Fading AM radio signal

[coachgrd]

Alright, let's see if someone can help me better understand AM radio signals. I listen to a lot of AM sports talk radio on the way to work. Each day I pass a long deserted AM station that was situated along a state route. They haven't used or broadcasted from this location for I bet 20+ years. The station itself (pole building) and antennas are still standing, complete with brush and weeds growing up all around them.

Each time I pass this old station, whatever AM station I have on fades out, only to come back on when I get a 1/2 mile beyond the old station. What gives? Did the last guy out forget to pull the plug on the way out the door?

As long on we're on the subject...
Is there are logical explanation why I can pick up AM stations from clear across the country at night but not in the day?

 

[jcliburn]

As long on we're on the subject...
Is there are logical explanation why I can pick up AM stations from clear across the country at night but not in the day?

The ionosphere changes at night, allowing a.m. frequencies to propagate great distances.

 

[k0ua]

Well it get kinda complicated.. so hang on.
I dont know why when you drive by the old AM station you experience loss of the desired signal, unless like you said the transmitter is still on, and you are experiencieng desense or fundamental overload of the first RF stage or mixer stage in your radio. If this is the case, not only your desired signal should go way down, but the background noise should also go way down also. Now to the meat and potatoes. of radio propogation. This will be quick and dirty: the ionosphere is divided into layers of ionized particles. D E F1 and F2. these layers are created by energy from the sun. They come and go, by solar radiation. The layer you are concerned with that deals with the AM broadcast band, .530 to 1710 Khz is the D layer and the F layers. In the daytime, you mostly rely on ground wave propagation not skywave, as the D layer absorbs these lower frequencys Unlike shorter HF waves that are not absorbed by the D layer and go on to refract on the F layers, to acheive long distances during the day. But at night the D layer disipates quickly as soon as the sun stops fueling the ionosphere, but the F layers usually converge into one F layer and will support propogation of the AM broadcast band signals. The F layer last much longer sometimes completely overnight. As soon as the sun comes back up the D layer forms quickly and absorbtion start again at these frequencys. So again no skywave, just groundwave propogation, which is short distance. Now all of that said.. It is all way way more complicated than I have let on, and while I know more, I dont know enough by a long shot. In fact some of the processes are not fully understood by anyone. For those of you wondering about the E layer.. well it is usually called "sproradic" E because it is usually um..sporadic:laughing: If you want to learn more, let me know and we will get you hooked up with getting on track to get your Amateur Radio License, and you can spend the rest of your life trying to understand radio propagation:laughing: There are many "hams" here on TBN. some are pretty easy to spot. some not. Bet you wanted a simple answer to a simple question didn't you
James K0UA

 

[timswi]

Well it get kinda complicated.. so hang on.
I dont know why when you drive by the old AM station you experience loss of the desired signal, unless like you said the transmitter is still on, and you are experiencieng desense or fundamental overload of the first RF stage or mixer stage in your radio. If this is the case, not only your desired signal should go way down, but the background noise should also go way down also. Now to the meat and potatoes. of radio propogation. This will be quick and dirty: the ionosphere is divided into layers of ionized particles. D E F1 and F2. these layers are created by energy from the sun. They come and go, by solar radiation. The layer you are concerned with that deals with the AM broadcast band, .530 to 1710 Khz is the D layer and the F layers. In the daytime, you mostly rely on ground wave propagation not skywave, as the D layer absorbs these lower frequencys Unlike shorter HF waves that are not absorbed by the D layer and go on to refract on the F layers, to acheive long distances during the day. But at night the D layer disipates quickly as soon as the sun stops fueling the ionosphere, but the F layers usually converge into one F layer and will support propogation of the AM broadcast band signals. The F layer last much longer sometimes completely overnight. As soon as the sun comes back up the D layer forms quickly and absorbtion start again at these frequencys. So again no skywave, just groundwave propogation, which is short distance. Now all of that said.. It is all way way more complicated than I have let on, and while I know more, I dont know enough by a long shot. In fact some of the processes are not fully understood by anyone. For those of you wondering about the E layer.. well it is usually called "sproradic" E because it is usually um..sporadic:laughing: If you want to learn more, let me know and we will get you hooked up with getting on track to get your Amateur Radio License, and you can spend the rest of your life trying to understand radio propagation:laughing: There are many "hams" here on TBN. some are pretty easy to spot. some not. Bet you wanted a simple answer to a simple question didn't you
James K0UA

Holy Cow that was complicated. I'm glad someone understands this.

