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Old 06-08-2013, 10:26 PM   #1
Nailhead OP
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Some FM Questions...

Frequency Modulation, not Flea Market...

First, I've wondered for quite some time why FM reception seems to get worse as the weather warms. In the winter, WPR comes in consistently, well...decent on my stereo receiver. About a week ago, its signal started its seasonal degradation to "almost unusable", and it'll be this way until September or so. Why would air temperature have any effect on FM radio reception? It's gotta be something to do with the sun and its differing position in the sky, right?

Second, I was listening to said radio station about a month ago, when several times, an Oregon Public Broadcasting station faded in and out. I'm familiar with "skip", but I was under the impression that was exclusively an AM phenomenon. What gives?

Anyway, just curious & thought there would be some radio experts around here somewhere.
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Old 06-08-2013, 10:56 PM   #2
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No expert, but it's probably summer foliage getting in the way.
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Old 06-09-2013, 08:31 AM   #3
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Christ, there are guys earning their PhD's who cannot give you a good answer.

I'm a radio amateur and it's still a mystery.

Usually what happens is that one of your RF signal paths either gets much worse or multipath gets BETTER and degrades your signal. I live in a deep valley and get radio via multpath and some days you are the windshield, some days the bug. With my amateur FM station, I get better signals in winter when the leaves are gone off the hills.

Also possible your station turned down the power due to repairs?

Do you have a real antenna or a chunk of wire?

If the latter, the former will help a lot - I put up a 6 element FM antenna for a friend and it made a huge difference.
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Old 06-09-2013, 08:38 AM   #4
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Humidity surely is a factor as well if the signal is weak imho. My family has always owned radio stations, by the way, and Dad is a broadcast design engineer who probably knows all there is to know about RF. So I probably ought to ask him. Part of the deal is that modern receivers can work with almost no signal, and the line between almost and no signal is a thin one.

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Old 06-09-2013, 08:53 AM   #5
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Cold winter air is denser and transmits FM signals better...


...or...

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Old 06-09-2013, 09:05 AM   #6
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It is called ducting, where VHF radio signals get refracted or bent toward the ground which increases the range. Very different from the shortwave long range propagation which is more like bouncing than bending of the radio waves. This bouncing occurs at very high altitudes. http://en.wikipedia.org/wiki/Tropospheric_propagation
Like MGOK said radio propagation is actually quite mysterious and can change minute to minute and can change when you move your antenna a few feet. I have done a bit of modeling of VHF and UHF propagation to design telemetry systems, but I have very little faith in my models, though they at least give me a starting point (and something to show when they ask why I thought it would work all the time and it doesn't)
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Old 06-10-2013, 08:30 AM   #7
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Possible that the station is still using a tube type transmitter and they have poor ventilation and ducting.

At least that's what happened to one of my FM stations a decade or so ago. Mama bird built a nest which caused the transmitter to overheat and cut power.
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Old 06-10-2013, 03:38 PM   #8
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Its cause of the damn Gophers.
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Old 06-10-2013, 06:16 PM   #9
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Seriously, FM is more affected by dense stuff like clouds, mountains, trees etc but can bounce off real hard stuff such as concrete and metal buildings. At least that is what I remember from my radio wave class 40 years ago.
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Old 06-10-2013, 09:02 PM   #10
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I do indeed have a tree between my receiver and the WPR transmitter in Douglas. I thought foliage, wood, etc., was transparent to RF, though.

Hmm...
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Old 06-11-2013, 03:52 AM   #11
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Quote:
Originally Posted by GSWayne View Post
It is called ducting, where VHF radio signals get refracted or bent toward the ground which increases the range. Very different from the shortwave long range propagation which is more like bouncing than bending of the radio waves. This bouncing occurs at very high altitudes. http://en.wikipedia.org/wiki/Tropospheric_propagation
]
OT but I discovered this at age 12 when I used to listen to an old tube radio. A Chicago AM pop station would often come on, clear as a bell - in Boston, nearly 1,000 miles away.

Edit - at night.
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Old 06-11-2013, 07:46 AM   #12
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Quote:
Originally Posted by garandman View Post
OT but I discovered this at age 12 when I used to listen to an old tube radio. A Chicago AM pop station would often come on, clear as a bell - in Boston, nearly 1,000 miles away.
After sunset, I suppose?
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Old 06-11-2013, 07:55 AM   #13
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Quote:
Originally Posted by garandman View Post
OT but I discovered this at age 12 when I used to listen to an old tube radio. A Chicago AM pop station would often come on, clear as a bell - in Boston, nearly 1,000 miles away.

Edit - at night.
AM condidiotns change drastically between daylight, dusk and night. As well, some AM radio stations go to half power at night if their license dictates.
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Old 06-11-2013, 08:03 AM   #14
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As well, some AM radio stations go to half power at night if their license dictates.
And some drop their power even further during 'critical hours'. For example one of the AM stations that I operate runs at a full 50kW from sunrise to sunset, then goes to between 474 and 90 watts depending on month of year.

Some go off the air all together to allow for the clear channel stations.
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Old 06-11-2013, 07:30 PM   #15
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1170 KVOO in Tulsa used to be used as a nav beacon. Dunno if anyone still uses AM for navigation.
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