# How far away can you listen to FM radio?

As is plainly obvious from my location field, I reside in the fair country of Japan. Occasionally, usually on Sundays, I catch a faintly glimmering radio station. As best as I can tell, it seems to come from Chinese lands, more specifically, Beijing. The reception varies between barely discernable and quite clear.

However, Beijing lies 1850km away. Japan is a mountainous country, and a few of those mountains stand between me and China. Often I cannot pick up stations broadcasting from 100km away.

I believe that someone is re-transmitting the signal from within Japan. My wife disagrees, however, she claims that the music we hear indeed comes straight from Beijing, crossing sea, mountains and dismembered constellations to reach us.

Who’s right? Could an FM radio station be heard that far away?

I was pretty sure that FM transmission is line of sight. Wikipedia pretty much confirms this except for extremely unusual conditions.

the Wiki article seems legit to me. Actually, line-of-sight is the optical path and not the geometric. Refraction by the atmosphere allows transmission slightly over the horizon. In radar work the line of sight is usually calculated using a hypothetical earth with 4/3 the actual radius. For two antennas, both 100 ft above the surface, the geometric line-of-sight distance is about 25 miles while the actual distance is about 28 miles.

As I recall the ducting mentioned in the article is more common over the oceans than on land.

I guess reflection should also be mentioned. I live in a mountainous region and reflections from them allow some us to get FM stations quite a bit further than line-of-sight.

For example, we get a classical music station in LA 130 mi. away as the crow flies. However, when you drive 10 mi. south of town, toward LA, it fades out.

What gives you the impression that the music is coming from China, Bejing to be precise? It is not plausible that this broadcast can reach your receiver from that distance. I know that AM radio can reach much farther than that under the right conditions, but FM, I think is out of the question. AFAIK, it is a line of sight broadcast and does not reflect off the atmosphere or anything else, unless the Chinese have some new secret technology in FM broadcasting… Of course, here in Colombia, we are one step behind the rest of the world.

First off, absolutely righteous Cake references in the OP. Well worked in, sir.

Yes, FM is largely LOS. But that line of sight could include enormously high broadcast towers or satellite coverage. Or even that wacky airplane studio concept that’s popular from time to time.

Heck could even be a ‘Radio Free Japan’ boat in the ocean offshore.

Tropospheric ducting is not rare of bodies of water, especially salt water. When there is a temperature inversion (warmer air over cooler air) the air refracts EM radiation toward the water, where it is reflected back upward, where the air then again returns it downward.

Calm cool weather, where the air temperature is significantly lower than the water temperature is the setting for this so called “tropo ducting”.

Long distance reception of VHF signals does happen occasionally thanks to E-band skip. A certain layer of the atmosphere will sometimes reflect higher-frequency RF. Ooften, what will happen is a signal will bounce back and forth between two layers before finally returning to earth. I’m a little vague on this because it’s been ages since I’ve thought about the matter. Seems I once heard a distant FM station, but I won’t swear to this. Many years ago a friend of mine (sans cable) once watched several minutes of a TV newscast before he realized it was coming from Oregon (we’re in Kansas).

This is, however, a rather rare phenomenon.

Would you believe, E-layer skip. I told you my memory was a little vague.

No cite. I didn’t feel like wading through the hits that a search of the term turns up for a clear, consise definition.

The underlined statement is a little extreme. If VHF and UHF didn’t reflect, radar wouldn’t work.

I thought the E-layer affected AM, and not FM.

Anyway, one thing I’ve noticed with a portable FM radio is that it will be unable to get a clear reception in one spot in a room, and if you step aside a yard or two, it will be fine. I don’t think it’s only LOS.

It’s the announcer who, in Mandarin, says things like “it’s now 10 a.m. in Beijing”. My wife just told me her mother claims she used to be able to hear North Korean broadcasts occasionally, although I think they were AM.

What opened my mind to the possibility that it might be a freak phenomenon, is that we can only catch this station a few times a year and reception is highly dependent on position. It might be very good at one place, but move 100m away, and it’s completely gone. That being said, 1850km is a long way.

My vote is for a repeater or rebroadcast for some Chinese government station. This would put the distance at a lot closer than 1850 kilometers. It’s not unusual for these stations to use repeaters or to rebroadcast their signals from transmitters that are closer to their intended audience. For example, I can occasionally hear a Radio Hanoi broadcast whose signal actually comes to me from Canada.

Robin

The type of modulation has nothing to do with the reflection off the ionized atmospheric layers. The crucial element is the wavelength (inversely proportional to frequency). The FM broadcast band is in the VHF frequency spectrum, short wavelength, and that and all higher frequencies, shorter wavelengths, are not reflected by the ionized layers.

As to the dead spots noted when moving a portable FM receiver around, despite rumors to the contrary, FM band EM waves are reflected. When there are reflections in a space there can be standing waves resulting from cancellation and reinforcement of the radio waves. This can lead to dead spots or, alternatively, areas of increased signal strength.

You know, I’ve heard this “FM is strictly line-of-sight” conventional wisdom for decades, and it seems to be contrary to obvious facts. I receive FM stations clearly from cities you can’t see from here (25 and 35 miles away.) If I drive down in the river valley, where you can’t see three blocks away, I can still get these stations on the car radio.

So, is the line of sight rule incorrect? There is not a clear line of sight from here to the answer.

Line-of-sight is a good first approximation. In the real world, other factors such as reflection, refraction, and diffraction have to be considered to more accurately predict RF propagation. Under normal conditions, VHF radio waves, like the FM broadcast band, do not bounce off the ionosphere.

That may mean nothing: the whole of China operates on Beijing time.

Several years ago, while I was driving through the empty wasteland of west Texas, I experienced some spectacular tropospheric ducting. Instead of an FM band that was empty safe for preachers and twangy trucker tunes, there was a station at every spot on the dial. From the commercials and station IDs I caught, the stations were all from central and western Florida. Clear signals, and full stereo separation too.

The rule really should be reliable reception is limited to line of sight. However, as mks57 said there are secondary effects that can make longer distance reception possible. For example, there is the tropospheric reflection that results in ducting. Air masses of different dielectric constants will result in a reflection at the boundry between them.

Even though reception sometimes is possible beyond line of sight, it isn’t a good idea to depend on it. You shouldn’t rely on it for emergency services or put a lot of money into a fancy FM receiver at long distances from the transmitter.

The FM station where I work can be picked up on its main frequency from Tallahassee east to Lake City, FL, where it starts to fade out. Not far beyond Lake City, it disappears. That’s the limit that the signal will travel until it is thwarted by the curvature of the earth.

However, the engineer occasionally gets requests for QSL cards from FM DX’ers on the Gulf side of the coast, several hundred miles past our southern transmission contour. There is another station on our frequency, and adjacent to it in South Florida. Our station has overpowered their local broadcasts on the same frequency. Those have to be occurrences of ducting, as the official pattern of transmission does not cover the water. The distance from here to there is nothing like the distance from the OP in Japan to anywhere in China, though.