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Author Topic: Factors influencing radio propagation  (Read 58111 times)

Irvine John Donald

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Re: Factors influencing radio propagation
« Reply #15 on: December 10, 2011, 11:06:40 AM »

Sorry Ric. You had suggested in another thread that FN may have fallen out the door while taking a sighting or his almanac blew out he window, to my suggestion it seemed liked AE was on her own.  Of course you meant these to be humorous banter. Or did you?    Not a radio message.
Respectfully Submitted;

Irv
 
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Ric Gillespie

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Re: Factors influencing radio propagation
« Reply #16 on: December 10, 2011, 11:18:28 AM »

My point was that it's possible to construct any number of explanations for a particular outcome, but the one for which there is some actual evidence is more likely to be the right one.
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Irvine John Donald

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Re: Factors influencing radio propagation
« Reply #17 on: December 10, 2011, 11:26:51 AM »

And a fine point it is.  Isn't this also Occam's Razor?  In competing hypothesis the one that raises the fewest newest assumptions is likely to be true.  Actual evidence provides an even greater likelihood.
Respectfully Submitted;

Irv
 
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Heath Smith

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Re: Factors influencing radio propagation
« Reply #18 on: December 10, 2011, 11:38:04 AM »

Quote
Once Bob has had a chance to get his paper written and published, any skeptic with the ability and inclination to buy the software and check his calculations will be able to do so.  Replication of results is the essence of scientific investigation.

This is a very interesting topic and I am looking forward to his report and would happily attempt to replicate the findings.

A post over on this thread suggests a particular piece of software (4NEC2) is being used for this analysis, is this still the case?

https://tighar.org/smf/index.php/topic,285.msg5763.html#msg5763

Quote
The back story for the 3105 Donut article in the October 2008 issue of TIGHAR Tracks is that the dimple became evident when the antenna modeling software was changed from one based on MININEC3 (NEC4WIN95) to one based on NEC2 (4NEC2).

I am not sure about others but I am not a skeptic. I am interested in attempting to validate the work of others. Just attempting to understand all of this hard data is very interesting to me and I am sure others interested in this topic.

I do have a question about the modeling. One key ingredient to this doughnut hole theory seems to be the signal strength threshold used to create this probability map. It is might understanding from reading over the various articles is that the signal registered a "5 out of 5" on the Itasca. If this is the case in the radio log, are we going to assumed that the threshold is 80% < x < 100%? Or are we assuming 5 out of 5 means  x = 100%? This would seem to have a great affect on the model / probability map.

Thank you in advance.
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Ric Gillespie

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Re: Factors influencing radio propagation
« Reply #19 on: December 10, 2011, 11:58:20 AM »

A post over on this thread suggests a particular piece of software (4NEC2) is being used for this analysis, is this still the case?

Yes, I believe so.

One key ingredient to this doughnut hole theory seems to be the signal strength threshold used to create this probability map. It is might understanding from reading over the various articles is that the signal registered a "5 out of 5" on the Itasca. If this is the case in the radio log, are we going to assumed that the threshold is 80% < x < 100%? Or are we assuming 5 out of 5 means  x = 100%? This would seem to have a great affect on the model / probability map.

Bob will have to answer that one.  The difficult part is that the "5 out of 5" was not a quantitative measurement but a qualitative judgement call by the Itasca radio operator.  "Gee, she sure was loud."
« Last Edit: December 10, 2011, 12:33:29 PM by Bruce Thomas »
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Harry Howe, Jr.

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Re: Factors influencing radio propagation
« Reply #20 on: December 10, 2011, 03:37:28 PM »


Not strictly a qualitative judgement call by the operator.  There was probably an audio meter with a needle and a dial with numbers and lines on it. For 5 out of 5 the needle would move all the way to the maximum position indicating a strong signal.
No Worries Mates
LTM   Harry (TIGHAR #3244R)
 
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Ric Gillespie

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Re: Factors influencing radio propagation
« Reply #21 on: December 10, 2011, 04:08:03 PM »

There was probably an audio meter with a needle and a dial with numbers and lines on it.
No, there was not.  It was purely a judgement call but everyone agreed that it was a very strong signal.
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richie conroy

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Re: Factors influencing radio propagation
« Reply #22 on: December 10, 2011, 04:48:51 PM »

i often wonder about the radio logs were amelia says she circling but cannot see u,

obviously she must have been circling land or she wouldnt have knowing which way she was going bearing in mind the weather was over cast

also why didnt she try flying back to were she seen the myrtle ship
We are an echo of the past


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John Ousterhout

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Re: Factors influencing radio propagation
« Reply #23 on: December 10, 2011, 07:52:25 PM »

Martin sez: "Her transmitter was crystal-controlled for three frequencies (500 kcs, 3105 kcs, and 6210 kcs).

The odds are that she was using her daytime frequency for a daytime flight--but strange things do happen.LTM,"

I agree that she was most likely using 6210 on departure from Darwin to Lae.  It worked for her at that time.

When she was approaching Howland, she was heard on 3105, then wasn't heard after saying she was changing to 6210.  The "donut hole" defines the likely distance she was at for these two frequencies to behave in this manner, meaning far enough for excellant propogation at 3105, but out of range for 6210, however, the Chater report indicates that Lae wasn't able to hear her on 6210 until 4 hours after takeoff, but this seems to conflict with the Darwin departure report, which heard her for 2 hours after takeoff (on 6210).  Were skip conditions just that different?  Why does the donut hole seem to apply to the flight to Howland, but not the flight to Lae?
Cheers,
JohnO
 
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Gary LaPook

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Re: Factors influencing radio propagation
« Reply #24 on: December 10, 2011, 08:15:24 PM »

Martin sez: "Her transmitter was crystal-controlled for three frequencies (500 kcs, 3105 kcs, and 6210 kcs).

