Variation in am/fm radio reception

Waddauno

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Mar 17, 2006
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I am currently in Jackson, WY. During the day, if one scans for reception on the ole car radio, you get about 4 fm stations and 2 or 3 am stations grabbed by the radio. At night, a whole different set of channels are available. I know that signal strength sometimes is a matter of licensing and can vary based on time of day.

My question, which is probably basic radio science: if nothing else changes re: the fcc, is there a difference in signal strength(availability?) based on the time day?
 
Waddauno said:
I am currently in Jackson, WY. During the day, if one scans for reception on the ole car radio, you get about 4 fm stations and 2 or 3 am stations grabbed by the radio. At night, a whole different set of channels are available. I know that signal strength sometimes is a matter of licensing and can vary based on time of day.

My question, which is probably basic radio science: if nothing else changes re: the fcc, is there a difference in signal strength(availability?) based on the time day?
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At night, there's less ionization in the atmosphere, so radio signals can travel further stronger. But this is slight; mostly, it's because a lot of stations are allowed to transmit stronger at night.

There's something else that happens mostly at night (and best in cold weather, for a variety of reasons), but mostly with low-frequency transmissions (AM). It's called skip. Radio signals can bounce back and forth in the ionosphere and travel far beyond what you would guess from line of sight.
 
epepke said:
mostly, it's because a lot of stations are allowed to transmit stronger at night.
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I think it's the opposite. Many stations are required to reduce their power at sundown, which reduces clutter allowing the remaining stations to reach a greater area.
 
Most AM stations must go off air at sundown, otherwise you'd be picking up all sorts of identical and nearly identical frequency stations. At night you can pick up stations from all over.
 
Yes, it's definately more apparent on the AM dial, given that I can get three stations at the best of times during day, and one of them is a low power local info broadcast. At night I've gotten seven or so -- unfortunately, at a certain time in the evening 2 of them are playing C2C. I got edumacated about reptile people that are living amoung us. The more pronounced stay in the shadows, apparently, but some look just like you and me and could be the guy in the next cubicle.

I was hoping to find "When Radio Was...."
 
I think people have it on it, but in fact FM is pretty much local-area all the time, and not quite line-of-sight but close to it. At night, you may get a bit of localish skip from over Idaho Falls way or something.

But AM at night can skip right around the planet. In the old days when I had a big tube reciever with a really quiet front end, I could get San Francisco on my radio in Ohio most nights. CKLW, KDKA, um, the country station from Cincinatti, WOWO were sure-in. KSLT (vhf) could be gotten on the TV too some days.
 
AM can definately travel. Get a map of North America. I'm about half way up Saskatchewan and have actually picked up AM stations from Texas at night.

We also have outstanding Northern Lights activity.
 
Radio propagation is a complex science. In a nutshell, AM frequencies tend to be absorbed by the D layer of the ionosphere unless they go pretty much straight up and back down. So the daytime range is maybe 75 miles. At night the D layer dissipates and that part of the signal going out at a lower angle is no longer absorbed. So you get a much longer range at night. (And these frequencies are affected by atmospheric noise which is far lower in winter than in summer. That's why you can often hear stations further away in winter than summer. Those winter nights are great for picking up AM stations on the other side of the country.) And for picking up AM signals from really, really far away you can make use of so called "grey line propagation". Grey line is the area between night and day and of course runs all the way around the globe. If you're in the grey line (sunset or sunrise) you can sometimes pick up a station also in the grey line half way around the world.
FM broadcast frequencies generally don't get absorbed or reflected by the ionosphere- they go straight through. So receiption is usually a bit beyond line of sight (4/3 Earth radius). On occasion around May and June the E layer of the ionosphere will reflect FM broadcast frequencies and you'll suddenly be picking up an FM station from 1000 or more miles away. (The same thing can happen with the lower several TV channels as well.....)
 
Cool, Hamradioguy, that made sense, thanks. I get a kick out of turning on the radio in the middle of the night and creeping up and down the frequency finding voices from who knows where. I'll have to try the grey line to see what I hear.
 
Dark Jaguar said:
Aren't we forgetting that only some of the planet is in "night" at any one time?
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Hello, half way... Yeah, half is dark alright.

Btw, some skip has in fact been measured going all the way around and coming back to the sending station.
 
Why, when I have a weak signal on my FM radio, does it get much clearer when I hold the aerial?
 
Waddauno said:
I was hoping to find "When Radio Was...."
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Both Sirius and XM satellite radio have old-time radio channels that harken back to a time when life in Wistful Vista was always simple, the FBI always got their man, and Jack Benny was always 39.

I started listening to Jack Webb in "Pat Novak for Hire" and almost bust a gut laughing. The style is so over the top that the show sounds like a parody of the detective dramas of that time.
 
Ladewig said:
Both Sirius and XM satellite radio have old-time radio channels that harken back to a time when life in Wistful Vista was always simple, the FBI always got their man, and Jack Benny was always 39.

I started listening to Jack Webb in "Pat Novak for Hire" and almost bust a gut laughing. The style is so over the top that the show sounds like a parody of the detective dramas of that time.
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They're great. I first heard this stuff one night driving across Colorado in the pitch dark. It was some sort of cheesy pre-twilight zone thing about drinking out of a cursed goblet story. The characters were so earnest, with very little time to set up the situation. I loved it, and despite myself could allow it to have some resonance.
 
Hamradioguy said:
Radio propagation is a complex science. In a nutshell, AM frequencies tend to be absorbed by the D layer of the ionosphere unless they go pretty much straight up and back down. So the daytime range is maybe 75 miles. At night the D layer dissipates and that part of the signal going out at a lower angle is no longer absorbed. So you get a much longer range at night. (And these frequencies are affected by atmospheric noise which is far lower in winter than in summer. That's why you can often hear stations further away in winter than summer. Those winter nights are great for picking up AM stations on the other side of the country.) And for picking up AM signals from really, really far away you can make use of so called "grey line propagation". Grey line is the area between night and day and of course runs all the way around the globe. If you're in the grey line (sunset or sunrise) you can sometimes pick up a station also in the grey line half way around the world.
FM broadcast frequencies generally don't get absorbed or reflected by the ionosphere- they go straight through. So receiption is usually a bit beyond line of sight (4/3 Earth radius). On occasion around May and June the E layer of the ionosphere will reflect FM broadcast frequencies and you'll suddenly be picking up an FM station from 1000 or more miles away. (The same thing can happen with the lower several TV channels as well.....)
Click to expand...
Let's not forget ducting. It is really weird to be standing on the beach in Southern California and talking to Hawaii on a 5 watt HT.
 
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