FM Broadcasting vs. AM Broadcasting

There's a lot of science behind this, most of which I don't understand, but I do understand that the lower frequencies associated with AM broadcast propagate better than the higher frequencies associated with FM. Which is to say, they travel farther. The reason has to do with the wavelength of the signal, the lower the frequency the longer the wavelength. I'm sure a Google search could explain it in much better detail.

http://books.google.com/books?id=ynL8MJoRm2MC&pg=PA286&lpg=PA286&dq=am+vs+fm+propagation&source=web&ots=SzVxyv9vbO&sig=rRsRRnIHyHtsKTFKVyPSP8UTgqo&hl=en&sa=X&oi=book_result&resnum=10&ct=result

I'm not sure this huge link is going to work, but I found lots of good links on the subject using the search term "am vs fm propagation"

Have fun!

Tom

One assumes that you are suggesting replacing AM with FM in the 550-1600 KHz band. First, the FCC (U.S.) doesn't permit the FM mode. 2nd, the wide bandwidth required for FM would only allow a few broadcast channels; if changed, which would never happen anyway.

FM signals, being higher in frequency, travel in a straight line. That is why you sometimes see FM transmitters on top of tall buildings. FM signals are blocked by hills, mountains and even large buildings. AM signals will follow the contour of the terrain. They will travel farther on less energy than FM. They will reach areas that FM can't get into.

I believe that FM is line-of sight, so that coverage is limited by the earth's curvature. AM is lower frequency, and the waves can bend slightly. Additionally the ionosphere reflects the waves (under some conditions), allowing them to follow the earth's curvature.

It's got much more to do with transmission frequency than modulation method.

FM requires much greater bandwidth than AM, which is why FM signals are to be found at VHF and UHF, where there is more room for them. The ionosphere is not reflective to VHF and UHF, so these transmission frequencies are 'local'.

It seems as though topics as this seem to get off-topic. Since jammo didn't say otherwise, we should assume the given info as we read it, and not read into it.

The comments otherwise are certainly valid. But only should be deened as off-topic.

Within the U.S., there are two FM modulation specifications: One as the initiator is probably citing (FM broadcast in the 88-108 MHz spectrum), and Narrow Band FM (NBFM) that is used in other specified and allocated locations in the RF spectrum. NBFM is limited to 15KHz which does not lend much at all in for broadcast (music, etc) over AM. Nevertheless, it is not provided for in the AM broadcast band.

I would doubt that the FCC would provide for FM in the existing FM broadcast band because of international agreements where it remains AM only.

I suppose that sometime in the future that certain modes of transmission may be allowed in certain allowed areas within the HF spectrum that would allow for long distance broadcast. But know too, that the propagated signal would be lended to the time of day may extend the transmitted distance, all dependant upon the ionosphere as described above.

Tell me, who here holds an FCC Radiotelephone General Class License? I do. For some 35 years. Used to be a 1st Class until the FCC downgraded all of us.

I find it interesting and too sad that over the time I've contributed to topics on CR4 that no one has ever given me an positive "good answer" to that which I'd commented on. Most all have ON-topic and to the point. I don't tend to do a lot of yak. Just to the point with the question info as known. I think the "usual" participants tend to favor one-another.

So many inquirees do not provide much info, which leaves a lot open to too many tangents. In those events, I believe that we should only prompt for more information before making much of any comment/resolve. My view.

Bill ML makes a good point that even NBFM requires much more spectrum than AM. AM BCB channel separation in the USA is 10 kHz, in EU, 9 kHz. Actual signal content is 6 kHz or less. The 15 kHz signal spectrum cited by Bill ML results in about 25 kHz channel separation in the vhf-FM band used in military radios such as SINCGARS (Single Channel Ground and Airborne Radio System). That radio is strictly voice communications. If you had 25 kHz channel separation in the AM BCB, clearly you would lose a lot of stations in a highly congested area such as Los Angeles. It wouldn't matter much in Peoria, or even Wichita. But there is something else that Bill ML might be able to shed some light on. Back when AM radio started, in the 1920s, the radios were not superheterodyne receivers. They tuned the front end, detected the output and amplified it and that was that. I'm guessing the "Q" of that tuning was tight enough that 9 or 10 kHz was all the separation you needed. In other words, the channel separation was based on the technology of the time, and while it would be technically possible to do wide band AM today, the standards are based on what was possible back at the beginning. Hope to hear back from Bill ML - sounds like a savvy guy.

The "Q" was probably sufficient because back then there weren't that many stations and their scarcity added to the separation.

FM (frequency modulation) signal flows in a straight line like a pool ball in a table the globe is round after a few miles the earth curves down and the FM signal keeps on going straight into outer space AM audio modulation is like the pebble dropped in a puddle of water is waves bounce of all ab-suctions including the ionosphere and bounce back to earth etc.

that was the way it was explained to me the first time 50 years ago

nick

Qc.ca.

Thank you all for your response.

As I can see from the answers given It seems it's the frequency of the carrier that is making the FM broadcasting limited to short distances but not the modulation itself. I do understand that FM needs greater band width than AM. Is it not possible to transmit FM modulated signal with low frequency carrier so that is can travel to long distances?

What is more important in practice? because when we learn in schools we can see the numbers only. Is the signal quality improved in FM not as important as the bandwidth gained in AM?

I really appreciate detailed answers and/or recommend articles/books, which can give me an insight to what is being done practically.

In theory yes, it is possible to transmit FM at lower frequencies, but in practical terms no, it is not. the problem is that you'd use up a huge chunk of the frequency band with your signal. the entire AM broadcast band is only 1090Khz wide. 25Khz channel separation that would only leave 43 channels to work with vs 109 for AM. Also FM receivers use phase locked loops to lock onto the carrier frequency and as such they lock onto the strongest signal at that frequency and lock out all others. Since low frequencies travel so far, that makes channel separation that much more important. AM receivers do not do that, they detect all signals on that frequency so it is possible to hear a distant station, even if you have some bleed from another station.

The solution to this is what is being rolled out on the FM band called HD radio. HD in this case does not stand for High Definition the way it does for TV, it stands for Hybrid Digital. Multiple stations are transmitted digitally in the guard bands between signals. By using digital encoding, the signals are far more bandwidth efficient. Such transmissions are possible on the AM band as well.

AM is a vertically polarized ground wave. It uses the ground plane and ionosphere(under certain conditions) to propagate and can travel over topographical changes around the earth. In the evening when the suns' energy is not exciting the ionosphere it will reflect the low frequency AM signal back to the earth. I believe this "skywave" effect diminishes as the frequency increases.

FM is a (usually) circularly polarized high frequency transmission which is "line of sight". Power output and elevation are the major factors that contribute to increasing the listening area for FM. Large structures and terrain such as mountains are detrimental to FM transmission. Certain weather conditions can 'duct' and/or 'E-scatter' FM signals far beyond their typical listening area but these unstable and short lived reception regions are small and localized. Humidity and temperature inversions can cause these effects.

AM HD has two side bands 3kHz on either side of the center frequency; that is the digital carriers. AM can only contain one digital 'channel' and the analogue bandwidth is reduced to 5KHz to accommodate the digital carriers. Digital carrier is the same output power as the analogue power output.

FM HD can contain 8 discrete digital carriers within the allocated bandwidth. The digital output power is only 1/10 of the analogue power output.

GA there, lots of good technical details.