 

[Alan L.]

Well it get kinda complicated.. so hang on.
I dont know why when you drive by the old AM station you experience loss of the desired signal, unless like you said the transmitter is still on, and you are experiencieng desense or fundamental overload of the first RF stage or mixer stage in your radio. If this is the case, not only your desired signal should go way down, but the background noise should also go way down also. Now to the meat and potatoes. of radio propogation. This will be quick and dirty: the ionosphere is divided into layers of ionized particles. D E F1 and F2. these layers are created by energy from the sun. They come and go, by solar radiation. The layer you are concerned with that deals with the AM broadcast band, .530 to 1710 Khz is the D layer and the F layers. In the daytime, you mostly rely on ground wave propagation not skywave, as the D layer absorbs these lower frequencys Unlike shorter HF waves that are not absorbed by the D layer and go on to refract on the F layers, to acheive long distances during the day. But at night the D layer disipates quickly as soon as the sun stops fueling the ionosphere, but the F layers usually converge into one F layer and will support propogation of the AM broadcast band signals. The F layer last much longer sometimes completely overnight. As soon as the sun comes back up the D layer forms quickly and absorbtion start again at these frequencys. So again no skywave, just groundwave propogation, which is short distance. Now all of that said.. It is all way way more complicated than I have let on, and while I know more, I dont know enough by a long shot. In fact some of the processes are not fully understood by anyone. For those of you wondering about the E layer.. well it is usually called "sproradic" E because it is usually um..sporadic:laughing: If you want to learn more, let me know and we will get you hooked up with getting on track to get your Amateur Radio License, and you can spend the rest of your life trying to understand radio propagation:laughing: There are many "hams" here on TBN. some are pretty easy to spot. some not. Bet you wanted a simple answer to a simple question didn't you
James K0UA

I was just gonna say skip.

 

[k0ua]

I was just gonna say skip.

and do you know why it is called "skip"?

James K0UA

 

[coachgrd]

Interesting, thanks.
Gary

 

[dmccarty]

and do you know why it is called "skip"?

James K0UA

That was funny.

I listen to a FM station 60 miles south of me. At the house reception can be good or bad. Seems to depend on the sun and cloud cover but not always.

On the way to work which puts more distance between me and the station reception gets worse as expected but always is worse under power lines. There are certain intersections that totally block reception due to the power lines. At some intersections the signal is blocked but if I move up a just bit, maybe 6-12 inches, I can get the signal again.

I doubt many people do this anymore but I am sure there are some, but people used to try to pick up AM stations at night and see how far away those stations were from their location. I think it was/is called DXing. I did this as a kid before cable. :laughing: Can't believe I remember this bit of trivia. :laughing:

I do remember picking up stations that were many states away from me on a hand held AM/FM TRANSISTOR radio.

Later,
Dan

 

[mrutkaus]

I go with K7LN!

Interesting question.

Mike
K4QET

 

[k0ua]

I go with K7LN!

Interesting question.

Mike
K4QET

A quick check with a field strength meter would tell the tale.. even something as simple as an antenna, and LC circuit (coil,cap) a 1N34A germanium diode and a sensitive uA meter would let us know if that "old" installation is radiating. I am having a little trouble with the resonant antenna dead-spot theory. maybe so, but having run a lot of resonant antennas next to each other, I haven't seen it. Anyone else care to weigh in on that aspect of his question?
James K0UA

 

[coachgrd]

I doubt many people do this anymore but I am sure there are some, but people used to try to pick up AM stations at night and see how far away those stations were from their location. I think it was/is called DXing. I did this as a kid before cable. :laughing:
I do remember picking up stations that were many states away from me on a hand held AM/FM TRANSISTOR radio.

Later,
Dan

Oh I did that. I used to lay in bed with the transistor radio, ever so slowly turning the tuning dial, trying see what baseball game I could pick up from here in NW Pa. Detroit was easy, I think they had a powerhouse station named CKLW. I could even pick up the Cardinals from St. Louis on occasion, KMOX maybe?

