# AM radio reception **URL:** \ **Category:** Factual Questions\ **Created:** [July 31, 2000, 8:39pm UTC](https://boards.straightdope.com/t/am-radio-reception/27602 "2000-07-31T20:39:10Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Montfort](https://avatars.discourse-cdn.com/v4/letter/m/d6d6ee/32.png) [@Montfort](https://boards.straightdope.com/u/Montfort)\ **Post date:** [July 31, 2000, 8:39pm UTC](https://boards.straightdope.com/t/am-radio-reception/27602/1 "2000-07-31T20:39:10Z")
In my car, driving around the Washington, DC suburbs, I can occasionnaly tune in 1210 WPHT radio out of Philadelphia, 130 or so miles away. This is great, since I listen to my Phillies that way when I’m not home listening to a RealAudio feed. I need rather optimal conditions to tune it in, usually, meaning no overhead power-lines above or near my car, and that I’m not under an overpass or a bridge. However, I get much better reception at night, than during the day. What’s the reason for this? Is it simply atmospheric (does sunlight affect AM radio waves?), or am I just benefitting from a reduction in traffic in the air? The difference is remarkable, as the distortion resulting from things above me, physically, that kills the signal during the day has a negligible effect at night. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [July 31, 2000, 8:45pm UTC](https://boards.straightdope.com/t/am-radio-reception/27602/2 "2000-07-31T20:45:24Z")
Amplitude modulation bounces off the D layer of the atmosphere at night and travels further. ---
**Author:** ![casdave](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@casdave](https://boards.straightdope.com/u/casdave)\ **Post date:** [July 31, 2000, 8:55pm UTC](https://boards.straightdope.com/t/am-radio-reception/27602/3 "2000-07-31T20:55:54Z")
Yes you are right, sunlight has quite an effect on radio waves, especially on am where it shows itself up as random noise superimposed on the carrier signal. Atmospherics, such as temperature layers can affect reception which can cause reception to occur at far greater distances than should be possible. What happens is that the radio waves head at an angle from earth toward space but they encounter atmospheric layers which refract the signals in a similar way to that in which light is refracted in water. If the refraction is great enough the signal may be directed downward at an angle toward the earth but now, because of the curvature of the atmosphere it will strike another lower layer at a more oblique angle and be redirected upward again . In this way a radio signal can be bounced between atmospheric layers and finally make landfall hundreds of miles away. In the overcrowded am band this often means that a distant staion will interfere with a olocal one which is why you have that rx/dx switch on your radio which narrows down the range of frquencies that go to make up one radio staion(makes it more selective)but can cut the ‘quality’ of the station. ---
**Author:** ![Arjuna34](https://avatars.discourse-cdn.com/v4/letter/a/eb8c5e/32.png) [@Arjuna34](https://boards.straightdope.com/u/Arjuna34)\ **Post date:** [July 31, 2000, 10:18pm UTC](https://boards.straightdope.com/t/am-radio-reception/27602/4 "2000-07-31T22:18:49Z")
In addition to the atmospheric effects, many AM stations change their transmitter power at night, which affects their range. I believe there are some major stations that increase their power to reach a much wider audience, while others reduce it to let them in. For example, I think 880 AM in NYC ([http://www.newsradio88.com](http://www.newsradio88.com) ) does this. Arjuna34 ---
**Author:** ![whitetho](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whitetho/32/1059_2.png) [@whitetho](https://boards.straightdope.com/u/whitetho)\ **Post date:** [July 31, 2000, 10:31pm UTC](https://boards.straightdope.com/t/am-radio-reception/27602/5 "2000-07-31T22:31:34Z")
> [@](#): > > \*Originally posted by Montfort \* > \*\*What’s the reason for this? Is it simply atmospheric (does sunlight affect AM radio waves?), or am I just benefitting from a reduction in traffic in the air? The difference is remarkable, as the distortion resulting from things above me, physically, that kills the signal during the day has a negligible effect at night. \*\* Think higher up–it is the ionosphere that is responsible for AM (mediumwave) signals travelling much farther at night than during the day. Below is the way I explain it in an article, [Building the Broadcast Band](http://www.ipass.net/~whitetho/buildbcb.htm), on my website: > [@](#): > > With the introduction of broadcasting, information