Perplexing Propagation: Newsletter Challenge (January 2017)

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

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

OK, it's HAARP then.

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.

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

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.

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.

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

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

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.

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.

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.

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.

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...

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.

"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.

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

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

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.

'pah' to all that hi-tech stuff.

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.

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

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.

...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.

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

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.

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.

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

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

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.

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.

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

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.

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.