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Perplexing Propagation: Newsletter Challenge (January 2017)

Posted December 31, 2016 5:01 PM
Pathfinder Tags: challenge questions radio

This month's Challenge Question: Specs & Techs from IEEE GlobalSpec:

Short-wave and AM transmitters are required to lower their transmission power and directivity of the signal, and, in some instances, are forced to go off the air at sunset. The reason for this FCC requirement: At night, the wave signals from these transmitters (like your local AM radio station) travel a longer distance than during the day. Keeping all the station’s parameters the same, why does the night help to increase the range of these radio waves? Does this also happen with FM transmitters?

And the answer is:

AM signals (including short-wave) are sent to space by the transmitter. How far an AM station’s signal travels depends on the station’s power, its transmission frequency, its antenna, the conductivity of the land around the antenna, and the ionospheric refraction. Some of these waves (the ground signals, as shown in the figure) do not go too far because the earth stops them. The signals that are transmitted to the sky (sky signals, in the figure) reach the ionosphere and are refracted back to earth, which in turn refracts them back to the ionosphere; this process lasts until the signal power is exhausted.

The ionosphere is a layer ranging from 30 miles to over 100 miles from the earth’s surface. The ionosphere is a heavily charged layer of ion molecules, and the number of ions is a function of the amount of sunlight received by the ionosphere. During the day, the production of ions increases, but during the night—in the absence of sunlight—the ionospheric ions decrease, thus, decreasing the width of this layer. A smaller ionospheric width means a longer distance from the surface of the earth, and this raises the refracting levels so the radio signal travels farther around the earth. This is a problem because the AM signal from a particular radio station may interfere with a faraway radio station with the same frequency in violation of allowed ranges for each station. This is the reason that at sunset U.S. AM radio stations must: (a) reduce power to decrease the range, and/or (b) directionalize their signals so they are sent in some directions more than others, or (c) go off the air until sunrise the next day.

For FM signals, on the other hand, because of their higher frequencies, ionospheric refraction is negligible. Having a short wavelength (high frequency), FM signals move through the ionosphere without appreciable interactions, as is depicted in the right hand-side figure.

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#1

Re: Perplexing Propagation: Newsletter Challenge (January 2017)

12/31/2016 5:16 PM

Anything to do with bouncing off the Ionosphere ? I don't know, it's fairly hard to see.

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#2

Re: Perplexing Propagation: Newsletter Challenge (January 2017)

12/31/2016 6:25 PM

The waves can't see the boundary markers at night....

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#7
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 4:27 PM

OK, it's HAARP then.

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#13
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 4:52 PM

I'm now waiting for the tin-foil hats to come out of the woodwork. You realise the REAL purpose of HAARP is mind-control and weather modification, yes?

And the flat-Earthers. Imagine how perplexing over-the-horizon communications must seem to someone who believes the horizon is infinite.

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#15
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 5:10 PM

You seem to be suggesting the Earth is not flat. I just looked outside and it jolly well is .

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#21
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 5:44 PM

Bollocks. If were flat you could see me waving. Okay, so there's a few trees in the way.

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#23
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 5:51 PM

Most people reckon my ego is so big I could see over any trees. Anyways,I can just drill a hole and pop up. A flat Earth csn have two sides.

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#3

Re: Perplexing Propagation: Newsletter Challenge (January 2017)

12/31/2016 7:47 PM

Ionized molecules in the ionosphere refract radio signals in the AM range back toward the ground. These signals may bounce between the ground and ionosphere multiple times resulting in transmission over long distances. FM radio is much shorter wavelength and does not bounce off of the ionosphere.

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#4
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 10:48 AM

During the daytime when the sun is shining, lower levels become ionized, but because the air is denser, recombination occurs more rapidly. At night, the higher levels where the air is very rarified remain ionized for a long period of time and refract the radio waves of the AM band (.535 - 1.610 MHz). It is not effective for FM bands (87.5-108 MHz) and these frequencies are blocked by the curvature of the earth.

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#32
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/02/2017 11:50 AM

GA! You bring up an interesting point: Do the waves reflect or refract (or possibly a combination of the two) at the ionosphere? I think I've always heard "reflect" in previous discussions, but if the ion density varies with altitude, then I think "refract" may be the more accurate term.

Also, generally speaking, reflectors generally work better when they are large compared to the wavelength, so it would seem like the smaller FM waves should reflect better, which is not the case.

A side note: FM/TV waves can also diffract. I heard (about 50 years ago) that people in Bishop, California (on the East side of the Sierra Nevada) could receive TV signals from Channel 6 TV (just below the FM band) in San Luis Obispo, Ca. (on the West side of the Sierra Nevada) , if they pointed their antennas at the tops of the Sierra mountains.

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#35
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/03/2017 4:10 AM

'Edge' diffraction, sometimes it's a real problem at UHF frequencies. The Tropo's been good over Christmas too.

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#36
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/03/2017 9:17 PM

Do the waves reflect or refract (or possibly a combination of the two) at the ionosphere? I think I've always heard "reflect" in previous discussions, but if the ion density varies with altitude, then I think "refract" may be the more accurate term.

