Part 15 Skywave

[BobOnTheJob]

This popped into my mind...has a compliant Part 15 AM station ever been received via skywave?

 

[WCWalker]

Yes. Back in 1999 I read a report from someone in the south that they had picked up WVRM 1620 aka Village Radio of Montclair, NJ in I believe FLA. That station passed an FCC inspection with its Rangemaster transmitter shortly after going on the air.

 

[rfry]

Receiving a useful or even detectable skywave from a functionally legal Part 15 AM setup over a path from NJ to FL is highly unlikely.

Even when using 100 milliwatts of transmitted power (ERP), the field intensity at the end of a 1500 km free space, unobstructed propagation path is about 1.5 µV/m -- which is below the medium wave radio noise floor almost everywhere but in a screen room.

And this doesn't include the path loss produced by incomplete reflection of the skywave from the ionosphere.

RF

 

[WCWalker]

Receiving a useful or even detectable skywave from a functionally legal Part 15 AM setup over a path from NJ to FL is highly unlikely.

Even when using 100 milliwatts of transmitted power (ERP), the field intensity at the end of a 1500 km free space, unobstructed propagation path is about 1.5 µV/m -- which is below the medium wave radio noise floor almost everywhere but in a screen room.

And this doesn't include the path loss produced by incomplete reflection of the skywave from the ionosphere.

RF

A guy here in KC with an interest in radio showed me a newspaper story about this event and I seem to recall it was from a FLA newspaper. It certainly is not a common occurrence but it did happen at least this one time.

 

[rfry]

It certainly is not a common occurrence but it did happen at least this one time.

People are free to believe that if they wish, however even in the overpowered example I posted it would be impossible in terms of the physics involved.

 

[WCWalker]

It certainly is not a common occurrence but it did happen at least this one time.

People are free to believe that if they wish, however even in the overpowered example I posted it would be impossible in terms of the physics involved.

Your Immenseness. It really did happen. I don't know if it was with one transmitter or four of the synched Rangemasters. But when I have a guy in a suburb of KC showing me a legitimate news article about how a station I was not even acquainted with at the time, was picked up on a skywave in FLA, then I am going to believe the story.

You sir, are free to believe whatever you wish.

 

[FreddyE1977]

What receiver were they using? (cause I wanna buy one!)

 

[pianoplayer88key]

Receiving a useful or even detectable skywave from a functionally legal Part 15 AM setup over a path from NJ to FL is highly unlikely.

Even when using 100 milliwatts of transmitted power (ERP), the field intensity at the end of a 1500 km free space, unobstructed propagation path is about 1.5 µV/m -- which is below the medium wave radio noise floor almost everywhere but in a screen room.

And this doesn't include the path loss produced by incomplete reflection of the skywave from the ionosphere.

RF

I was thinking about something regarding that "below the noise floor". It seems that sometimes signals CAN be detected "below" the noise. I've seen websites mentioning that some CW and efficient digital modes could be detected 30 dB or so below the noise, for example.
Some time ago I did an experiment with an audio editor and one track white noise, the other either a ~2-4 kHz (single frequency, don't remember which now) sine wave or some program material that wasn't dynamic range compressed. I basically found that I was still able to faintly detect the sine wave about 36-40dB below the noise, and the program material was still somewhat readable at around -10-15 dB or so, IIRC.
I also experimented with an oscillator, Tecsun PL-606 and Sony SRF-59. (I used the PL-606 for the signal strength readings, and the SRF-59 because it was easier to dig the weak signal out on that.) I started with a live broadcast signal that was indicating about 30 dBµ and quite readable (but a little noisy - S/N was in the mid to upper teens when the PL-606 was set to 1 kHz bandwidth). With the oscillator, I was still able to barely hear audio on the SRF-59, even when the PL-606's signal indicator was showing 87 dBµ.

