in your bunker... (I dont understand why)

but I am so annoyed, because in my region... ESPN is on AM radio on 1250Hz, and every time i go near a power line in my car... I get so much static the station is unusable.
Is there a antanna filter or booster or somehting to help my signal interference?

 

[BrownTown]

Well, power circuits have a frequency of 60 hertz. Id say the problem is most likely due to the fact that the power lines have transients at pretty much every multiple of 60, and that the transients in the frequency range you are using are large enough to overpower the signals from the radio transmitter.

At least thats the only explanation i can come up with that makes any sense...

 

[MAW1082]

The entire problem with AM is static. Due to background radiation, other sources on earth etc. there is alway interference with the amplitude portion of a signal.

FM is noiseless because the modulation scheme (frequency modulation) uses a shift in frequency of teh transmitted signal to symbolize a shift in amplitude of the original baseband signal.

AM is still useful, especially for simple communication, but FM is far superior for transmitting analog sound / music.

 

[Stas]

don't AM waves travel further than FM?

 

[Gibsons]

Originally posted by: Stas
don't AM waves travel further than FM?

AM gets reflected off the ionosphere. At night you can hear clear channel stations (meaning those with no other stations on the same frequency; not to be confused with the Clear Channel corporation) from a LONG way away. I've heard a Dallas station (820) in Atlanta, longer distances are certainly possible. FM signals don't bounce off the ionosphere, so they are mostly restricted to line-of-sight transmission.

 

[theMan]

yeah, also, shortwave radio travels even further than AM, you can receive signals from across the ocean.

 

[gsellis]

Originally posted by: Gibsons

Originally posted by: Stas
don't AM waves travel further than FM?

AM gets reflected off the ionosphere. At night you can hear clear channel stations (meaning those with no other stations on the same frequency; not to be confused with the Clear Channel corporation) from a LONG way away. I've heard a Dallas station (820) in Atlanta, longer distances are certainly possible. FM signals don't bounce off the ionosphere, so they are mostly restricted to line-of-sight transmission.

I used to tune in nightly to a Nashville station from Orlando in the early 80's. Yep, skip is fun.

 

[DrPizza]

There are also high quality AM tuners that you can purchase.. I have a friend who loves to listen to various talk shows on AM radio far more than listen to the fm stations (I was amazed at the variety of shows available - much of the information is quite useful)

And, for what it's worth, there is a noticeable difference between the refraction for AM and FM signals - noticeable if you live in a hilly/mountainous area

 

[theMan]

i could recieve new orleans station am 870 in the mornings before my local lansing, Michigan station came on the air at 8AM. thats like 1000 miles isnt it?

 

[Smilin]

Think of what an AM signal wave looks like
Think of what an FM signal wave looks like.

If you add a spike of noise to an AM signal it actually changes the amplitude and distorts the signal being carried.

If you add a spike of noise to an FM signal it again changes the amplitude but that's not where the signal is being carried.

For FM it takes consecutive spikes of noise to really cause problems.

 

[josh609]

Originally posted by: theman
yeah, also, shortwave radio travels even further than AM, you can receive signals from across the ocean.

Yes. I was down in Costa Rica on a missions trip and i had a short wave radio with me, and i was able to hear a radio station from Bulgaria. I think i also heard somthing from China, or some asian continent.

 

[pm]

So, why use AM modulation at all on the "AM band"? Why not use FM modulation? Is there something inherrent to those AM frequencies that means that FM can't be used (ie. bandwidth)? If FM is inherently less noise-prone than AM (and it is) then why does anyone use AM at all nowadays?

Edit: well, I got a huge earfull from all the engineers sitting around me on why you can't do FM on AM bands:

1. cost for broadcasting... people have investments in AM broadcasting statoins
2. legacy radios in people's houses wouldn't work any more.
3. not enough frequency bandwidth (you'd need to reduce the number of channels/stations to do it... even allowing for mono-FM)
4. breaks cross-border stations (so AM radio receivers in Mexico and Canada would be affected by FM "noise" which would be worse than white noise to deal with).

and the cacophany of responses from all the guys sitting around me probably mentioned a few other things that I forgot.