That's a good point. It's called reflection, but I suspect it's like a mirage, which looks like a reflection but is really a refraction. Reflection requires a discontinuity from one medium to another.

Here is a good explanation: Ionospheric Propagation Explained

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#30
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 10:42 PM

Used to listen to Sydney AM radio stations at night, while at boarding school - 300 km away - on my crystal set.

No chance during the day.

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#37
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/03/2017 9:30 PM

My fascination as a kid was trying to find the farthest AM radio station I could pick up at night. I once received one 2000 miles away.

When I could afford it, I bought a shortwave radio that covered bands that have much farther range, even in the daytime.

When the internet came along, it seemed to remove the wonder of receiving signals from far away.

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#39
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/03/2017 11:10 PM

Same here! I remember hearing XERB (Mexico) and several of the "W" eastern stations when listening from northern Colorado, in the late '40s or very early '50s.

I definitely remember listening to KOA Denver and KOB Albuquerque using a homemade crystal radio with a very long wire antenna in northeastern Colorado in the late '40s, but that may not have been due to any ionospheric effects.

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#41
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/04/2017 5:33 PM

While serving in the US Army Signal Corps at Camp Gorden in Augusta, Georgia in 1956, I listened to the Dick Saint Clair Polka Parade broadcast from KFI 640 AM in Los Angeles. I believe it was a 50KW all clear channel. Perfect reception during the late afternoon hours, if memory serves. Approximately 3000 miles coverage.

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#42
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/04/2017 8:26 PM

Welcome to CR4!

Do you remember anything about the radio you used? Besides the crystal radios we were making in the late'40s, I remember sitting around Saturday evenings listening to the Lone Ranger and Sargent Preston of the Canadian Mounties on a floor-standing RCA Victor...

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#43
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/05/2017 1:08 AM

Not in detail. We carried little, a bit bigger than a cigarette pack, battery powered radios sporting "acorn" tubes. Tuning range through the AM band 550 1400 kc. Carried in our fatigue jacket pockets.

Cannot remember its cost. On army pay of those days they where expensive.

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#44
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/05/2017 4:07 AM

All the older gear I grew up with, dad was an R.O. in ww2 and wanted to pass on the skills, like my 19 set, B40's and a Hand built Codar were thermionic.

I have friends both sides of the Atlantic rebuilding such gear even now, but after hybrid then solid state I like a lot of people don't have the space for valve based equipment anymore. My current HF rx is a 'modern' (1990's) transceiver I rescued damaged from a skip (dumpster).

When the University radio club was shutdown, they wanted the shack space for an office and with the internet interest had insufficient members to fight the student union's bureaucracy, the union threw thousands of pounds worth of gear out having stored it in a damp cellar under the bar for several years. The RSGB young radio amateur of the year was an undergrad when the club closed and made it very clear he had no interest in being a member or even doing anything to do with ham radio when spotted by a member at the student societies 'bun fight', so perhaps it wasn't surprising they couldn't recruit new members.

Most people have little idea of how useful ham radio can be in the event of a natural, or other, disaster, when the internet and other telephony either fails or is taken off line on governmental orders, perhaps that's a good thing?

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#5

Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 1:03 PM

"Does this also happen with FM transmitters?"

Yes, by means of a transient phenom in the Ionosphere's E-layer called 'sporadic E'. Where the ionosphere is normally transparent to FM frequencies (88-108 MHz), patches or clouds of unusually high electron density forming in the E region can refract signals as high as 250 MHz back to Earth much in the same manner as the regular E- and F-layers do with lower-frequency AM and shortwave signals, making 'skip' at these higher frequencies sometimes possible over thousands of km.

At the very bottom the EM spectrum the space between the Earth's surface and the ionosphere form an approximately spherical resonant cavity that rings like a bell, but in this case one struck by lightning about 50 times/second worldwide, resulting in the so-called Schumann resonances. The fundamental frequency is around 7 Hz or so.

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#6
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 4:08 PM

Before satellites, with enough brute force, they bounced microwaves off the troposphere...

http://wikimapia.org/6889487/Adak-White-Alice-Communications-Station-Site

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#11
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 4:41 PM

Yes, and troposcatter is still used, albeit over shorter distances than the early days and using much smaller gear. Typical operating frequency is around 2 GHz but not limited to that. North Sea and GoM oil platforms use it, some widely dispersed island chains and of course, the military.

Ham operators on the VHF bands get real excited about sporadic-E because they can make DX (long-distance) contacts on a band that's normally limited to LoS.

Sporadic-E also works with VHF TV signals. When I was a teenager I was able to receive a (analog) VHF TV station's broadcast from about 800 miles away.

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#12
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 4:49 PM

'pah' to all that hi-tech stuff.

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#14
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 4:58 PM

With a good temperature inversion, though, I bet you could hear some interesting stuff from distant points on the ground, you know? After reflecting off the top of the layer?

