As a kid in Baltimore, I used to sit for hours on end at night, slowly turning the tuning dial on my AM radio, logging all the radio stations I could identify. I think the farthest I received was from CKLW, in Windsor, Canada. How far can AM signals skip? Is there some type of world record for receiving long distance AM broadcasts?
If by AM you mean signals in the .7 to 1.8 Mhz range, they can go around the world in good conditions. At slightly higher frequencies it’s not too uncommon for a ham sending CW to hear his own signals back in his receiver after they circle the earth. They are weak of course and conditions need to be ideal. It’s certainly not an every day, or evening actually, occurrence.
There’s an entire hobby devoted to trying to see how far away you can pick up a signal. It’s called DX-ing, since DX is an old abbreviation for “distance”. I couldn’t find a world record for an AM radio DX in my admittedly short google search, but many clubs and individuals keep records of their longest distances received. Not only do they go for long distance in general, but they also try to receive distant signals from known weaker stations.
DX isn’t limited to the AM radio band either. People will try to DX on the FM band, TV band, and various shortwave bands used by HAM radio operators and the like. With an AM radio station a DX-er will just be trying to receive the signal, but with a HAM operator, two operators will try to carry on a conversation, either with voice or with code. Sometimes they’ll even send postcards to each other to confirm their DX.
AM in the medium wave broadcast band is intended for covering a small region or in the case of clear channels a region of states or provinces (like a third or so of the nation). A station like CKLW being heard by you would be a nightly thing except during times of poor or good radio propagation (in poor conditions you would hear noise, in very good conditions you would hear farther away stations as well on top of it). In very good conditions, especially caused by good conditions in the 11 year solar cycle, stations from Canada and Mexico could be heard in the other.
AM in the shortwave broadcast band travels around the world intentionally.
AM in two way communications can travel around the world depending on the power and frequency and the radio propagation conditions.
As GaryM noted CW (morse code) in the short wave ham radio bands can travel around the world during excellent propagation conditions. this mode of radio is very efficient and in rare propagation conditions have communicated worldwide with less than 1 Watt of power.
For broadcast radio, I remember picking up XERB (The Mighty 1090) while north of Seattle some nights.
The number one station in the evening with most of the teenagers where I lived in Western Pennsylvania was WLS in Chicago. After Dick Biondi left for Los Angles, we could still pick him up sometimes.
I don’t remember picking up a station farther away than Los Angles.
I spent a few more minutes googling (it’s lunch time).
The DX world record for a TV signal is 10,800 miles (according to Wikipedia).
The world record for FM is 4,302 miles (also according to Wikipedia).
I still haven’t found a world record for AM broadcast, but I did find some sites that mentioned that if you are on the east coast of the U.S. it is actually easier to receive long distance AM signals from Europe and Africa than it is from a distant U.S. station. This is because all U.S. stations broadcast on 10 kHz increments up and down the AM radio band, and the way AM works is that if you end up with two signals on the same frequency, they add together and there’s no way to separate them. European and African stations broadcast on 9 kHz increments, which means they will often be on different center frequencies than the U.S. stations, which reduces interference and makes it easier to receive the signal.
Similarly, folks on the west coast of the U.S. can often pick up signals from Asia and Australia.
It’s not that uncommon for DX’ers on the east coast of the U.S. to pick up stations from the middle east.
I’ve picked up Quebec AM radio (Francophone, of course) in Virginia on more than one occasion.
I have been able to pick up WABC AM out of NYC from my home in SC. As far as FM, I have been able to pick up 99 Kiss Country out of Asheville, which it a good 3 1/2 hour drive from here.
the propagation of radio beyond line of sight depends on the signal being reflected off the ionosphere and the earth. the reflection off the ionosphere will depend greatly on the sun, reflection will depend on which layer is energized which changes with day/night, season and point in the 11 year sunspot cycle.
signals are reflected off the earth. oceans and flat land reflect well. wide mountain ranges can scatter reflections rather than send it on.
the frequency of the signal plus all the above will govern what signal is received where and when.
Where is XERB from Seattle?
It was the “border blaster” Mexican station on which Wolfman Jack gained his fame:
So, someplace just over the U.S. / Mexican border (sounds like it was probably in or near Tijuana).