I wonder if you can do anything useful with the AM separators... maybe throw some digital packet-based stuff in there.

 

[Molondo]

I thought EMI had a big part....?

 

[brentkiosk]

There are a couple of issues here. As has been pointed out, a lot of the power line noise has to do with the type of detection used in AM. Most of the noise appears as amplitude variations, so an AM radio is likely to detect it. FM detection if perfect does not notice amplitude variations, so does better.

But there are other reasons. The AM band runs from about .55 to 1.6 MHz and the FM band from about 88 to 108 MHz. The noise amplitude at the 100 times higher frequency range for FM from all sources is just plain much less, so even AM signals (such as aircraft radios) in the VHF range suffer much less from noise.

The greater distance for AM as compared to FM signals is mostly due to the frequency difference, not the type of modulation. As someone said, long distance involves reflection from the ionosphere - a group of ionized layers in the upper atmosphere caused by radiation from the sun. Over the horizon communication by radio/TV is accomplished when the radio waves go up to the ionosphere at an angle and are reflected back down to earth. Far distances inviove reflection from a higher layer (or multiple reflections). Longer waves reflect more than shorter waves (there's more to this. Time of day matters, and where we are in the sunspot cycle comes into play as well.). The waves in the AM band are roughly 300 meters long - in the FM band it's about 3 meters. The short FM band waves are not reflected much at all, so one rarely hears stations at a great distance. The longer AM band waves reflect a lot. In fact, during the day, they reflect from a low layer of the ionosphere and don't get far. At night (no sun to make more ions) the low layer dissipates and the reflect from a more persistant high layer and that's when you hear distant stations.

 

[MAW1082]

Originally posted by: pm
So, why use AM modulation at all on the "AM band"? Why not use FM modulation? Is there something inherrent to those AM frequencies that means that FM can't be used (ie. bandwidth)? If FM is inherently less noise-prone than AM (and it is) then why does anyone use AM at all nowadays?

Edit: well, I got a huge earfull from all the engineers sitting around me on why you can't do FM on AM bands:

1. cost for broadcasting... people have investments in AM broadcasting statoins
2. legacy radios in people's houses wouldn't work any more.
3. not enough frequency bandwidth (you'd need to reduce the number of channels/stations to do it... even allowing for mono-FM)
4. breaks cross-border stations (so AM radio receivers in Mexico and Canada would be affected by FM "noise" which would be worse than white noise to deal with).

and the cacophany of responses from all the guys sitting around me probably mentioned a few other things that I forgot.

I wonder if you can do anything useful with the AM separators... maybe throw some digital packet-based stuff in there.

The band that Am radio is allowed to use was designated to it by the FCC. The same exact thing for FM.

Anyone interested in AM/FM radio should read about how the two were invented / discovered by Edwin Howard Armstrong. It's the classic story of man versus powerful corporation and government. This struggle decided which band AM/FM was each allowed to use.

 

[stardrek]

Originally posted by: brentkiosk
There are a couple of issues here. As has been pointed out, a lot of the power line noise has to do with the type of detection used in AM. Most of the noise appears as amplitude variations, so an AM radio is likely to detect it. FM detection if perfect does not notice amplitude variations, so does better.

But there are other reasons. The AM band runs from about .55 to 1.6 MHz and the FM band from about 88 to 108 MHz. The noise amplitude at the 100 times higher frequency range for FM from all sources is just plain much less, so even AM signals (such as aircraft radios) in the VHF range suffer much less from noise.

The greater distance for AM as compared to FM signals is mostly due to the frequency difference, not the type of modulation. As someone said, long distance involves reflection from the ionosphere - a group of ionized layers in the upper atmosphere caused by radiation from the sun. Over the horizon communication by radio/TV is accomplished when the radio waves go up to the ionosphere at an angle and are reflected back down to earth. Far distances inviove reflection from a higher layer (or multiple reflections). Longer waves reflect more than shorter waves (there's more to this. Time of day matters, and where we are in the sunspot cycle comes into play as well.). The waves in the AM band are roughly 300 meters long - in the FM band it's about 3 meters. The short FM band waves are not reflected much at all, so one rarely hears stations at a great distance. The longer AM band waves reflect a lot. In fact, during the day, they reflect from a low layer of the ionosphere and don't get far. At night (no sun to make more ions) the low layer dissipates and the reflect from a more persistant high layer and that's when you hear distant stations.