Do you get stable temp inversions near the coast? Ones that act like an acoustic waveguide? We get them here sometimes. There are train tracks a few miles from here, and on some (usually very cold and still) nights the trains sound like they could come right through the door.

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#16
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 5:12 PM

I'm listening to the rail tracks and will report back ver.........

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#19
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 5:28 PM

Kris! Kris! Wake up! That's not a railroad track, mate! That's your arm!

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#38
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/03/2017 9:39 PM

I've noticed that too in a temperature inversion.

Another factor is the wind, which blows faster (and a different direction) at altitude. Sound propagating in the direction of high-altitude winds would tend to refract downward.

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#40
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/03/2017 11:18 PM

...and not only sound waves.

Ever seen "looming"? In late summer 1970, I was on the Upper peninsula of Michigan, and for a short while was able to watch cars and trucks driving along Highway 61, across 75 miles of Lake Superior. I'm not sure whether those vehicles were in The US or Canada...

Clearly a temperature inversion effect.

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#45
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/05/2017 2:00 PM

That sounds like a Superior mirage. I've never seen one that I know of but I see the Inferior mirages quite often on roadways. I would suspect that a number of the UFO sightings could be due to Superior mirages refracting the headlights of moving traffic on the ground.

https://en.wikipedia.org/wiki/Mirage

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#46
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/05/2017 2:46 PM

Yep! That's it. The term "Looming" is the term used in 1970 by the professor of meteorology at MTU, Houghton, where I was attending a summer workshop for Physics teachers.

The image I saw was definitely right-side-up. I was not aware at the time of the pillars mentioned by Wiki as commonly associated with superior mirages, but now I understand that is why the vehicles I remember appeared to be driving across a long bridge, when there are none in that vicinity.

I think I may have a photograph of it somewhere, but it may just be engraved in my mind, as it was such a striking image.

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#17
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 5:17 PM

Seems somebody made an audio recording at one of Hythe's mirrors. Nothing outstanding but you can hear a distant airliner, wind noise, sheep and a few birds. Hard to say whether these were ambient sounds you'd hear anyway or distant ones recorded at the focus.

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#18
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 5:25 PM

I told Mrs K it was prudent to keep the windows closed on 'frisky friday'.

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#20
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 5:29 PM

Good plan! Sounds like it worked!

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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 5:47 PM

It did for me, but them sheep were not too happy Mrs K was well unhappy.I've no idea what happened to the pilot landing at Lydd International. Yes, It really is called that, even though my driveway is longer than the runway. Not mentioned much, but it's 'international' because they fly to France. Brexit may have ended that status.

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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 5:54 PM

Next time you'll wear camo wellies and go at night.

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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 6:01 PM

You think I didn't ?

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#26
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 6:04 PM

<splarf!!!>

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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 6:05 PM

You realise of course we've totally derailed this thread.

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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/02/2017 3:52 PM

You're quite right. We best behave for a while.

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#28
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 7:11 PM

I spent some time on Adak and got a chance to walk up and look at White Alice before they sold it for scrap.

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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 7:39 PM

Man, that's way out there. Service-related? Naval Air Facility?

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#33
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/02/2017 3:39 PM

As an example of either atmospheric ducting of sporadic-E, in the early '60s at Purdue (Lafayette, IN) while waiting for the dormitory dining room to open some of us were watching cartoons from a station in Bloomington, IN when an interfering signal from a Louisiana station overrode the nearer station.

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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 4:29 PM

Much more tech than my post, but where is my GA !

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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 4:34 PM

Just one?

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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/01/2017 4:39 PM

ROFL !

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#31

Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/02/2017 9:58 AM

The ionosphere cools at night, causing the radio waves to refract (not reflect), giving the signal more range. The type of modulation has no influence. Higher frequencies have less problem with this, and there's minimal effect above 30 MHz.

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#47

Re: Perplexing Propagation: Newsletter Challenge (January 2017)

01/24/2017 11:54 PM

short wave is reflected at the ionosphere; FM transmitters use Ultra short wave; they are not intense enough to reach the ionosphere.

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#48

Re: Perplexing Propagation: Newsletter Challenge (January 2017)

02/28/2017 1:14 PM

There is another side do this Challenge. Yes, most of US AM radio stations must reduce their transmission power at sunset. However, there are certain AM radio stations called Clear Channel Stations that are authorized to increase their transmission power up to 50,000 watts at night. A city I used to live in had a clear channel station. It is WBT in Charlotte, NC (I think it is the only radio station with a 3 letter call sign). The range for WBT was several hundred miles at night.

Several others are WPTF Raleigh, NC, WOWO Ft. Wayne, IN and KDKA Pittsboro.

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#49
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Re: Perplexing Propagation: Newsletter Challenge (January 2017)

03/01/2017 1:33 AM

KOA Denver and KOB Albuquerque have three letter call signs, and have been around long enough, that I suspect they may be clear channel stations. KCRA Sacramento has 4 letters, but is a Clear Channel Station.

Another factor is the directional properties of the signals. I'm pretty sure that KCRA directs its signal mostly north and south.

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