about skywave signals suddenly became important. However, a full understanding of what was taking place did not exist in the early twenties. It was obvious the sun was involved, since in most cases skywave signals appeared only at night. Eventually it was determined that the ionosphere is composed of layers, each with distinctive characteristics. What became known as the “E” and “F” layers are responsible for reflecting radio signals back to Earth. (Unlike groundwave signals, the strength of reflected skywave signals are essentially the same across the entire 200 meter to 600 meter band.) > > Due to the ionizing effect of the sun, these reflecting layers actually are more concentrated, thus more effective at reflecting radio signals, in daylight hours than at night. Therefore, in theory skywave signals should be even stronger during the daytime than at night. However, it turned out that a inner “D Layer” also existed. And the D Layer absorbs signals in the wavelengths that happened to be assigned for broadcasting, blocking them before they have a chance to reach the reflective outer layers. But unlike the E and F Layers, the D Layer only exists during daylight hours, which is why skywave signals disappear during the day but return at night. An analogy is that, when talking about the wavelengths assigned to broadcast stations, the E and F Layers act as a mirror reflecting signals back to Earth, while the D Layer is a curtain drawn in front of the mirroring layers during daylight hours. ---
**Author:** ![RealityChuck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/realitychuck/32/195_2.png) [@RealityChuck](https://boards.straightdope.com/u/RealityChuck)\ **Post date:** [July 31, 2000, 10:51pm UTC](https://boards.straightdope.com/t/am-radio-reception/27602/6 "2000-07-31T22:51:56Z")
> [@](#): > > I believe there are some major stations that increase their power to reach a much wider audience, while others reduce it to let them in. For example, I think 880 AM in NYC ([http://www.newsradio88.com](http://www.newsradio88.com) ) does this. Some stations may be allowed to broadcast with more power during the night (though usually it’s the other way around – the stations have to shut down at night). Not 880, though. It’s a clear channel station and broadcasts at the legal limit (50,000 Watts, I believe) all the time. “Clear channel” means no other station can interfere with its signal. It just seems to be broadcasting with more power because the signal travels further due to the atmospheric conditions. ---
**Author:** ![Arjuna34](https://avatars.discourse-cdn.com/v4/letter/a/eb8c5e/32.png) [@Arjuna34](https://boards.straightdope.com/u/Arjuna34)\ **Post date:** [July 31, 2000, 11:51pm UTC](https://boards.straightdope.com/t/am-radio-reception/27602/7 "2000-07-31T23:51:21Z")
> [@](#): > > \*Originally posted by RealityChuck \* > \*\* > > > [@](#): > > > > I believe there are some major stations that increase their power to reach a much wider audience, while others reduce it to let them in. For example, I think 880 AM in NYC ([http://www.newsradio88.com](http://www.newsradio88.com) ) does this. > > Some stations may be allowed to broadcast with more power during the night (though usually it’s the other way around – the stations have to shut down at night). Not 880, though. It’s a clear channel station and broadcasts at the legal limit (50,000 Watts, I believe) all the time. “Clear channel” means no other station can interfere with its signal. It just seems to be broadcasting with more power because the signal travels further due to the atmospheric conditions. > > \*\* Thanks for the correction. I guess with 50kW, you don’t need to increase your power! Arjuna34 ---
**Author:** ![justwannano](https://avatars.discourse-cdn.com/v4/letter/j/7ab992/32.png) [@justwannano](https://boards.straightdope.com/u/justwannano)\ **Post date:** [August 1, 2000, 12:32am UTC](https://boards.straightdope.com/t/am-radio-reception/27602/8 "2000-08-01T00:32:44Z")
Us older Dopers will remember radio in its hey day. Well maybe not as much as our parents when it was in its infancy. Mom still talks about Orson Wells “War of the Worlds.” She really heard the original broadcast. Am was the Door to the outside world. Imagine hearing “Beacon Street” on KOMA Oaklahoma City 1030 IIRC That was a big deal to a 13year old in 1959. WLS 890 Chicago. WLS stands for Worlds Largest Store. Owned by Sears when it started. Had that set on my car radio. Of course WHO Des Moines was the clear channel for the farmer types and is still popular last time I heard. Yes AM came in better at night. They increased their power at night and the smaller stations would usually go off the air at midnight. The equipment wasn’t as good back then so staying on frequency was always a problem. You must remember all the stations were operating on vacuum tubes back then. Heat is what made a vacuum tube work but it was always the ultimate cause of failure. I was in awe of 50,000 watts when I started in Electronics school. The Local station broadcast 1,000 watts. Hell 20 years later some CBers were aproaching 1,000 watts. ---