Side question- is it the frequency or the modulation that lets AM go so much farther than FM in the U.S. ? Is it because it’s amplitude modulated or is it because of the frequency? Would am FM signal at 1200 kHz go as far as a regular AM 1200 khz signal at the same wattage?
There are some variables in the way AM and FM receivers detect the signal, but that aside, the distance is generally determined by frequency.
Hams generally prefer single side band modulation as it puts more of the power available into carrying the intelligence of the signal. AM actually consists of both upper and lower sideband at the same time. With the correct receiver design only one is needed. Hams can use AM on most of our frequency allocations, but most use either CW or SSB.
There was and maybe still is a station located just across the river from Del Rio, Texas that could be heard almost anywhere in the west. That station (XERF??) had a lot of very fundamentalist religious programs; legend had it that they once offered autographed pictures of Jesus Christ----I never heard that personally but I did hear a lot of off the wall stuff from them.
It’s mostly the frequency.
There are some other issues though, because the type of modulation isn’t the only difference between the FM and AM radio bands.
AM and FM only refer to the modulation type. AM is amplitude modulated, which is kinda like varying the volume control to encode the signal. FM is frequency modulated, which means the frequency actually shifts up and down a bit to encode the signal. In order for FM to be received, the receiver has to be able to track the shifting frequency. When the signal gets really weak and there’s a lot of interference, FM receivers will often fail to track on the signal, which means FM will cut out sooner than AM.
On a slightly aside note, the frequency tracking aspects of FM are also what make it much more immune to noise than AM. An FM receiver will track on the loudest signal and will completely ignore smaller amounts of noise. With AM, the signal and any noise on the same frequency are indistinguishable from each other and cannot be separated. So, you get better broadcast quality with FM but it doesn’t work as well with really weak signals. (It’s actually a bit more complicated than this based on receiver type but I’ll just leave it at that for now)
AM radio in the U.S. is also vertically polarized, where FM signals are circularly polarized. If you’ve got a simple single wire antenna, you can picture vertically polarized as holding the antenna straight up and down and horizontally polarized as holding it sideways. Circularly polarized requires a fancier antenna, but basically you can picture circularly polarized as having both a vertical and a horizontal component. Vertically polarized signals bounce off of the ground and sky better. On an unrelated note (but possibly of interest) TV signals are horizontally polarized.
Mostly though, it’s the frequency, and it is due to how well that particular frequency bounces off of different layers in the atmosphere and how much water in the air tends to absorb that frequency (2.4 GHz for example is a fairly bad frequency to transmit on due to water absorption).
If all you changed was the modulation type, a vertically polarized FM signal at 1200 kHz would go as far as a 1200 kHz AM signal, though your receiver might have a slightly harder time picking it up at the far end of it.
Mexican stations also advertised goat gonad implants for that boost. Lots of product providers went there and built stations to sell their products.
While on a trip to the Northeast, I picked up WWL 870 AM out of New Orleans while driving through Manhattan New York. The Saints game was on…they were in the playoffs, and while driving through Times Square, the game was on the big screen there, and we were listening to the game on our home station.
Yes and the vertical polarization is important for compatibility with groundwave propagation where the signals refract with the curvature of the earth. That strong groundwave prop is what the designers are going for in a local broadcast radio system. The long distance skywave propagaion is an unintended and unwelcome side effect. The wikipedia goes discusses this:
Marginally related anecdote:
I’ve recently become more interested in HF QRP. I’m an Amateur General and got myself a Yeasu FT-817ND. This is a pretty amazing radio that transmits up to 5watts on all of HF, 6m, 2m + 70cm. That 5watt limit is pretty low, especially considering the inefficiencies of HF antennas. My antenna is some salvaged wire from Cat5 strung in a rough dipole with about a 35 foot wingspan in the beams in my attic. I’ve worked stations over 5000 miles away both in Europe and South America on single side band, 15 and 10 meters. I’ve worked coast to coast from Chicago on 40 and 18 and 20m, too. My alarm clock can’t run on five watts!
What can I say, I still can’t get over it.
I am in Nova Scotia and routinely hear AM broadcast stations from Europe. Depending on conditions, ZIZ in St Kitts/Nevis comes in pretty well(it’s on 555). Radio Reloj in Cuba sometimes overrides the US flamethrowers.