Why FM broadcast can't travel too far like the AM ones?

[ofosot69]

I need to know why an FM broadcast can't travel too far (from one side of the earth to other) like an AM broadcast that could travel thousands of miles after getting reflected from the ionosphere.

Why can't FM signals be sent to the ionosphere for high range broadcast?

 

[exkalibur]

AM signals are generally much higher transmit power, firstly. Also, frequencies in the 88-108 VHF range aren't affected nearly as much by atmospheric conditions which is why MF signals can travel extremely long distances.

Radio propagation - Wikipedia is a fairly good tl;dr

 

[John_S]

Actually, VHF/UHF is more affected by atmospheric conditions than AM medium wavelengths. VHF/UHF freqs just don't propagate the same way unless operating in the lower portions...like 6 meters...VHF low band and down towards 10 meters. That's those bands are so interesting to operate. They're affected by both HF and VHF propagation to some extent.

 

[pb_lonny]

Different frequencies travel differently, this is why amateur operators use 2m to talk across the city and 80m to talk across the world...

 

[mmckenna]

I need to know why an FM broadcast can't travel too far (from one side of the earth to other) like an AM broadcast that could travel thousands of miles after getting reflected from the ionosphere.

Why can't FM signals be sent to the ionosphere for high range broadcast?

It's not FM modulation versus AM modulation. It's the frequencies used.

The ionosphere is what controls this.

AM Broadcast radio frequencies are from 540KHz to 1700KHz. Those frequencies are low enough that they will bounce off the ionosphere and travel long distances. At night, the layer of the ionosphere that reflects the signal is much higher in the sky, so it bounces farther. During the day, the suns energy charges the layer of the ionosphere much closer to the surface of the earth, so they signals bounce off a layer much closer and don't travel as far.

FM Broadcast uses frequencies 87.8MHz to 108MHz. Those frequencies are high enough that they pass through that charged level of the ionosphere rather than bouncing off it most of the time.

RF power level doesn't play into it usually. A 1000 watt AM broadcast station can still travel thousands of miles, where a 100,000 watt FM broadcast station usually cannot.



However….
Sometimes the atmosphere will do a thing called ducting, where the signals can get trapped between two layers and FM Broadcast, as well as TV and other bands, can bounce along in that duct and travel long distances. Some hobbyists like to "DX" FM broadcast and TV signals from across the Atlantic ocean. It can be a very rare occurrence and short lived, but it does happen.


If you want to learn more, do some googling on "Maximum Useable Frequency" and "Propagation Ducting".

 

[slicerwizard]

I need to know why an FM broadcast can't travel too far (from one side of the earth to other) like an AM broadcast that could travel thousands of miles after getting reflected from the ionosphere.

Why can't FM signals be sent to the ionosphere for high range broadcast?

If only there was a way to get answers to questions like that in an instant.

 

[jwt873]

Broadcast FM and TV signals DO travel great distances. It just doesn't happen that often. Some radio hobbyists enjoy seeking out distant TV and FM stations. It's called TV and Broadcast FM DXing.

Propagation is a complex subject.. If you truly want to get acquainted with it, this link has a great primer --> https://iaeme.com/MasterAdmin/Journal_uploads/IJECET/VOLUME_5_ISSUE_11/40120140511003.pdf

 

[nd5y]

If only there was a way to get answers to questions like that in an instant.

If you want correct answers then you shoudln't use AI crap.

 

[nd5y]

A related issue is the reason that you can hear distant AM broadcast stations at night but not in the daytime is D-layer absorption.

Global D-Region Absorption Prediction Documentation | NOAA / NWS Space Weather Prediction Center

www.swpc.noaa.gov

This map (updated every few minutes) shows the current predicted highest absorption frequency.

D Region Absorption Predictions (D-RAP) | NOAA / NWS Space Weather Prediction Center

www.swpc.noaa.gov

 

[nd5y]

A related issue is the reason that you can hear distant AM broadcast stations at night but not in the daytime is D-layer absorption.

The other related issue is that you can hear stations in the lower end of the AM broadcast band a lot farther in the daytime than the higher end of the band because groundwave propagation is better at lower frequencies.

 

[exkalibur]

If you want correct answers then you shoudln't use AI crap.

But it gave a correct answer.

 

[slicerwizard]

If you want correct answers then you shoudln't use AI crap.

Lookie here, Mr. "shoudln't", I'm no LLM/GPT/AI fanbio (or Microsoft fanboi either), but Copilot has, for the most part, been providing good results for the last month or so. And it did far better than Google searches a couple of days ago when I needed to get up to speed on some Powershell scripting.

The trust (not really), but verify (absolutely) model has been working fine for me. But you go and do your own thing.

 

[sunwave]

Basic insight:
MW band has a lower angle of radiation. It is like skipping a rock across the surface of water. FM broadcast is in the VHF portion of the RF spectrum. It's very limited to local within the range of 25-35 miles. It's take off angle is high enough to go straight through the ionosphere but its not a 90 degree angle upwards. The angle is low enough still for the troposphere to reflect a FMBC signal back to earth during tropospheric ducting season (April-May). Which is basically a temperature inversion when cool air meets warm air.