So, is the noise level REALLY the limit, or is it really true that signals can be detected below the noise level? I had an idea for an experiment, but I don't have the equipment or location necessary to do it.
Take a good quality field intensity meter, high-end radio (Collins, Drake, just to give you a couple brand name ideas), beverage antenna (or an equivalent-sensitivity omnidirectional antenna or loop antenna) and signal generator (with audio input capability and the ability to adjust the output level) to some area where the local man-made noise level is well below the wintertime atmospheric noise level, and preferably not within the 0.5 mV/m groundwave contour of any AM broadcast station. Using the radio + antenna, find a frequency of your choice that has absolutely NO signal detectable (using the BFO / CW mode to be extra sure). Measure the noise level of that frequency using the FIM and note it. Then, set up the signal generator to transmit something on that frequency and set it up so it's easily readable in the receiver with the large antenna in place.
What I'm basically wanting to know is.... When turning the transmitter's output down, does the signal disappear BEFORE the FIM's indicator drops to the noise floor (as measured when no signal was transmitted), right AT that time, or AFTER? If after, how far do you have to drop the output on the transmitter before it's no longer detectable?

 

[rfry]

I was thinking about something regarding that "below the noise floor". It seems that sometimes signals CAN be detected "below" the noise. I've seen websites mentioning that some CW and efficient digital modes could be detected 30 dB or so below the noise, for example. (etc etc)

That is as may be.

However such circumstances would not apply to a casual listener using a typical AM broadcast receiver trying (or happening) to receive a functionally compliant "Part 15 AM" skywave signal over a 1500 km, free-space path -- no matter what a "legitimate" news article allegedly stated.

Given the ambient r-f noise in the MW band at most locations, useful reception of the program modulation of a MW AM signal over a 1500 km skywave path requires much more radiated power than produced by a transmitter and antenna system radiating 100 milliwatts at all elevation angles toward the receive location.

RF

 

[radioboy]

A few years ago I had a Part 15 AM station that operated as a daytimer on 1160kHz.

One Sunday morning, just a few minutes after signing on around 7:15 a.m., a listener reported
picking the station up in his car around Crawfordville, Fla., roughly 25 miles away. Seems like it
was around mid-September or so. He described the programming he heard, which included a local weather forecast. Naturally, he didn't pick the signal up for long.

 

[druidhillsradio]

I would have to agree with Rich, very unlikely if all was legal. However that being said, using resonant 1/4 wave vertical at the operating frequency, with 120 ground radials, likely, but not very often. Also, not legal.

 

[stormy01]

I'm going to make it simple: 100 milliwatts of power can't even get high enough to hit the ionosphere for it to bounce such a signal back down to the earth. The ionosphere starts at about 43-50 miles (70-80 km) high and continues for hundreds of miles (about 400 miles = 640 km). http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Atmosphere.shtml We're doing good if we get 1-2 miles of range! And in case anyone forgot, the effective radiated power of a *Legal* Part 15 operation is far, far less than 100 mW!!!

 

[druidhillsradio]

I'm going to make it simple: 100 milliwatts of power can't even get high enough to hit the ionosphere for it to bounce such a signal back down to the earth.

Not true. Amateur radio operators would differ with you.

 

[pianoplayer88key]

I was curious how strong a 100 mW signal with an efficient (exceeding part 15, I know) antenna be on a single hop. I'm not factoring in ground & ionospheric absorption losses, rather I'm just calculating based on free space, so I'd expect the actual field to be lower.
As for the antenna, I'll use KSTP's daytime stick as an example. It has an efficiency of 511.77 mV/m @ 1 km for 1 kW, so downconverting to 100 mW results in approximately 5.12 mV/m @ 1 km. For the bounce, I'll assume 75 km to the ionosphere with a mirror-like bounce (obviously not real-world), for a total path of 150 km. So, if I calculated free-space loss correctly, the field would be approximately 34.12 µV/m at 150 km.
I've heard signals not much above this level in semi-rural southeastern San Diego county with a portable radio (Tecsun PL-606) and its 4" ferrite bar. (They were faint, but definitely identifiable; possibly even below that level as for a few of them, per Radio-Locator, I was more than twice as far as the predicted 150 µV/m contour, yet they were almost seek-stoppers.) I expect that a quality communications receiver with a beverage antenna in a low-noise environment could easily hear signals tens of dB below that. (I also figure that the same elaborate setup would get "full-quieting" (NO noise audible - similar S/N to listening in HD in a strong signal area) on a signal to which my GE Superradio III, for example, would be completely deaf.)