I wish life were that simple now.

 

[eepete]

K0UA "skip" explanation right on the money. My best skip ever was doing CW on a double E skip at 50 MHz from NC to CA on a home built 100W PA :thumbsup:. Got a kick out of a EE doing a double-E skip :laughing:.

I remember listening to Texas and Chicago (WLS) AM radio as a kid in New England, then when I got my first ham receiver (around 8th grade) listening to BBC and other stations from around the world. Had fund computing and building the antennas, learning about radio propagation stuff. The internet and cheap phone service had made the planet smaller, and we lost a little of the magic in there somehow. But I don't want to go back. I'm not sure what's equally fun that gets kinds into math and science and mechanical stuff these days. Back to the OP now!

The AM station goes away by the old AM towers is a puzzler. The part that bothers me is the "1/2 mile past before the signal comes back". If the towers were blocking something, after you're a wavelength away you should be good. So for AM, once your past 1000 feet or so the towers should have no effect. Need more geek data to solve this one.

The lay explanation here is "For something to block you, it's got to be between you and the something." When you're as little as 1/4 mile away, those old antennas are not between you and the station .

Pete WA1YYN

 

[Freds]

Must be the old WRIE station on Rt 19. I hear it's haunted.
lol ;^)

 

[k0ua]

I remember listening to Texas and Chicago (WLS) AM radio as a kid in New England

Wow. does that ever bring back some memories. "89 WLS" sung with a very pleasant female voice. way back in the late 60's and early 70's. Yep my dial was tuned to 890 Khz.. we called em Kilocycles back then It was a nice "clear channel" 50 Kilowatt rocker back then. I think it is talk radio now.
Yeah I dont know about the signal degradation thing either, need to see it. I go by an old abandond AM station tower within 150 foot of the antenna every day, and no degradation of a 560khz signal I listen to. and it is not all that strong either. Of course it hard to believe that the transmitter is still on on his station, who would pay the electric bill for even a killowatt station.? Unless it is really in use. I go by another station on 900Khz, and it knocks down my AM radio into the dirt for at least 1/8 of a mile. Not only the desired off channel signal goes in the dirt, but also the background noise. as the reciever front end is overloaded badly. (dodge company car).

As for the E skip on 6 meters, that is a lot of fun, also really cool to work tropo scatter on 2 meters.. I worked 25 states one spring/summer/fall at one location in Southwest Missouri when I was about 19 the antenna was a homemade pair of 5 element beams made out of #10 aluminum wire and the booms were 1/2 inch conduit. We pulled the wire to stretch/ and work harden the wire as it was very soft to begin with. Phasing harness was pulled apart zipcord. Best DX was a contact from my 25 watt rig to another 25 watt mobile in Albany NY. on 146.52 simplex. Oh, well we are probably boreing the heck out of the others, with all of my old time memories..
73
James K0UA

 

[coachgrd]

Must be the old WRIE station on Rt 19. I hear it's haunted.
lol ;^)

That's it Fred.

 

[Boiler74]

I read in a book once that said the larger AM stations can "crank up" their power at night as well, allowing them to reach further. That this requires a more expensive license, so the smaller stations go off air at night, and only the big boys with deep pockets can afford to broadcast at night with the higher power. Is this not true then?

Just curious.

Josh

 

[eepete]

When I was going to Purdue in the 70's I worked at WBAA, their AM radio station. At night, they cut the power back from 5KW to 1KW and kicked in the two side towers so that there was a pattern to the signal. They cut back because of the skip that pops up at night. The pattern was to avoid interfering with other stations on the same frequency.

AM radio has always had some high power "clear channel" frequency assignments. WLS in chicago, WBZ in Boston, and more such as Texas and Atlanta were there too.
EDIT: K7LN cleared up my incorrect Clear Channel recollections in the next post. Tnx K7LN!

A buddy of mine and I used to ride around the cornfields of Indiana once a week and record the field strength of the station when it changed over to the pattern to be sure everything was OK. It all seemed so simple back then...

Pete

 

[k0ua]

Clear channel stations are as the name implied; clear channels. They were 50KW stations with a single non directional antenna and operated at the same 50KW day and night.

what about the stations with callsigns that begin with XE
all bets are off with them
James K0UA