I would like to throw in that during the day, because of the presence of a lower ionosphere the transmitions that take place on AM bands are said to propagate using groundwave reflection rather then ionic reflection. Groundwave reflection means that the frequencies are reflected via diffraction from the earths surface. This also explains why power stations/lines/junctions would have a much greater affect on the quality. Because the frequencies are bouncing so close to the earths surface, they run into more items that can affect the signal.

 

[RU482]

Originally posted by: Gibsons

Originally posted by: Stas
don't AM waves travel further than FM?

AM gets reflected off the ionosphere. At night you can hear clear channel stations (meaning those with no other stations on the same frequency; not to be confused with the Clear Channel corporation) from a LONG way away. I've heard a Dallas station (820) in Atlanta, longer distances are certainly possible. FM signals don't bounce off the ionosphere, so they are mostly restricted to line-of-sight transmission.

hahaha, I think I was listening to 820 out of Texas on my way home tonight (In northern Iowa!!!)

 

[jlbenedict]

AM = Amplitute Modulation
FM = Frequency Modulation

thats all you need to know

 

[CTho9305]

So, why use AM modulation at all on the "AM band"? Why not use FM modulation? Is there something inherrent to those AM frequencies that means that FM can't be used (ie. bandwidth)? If FM is inherently less noise-prone than AM (and it is) then why does anyone use AM at all nowadays?

Edit: well, I got a huge earfull from all the engineers sitting around me on why you can't do FM on AM bands:

1. cost for broadcasting... people have investments in AM broadcasting statoins
2. legacy radios in people's houses wouldn't work any more.
3. not enough frequency bandwidth (you'd need to reduce the number of channels/stations to do it... even allowing for mono-FM)
4. breaks cross-border stations (so AM radio receivers in Mexico and Canada would be affected by FM "noise" which would be worse than white noise to deal with).

and the cacophany of responses from all the guys sitting around me probably mentioned a few other things that I forgot.

None of those are really technical reasons though.

Originally posted by: jlbenedict
AM = Amplitute Modulation
FM = Frequency Modulation

thats all you need to know

I sometimes wonder what prompts people to offer such useless replies in a thread - especially one that already has good, informative replies in it.

 

[Calin]

Originally posted by: MAW1082
The entire problem with AM is static. Due to background radiation, other sources on earth etc. there is alway interference with the amplitude portion of a signal.

FM is noiseless because the modulation scheme (frequency modulation) uses a shift in frequency of teh transmitted signal to symbolize a shift in amplitude of the original baseband signal.

AM is still useful, especially for simple communication, but FM is far superior for transmitting analog sound / music.

FM waves are not reflected as much by the stratosphere (?) so they can not be used for long distances. Also, I suppose you need more complex devices to emit/receive them, so more expensive emitters/receivers.
Also, I think high frequency waves have a higher transmission losses, needing more power to transmit

 

[Rubycon]

I like the sound of AM especially for voice.

The sferics (lightning) noise in a summer evening while listening to a ball game is soothing.

I guess I'm different.

 

[Calin]

No, maybe it is just that the AM transmission adds some kind of "pleasant" errors to the sound. I too like the feeling of listening to AM radio with a huge antenna

 

[sciencewhiz]

Originally posted by: pm
3. not enough frequency bandwidth (you'd need to reduce the number of channels/stations to do it... even allowing for mono-FM)

Current FM stations use 200khz bandwidth and there is 20 mhz availible. There's only 1.2 mhz AM bandwidth availible.

 

[Smilin]

Originally posted by: Calin
No, maybe it is just that the AM transmission adds some kind of "pleasant" errors to the sound. I too like the feeling of listening to AM radio with a huge antenna

Agreed.

Somewhat similar to the warm noise caused by the limitations of vinyl recordings.