Anyone care to calculate approximate part 15 skywave field using more life-like and legal examples?

 

[rfry]

Anyone care to calculate approximate part 15 skywave field using more life-like and legal examples?

A very good, functionally legal Part 15.219 AM installation on 1700 kHz might produce an inverse distance field of 0.1 mV/m at a radius of 1 mile in the horizontal plane.

The chart linked below is based on a transmit system producing 100 mV/m at that distance. In order to apply it to the Part 15 system defined above, the skywave field intensity values in the chart will need to be multiplied by 0.1/100 = 0.001. A 3-meter vertical, unloaded monopole radiator on 1700 kHz has a relative field at angles above the horizontal plane equal to the cosine of the elevation angle, as described in the chart.

Applying this chart for the highest skywave fields occurring about 140 miles from the transmit site for the Part 15 system gives the values shown below. The skywave field is less for distances closer and further than 140 miles.

Skywave Field,
Refl Coef. µV/m
1.0 0.4
0.5 0.2
0.2 0.08
0.1 0.04

Given that the ambient r-f noise level in the MW band is considerably higher than these incident fields, it would take some heroic receiving equipment to produce a demodulated signal from such fields with a program signal-to-noise ratio that one would want to listen to for long -- if it was detectable at all.

Note that the experience of ham radio operators where short-wave transmit systems with transmitters of 100 mW and even less might provide useful results does not apply to Part 15 AM, for several reasons. The ham systems have:

• much more efficient antennas, resulting in higher radiated powers
• generally lower ambient r-f noise
• more sensitive receiving equipment
• more efficient modulation performance such as single sideband, or even CW (Morse code)

CHART http://i62.photobucket.com/albums/h85/rfry-100/TermanFig55.jpg

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[radiomonkey2]

This popped into my mind...has a compliant Part 15 AM station ever been received via skywave?

Do a search for MEDFERS (Medium Freq.). A number of hams used to operate beacons in the X-band before it became crowded, I thing they regularly were heard for hundreds of miles.

http://www.ka7oei.com/psk_medfer.htmlhttp://www.ka7oei.com/psk_medfer.html[/url]

This page details a contact made at 800 mi.

 

[rfry]

Hams with high motivation, using very good equipment and very narrow-band (low data rate) radiated emissions may be able to detect signals over a skywave path from a functionally compliant Part 15.219 AM system. The noise bandwidth of the signal path will be important.

But that will be impossible for the average operator using a functionally compliant Part 15.219 setup with amplitude modulation of voice/music program sources, for reception by typical consumer-level receivers with loopstick antennas.

The link below leads to an analysis of this for a 200 km path, using the Friis equation. The tx power was set to 70 mW, and the antenna system gain to -20 dBi -- the approximate gain of an earth-mounted, 3-m vertical monopole at an elevation angle needed for a distant skywave path. The gain of the receiving loopstick was taken from the graph that appears at the bottom of that page. The calculation assumes 100% reflection from the ionosphere (which isn't likely).

The voltage across the receiver r-f input terminals is extremely low, and far below the ambient r-f noise level in the AM broadcast band.

http://i62.photobucket.com/albums/h85/rfry-100/FRIIS_Part15AM_Skywave.gif

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[Tom Wells]

But there's always magic, which is an unquantifiable term of the equation.
Highly motivated is perhaps the closest technical term for "magic".

 

[WCWalker]

Looks like the books on Physics may be rewritten. It is apparent the people that claim to know the laws of Physics are using a flawed system when trying to analyze why or why not a Part 15 AM signal behaves a certain way that defies their view of the laws of Physics.

http://news.yahoo.com/physics-atom-smashers-antimatter-surprise-232412931.html

 

[rfry]

It is apparent the people that claim to know the laws of Physics are using a flawed system when trying to analyze why or why not a Part 15 AM signal behaves a certain way that defies their view of the laws of Physics.

The concept in the link posted by William C. Walker even if/when proven as true has nothing to do with the proven physics applying to the performance of Part 15 systems operating in the AM/FM broadcast bands, or any other system radiating/receiving electromagnetic fields.

RF