I need to know why an FM broadcast can't travel too far (from one side of the earth to other) like an AM broadcast that could travel thousands of miles after getting reflected from the ionosphere.

Why can't FM signals be sent to the ionosphere for high range broadcast?

 

[prcguy]

Basic insight:
MW band has a lower angle of radiation. It is like skipping a rock across the surface of water. FM broadcast is in the VHF portion of the RF spectrum. It's very limited to local within the range of 25-35 miles. It's take off angle is high enough to go straight through the ionosphere but its not a 90 degree angle upwards. The angle is low enough still for the troposphere to reflect a FMBC signal back to earth during tropospheric ducting season (April-May). Which is basically a temperature inversion when cool air meets warm air.

FM broadcast antennas are engineered to put the most signal where the customers are and that's at the ground out to the horizon. They don't waste power up into the sky. The only reason FM broadcast 88-108MHz doesn't bounce off the ionosphere is because at that frequency it goes right through it at any angle. The only long distance mode you should encounter for FM broadcast is tropo ducting which happens at much lower altitudes between different atmospheric layers separated by temperature.

 

[T680]

It's not FM modulation versus AM modulation. It's the frequencies used.

The ionosphere is what controls this.

AM Broadcast radio frequencies are from 540KHz to 1700KHz. Those frequencies are low enough that they will bounce off the ionosphere and travel long distances. At night, the layer of the ionosphere that reflects the signal is much higher in the sky, so it bounces farther. During the day, the suns energy charges the layer of the ionosphere much closer to the surface of the earth, so they signals bounce off a layer much closer and don't travel as far.

FM Broadcast uses frequencies 87.8MHz to 108MHz. Those frequencies are high enough that they pass through that charged level of the ionosphere rather than bouncing off it most of the time.

RF power level doesn't play into it usually. A 1000 watt AM broadcast station can still travel thousands of miles, where a 100,000 watt FM broadcast station usually cannot.



However….
Sometimes the atmosphere will do a thing called ducting, where the signals can get trapped between two layers and FM Broadcast, as well as TV and other bands, can bounce along in that duct and travel long distances. Some hobbyists like to "DX" FM broadcast and TV signals from across the Atlantic ocean. It can be a very rare occurrence and short lived, but it does happen.


If you want to learn more, do some googling on "Maximum Useable Frequency" and "Propagation Ducting".

Thanks for the info. I bought my first radio with LW capabilities a few months ago and hadn't thought much about it before. I've lived in the flatlands of the Midwest most of my 65 years and haven't heard of anyone out here using it, but after learning about it LW seems like it would be ideal out here before FM became prevalent. Why wasn't it used? I must be missing something.

 

[mmckenna]

Why wasn't it used? I must be missing something.

LW broadcasting was done in Europe and other parts of the world for quite a long time. It fell out of favor since there were better mediums for broadcasting.

Long wave travels well, but it can really be hit or miss depending on the atmospheric conditions. It would require a lot of power ot be effective. I think some of the LW stations that used to do this in Europe ran in the million watt range and had substantial antenna systems.

If you have long wave capability, you might want to see if there are any aeronautical navigation beacons still running in your area. It's slow speed Morse code, but pretty easy to decode. Most of those long wave beacons have been shut down, but there's still some running. I used to try to catch them when I could. Sort of a fun aspect of the hobby.

 

[T680]

LW broadcasting was done in Europe and other parts of the world for quite a long time. It fell out of favor since there were better mediums for broadcasting.

Long wave travels well, but it can really be hit or miss depending on the atmospheric conditions. It would require a lot of power ot be effective. I think some of the LW stations that used to do this in Europe ran in the million watt range and had substantial antenna systems.

If you have long wave capability, you might want to see if there are any aeronautical navigation beacons still running in your area. It's slow speed Morse code, but pretty easy to decode. Most of those long wave beacons have been shut down, but there's still some running. I used to try to catch them when I could. Sort of a fun aspect of the hobby.

I'm pretty close to O'Hare but haven't heard anything when scanning or slowly scrolling through the band.
I wonder who still listens to long wave that generates enough demand to keep offering it on radios? I see it on more radios than I do the weather band, and I would guess there are more people that could listen to the WB than LW.

 

[nd5y]

I wonder who still listens to long wave that generates enough demand to keep offering it on radios?

People outside of North & South America where all the LW broadcast stations are located.

 

[T680]

People outside of North & South America where all the LW broadcast stations are located.

No wonder why I wasn't familiar with it.

 

[MikeinDestin]

Note that going into the LW part of the spectrum will bring much noise to anything received. Especially the longer your antenna is (which is what you want for LW). At least more noise than the AM band. One advantage of the lower bands is no matter how bad conditions are, there's always someone you can chat with on the Amateur bands. Or hear. Try that with 6 Meters. Not knocking VHF, that's certainly fun, and doesn't require such big antennas. Even mobile antennas work well.