**Author:** ![Doug\_Bowe](https://avatars.discourse-cdn.com/v4/letter/d/df788c/32.png) [@Doug\_Bowe](https://boards.straightdope.com/u/Doug_Bowe)\ **Post date:** [August 1, 2000, 1:37am UTC](https://boards.straightdope.com/t/am-radio-reception/27602/9 "2000-08-01T01:37:57Z")
We were taught to refer to the bouncing AM atmosphere as the _Kennelly-Heaviside layer_. Remember, also, that at sundown all the “daytimer” stations sign off. This reduces interference. There still may be a hobbiest or two who try to pull in distant stations. They’re called DX’ers. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [August 1, 2000, 2:28am UTC](https://boards.straightdope.com/t/am-radio-reception/27602/10 "2000-08-01T02:28:15Z")
KAAY 1090 had 50 kW in Arkansas. Beaker Street was heard in Central America I believe. The FCC made them use a pattern antenna later on. KAAY was sold and became a religious station. ---
**Author:** ![justwannano](https://avatars.discourse-cdn.com/v4/letter/j/7ab992/32.png) [@justwannano](https://boards.straightdope.com/u/justwannano)\ **Post date:** [August 1, 2000, 3:39am UTC](https://boards.straightdope.com/t/am-radio-reception/27602/11 "2000-08-01T03:39:32Z")
carnivorousplant I forgot about KAAY Little Rock.I used to listen to them too.And I believe you are right thats where I heard Beacon Street.Used to fall asleep with the transistor radio under my pillow.Mom never caught me. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [August 1, 2000, 3:45am UTC](https://boards.straightdope.com/t/am-radio-reception/27602/12 "2000-08-01T03:45:06Z")
Beaker Street. Like LSD made in a beaker. Clyde Clifford does it on an FM station now. AKA Dale Sidenschwartz, he also does a medical interview program on an educational station. We are old. ---
**Author:** ![justwannano](https://avatars.discourse-cdn.com/v4/letter/j/7ab992/32.png) [@justwannano](https://boards.straightdope.com/u/justwannano)\ **Post date:** [August 1, 2000, 3:56am UTC](https://boards.straightdope.com/t/am-radio-reception/27602/13 "2000-08-01T03:56:52Z")
Thread hijack Hows the Hip End Hijack ---
**Author:** ![Montfort](https://avatars.discourse-cdn.com/v4/letter/m/d6d6ee/32.png) [@Montfort](https://boards.straightdope.com/u/Montfort)\ **Post date:** [August 1, 2000, 4:49am UTC](https://boards.straightdope.com/t/am-radio-reception/27602/14 "2000-08-01T04:49:10Z")
I suspected that stations shutting down at night was probably a factor, too, since I had heard stories from my dad tuning in St. Louis stations as a kid growing up in Lebanon, PA. But, I figured that was a thing of the past, since I don’t know of any stations nowadays that turn off at sundown. Admittedly, I don’t listen to the AM band much, though. ---
**Author:** ![NothingMan](https://avatars.discourse-cdn.com/v4/letter/n/b77776/32.png) [@NothingMan](https://boards.straightdope.com/u/NothingMan)\ **Post date:** [August 1, 2000, 6:02am UTC](https://boards.straightdope.com/t/am-radio-reception/27602/15 "2000-08-01T06:02:47Z")
_Just to reiterate what has been said . . ._ Our sister AM station downstairs by FCC law must reduce their signal by %75 at dusk. This allows the more powerful AM stations to be heard better at night. Also AM signals have been known to bounce between layers of the atmosphere for thousands of miles. I remember one road trip down the east coast to Florida where we picked up station out of Chicago !! Talking about wattage . . . my station operates at 100,000 watts tying us for the highest in the state (NC). I do know of some stations in the midwest that are permitted to go up to as much as 250,000 watts ! Now **that’s** having an audience ! - **NM** ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [August 1, 2000, 8:35pm UTC](https://boards.straightdope.com/t/am-radio-reception/27602/16 "2000-08-01T20:35:01Z")
> **[BeakerStreet.com is for sale | HugeDomains](https://www.hugedomains.com/domain_profile.cfm?d=beakerstreet&e=com)** > > Get your very own domain easily. Fast and professional customer service. justwannano: The hip is fine. Beaker Street is alive and well if not quite as subversive to young minds. 🙂