The odds are that she was using her daytime frequency for a daytime flight--but strange things do happen.LTM,"

I agree that she was most likely using 6210 on departure from Darwin to Lae.  It worked for her at that time.

When she was approaching Howland, she was heard on 3105, then wasn't heard after saying she was changing to 6210.  The "donut hole" defines the likely distance she was at for these two frequencies to behave in this manner, meaning far enough for excellant propogation at 3105, but out of range for 6210, however, the Chater report indicates that Lae wasn't able to hear her on 6210 until 4 hours after takeoff, but this seems to conflict with the Darwin departure report, which heard her for 2 hours after takeoff (on 6210).  Were skip conditions just that different?  Why does the donut hole seem to apply to the flight to Howland, but not the flight to Lae?
There were no "skip conditions" near Howland. The "critical frequency" on that date and in that area was above Earhart's highest frequency of 6,210 kc. For radio transmissions below the "critical frequency" there is no skip zone so no donut hole. You can look this up in any textbook on radio propagation.
See:
http://en.wikipedia.org/wiki/Critical_frequency
http://www.radio-electronics.com/info/propagation/ionospheric/maximum-lowest-critical-optimum-usable-working-frequency.php
http://www.spacew.com/www/fof2.html

For example, I have attached a map of the critical frequency right now, 0315 Z (1545 Itasca time, late afternoon near Howland) on December 11, 2011. As you can see, the critical frequency is 11 megahertz (11,000 kc) well above the maximum frequency of 6,210 kc of Earhart's radio. So right now, any radio signal sent out at a frequency below 11,000 kc in the vicinity of Howland will be reflected back down from the ionosphere with NO SKIP ZONE and no donut hole. On the morning of July 2, 1937 the critical frequency was a bit lower, 9,000 kc, but still well above Earhart's frequency so no donut hole.
gl
de KA9UHH
« Last Edit: December 10, 2011, 10:45:13 PM by Gary LaPook »
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Gary LaPook

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Re: Factors influencing radio propagation
« Reply #25 on: December 11, 2011, 03:32:06 AM »

Let’s say you are staying in a tacky motel room. You lie down it the bed and, looking up, you
see that the ceiling is covered with a mirror. You reach into your bag and pull out a long
cylindrical object. You turn it on.













You shine your flashlight straight up and the spot of light is reflected off the mirror straight
back down, illuminating a spot surrounding the flashlight. You then slowly deflect the
flashlight beam from straight up and see that the reflected spot moves down the bed, passes
your toes, lights up the footboard, then moves across the floor and eventually the reflected
spots falls on the end table near the door. You are looking for your cellphone but the reflected
spot of light is now too dim to adequately illuminate the end table all the way across the room
so you now aim the flashlight directly at the end table and there it is, the missing cellphone.

This is an analogy to explain the TIGHAR “donut hole.”

see: http://tighar.org/Publications/TTracks/2008Vol_24/donut.pdf

Radio waves at the frequencies being used by Earhart behaved exactly like the flashlight beam.
They traveled up into the ionosphere about 160 nautical miles (300 kilometers ) to the “E
layer” which then acts like a mirror and reflects them back down to earth, just like the
flashlight beam. The signals that went straight up came straight back down and the part of the
signal that went up at a slight angle came down near the location of the transmitter. At
progressively lower angles, the reflected signal moved progressively further away from the location of
the transmitter. This process has the technical name of “near vertical incidence skywave
propagation.” It should be obvious that there is no “hole” in the pattern, it is continuous from
the location of the transmitter outward, but growing progressively weaker further from the
transmitter since the signals have further to travel. However, most of the path length is the
distance up 160 nautical miles and then back down the same 160 NM so even at the location of
the transmitter, the received signals have traveled 320 NM. Using trig, or the Pythagorean
theorem, you can calculate how far the signals would travel to arrive at various distances from
the transmitter. (We can ignore the curvature of the earth in this calculation.) At 80 NM from
the transmitter, the signals would have traveled a total of 330 NM, at 210 NM from the
transmitter the signals would have traveled 380 NM. You will note, that because most of the
travel distance is due to traveling up and back to the “E layer,” that the total distance
doesn’t change very much even though the distance away from the transmitter increases a lot
more. This extra travel distance caused the signal strength to get weaker but not very much.
Using the analogy, you wouldn’t notice much, if any, difference in the brightness of the
reflected flashlight beam when it was shining on your toes compared to when it came straight
back down to shine on the location of the flashlight.

So you can see that the signal should actually be a little bit stronger inside  the “TIGHAR donut
hole” than in the donut itself, so how does TIGHAR come up with the hole?

Brandenburg and Varney analyzed the transmission pattern of the antenna on Earhart’s plane
and calculated that it put out a very weak signal straight up, and nearly straight up, but got
much stronger at lower angles.  So there would only be a very weak signal to start
with going straight up, or nearly straight up, that would be available to be
reflected back down near the airplane. So TIGHAR claims, that due to the weak signal that
would be reflected back within 80 NM of Earhart’s transmitter, that Itasca couldn’t hear her
when she was within that distance. But as she got further away, the much stronger signal
transmitted at lower angles by the antenna, even when combined with the slightly longer path distance and
path losses, resulted in a strong signal at Itasca. So the reflected signal from the ionosphere would be strong
everywhere within the 80 NM proposed "donut hole" except just very near the transmitter so there would only
be a much smaller hole there, certainly less than 38 NM.
To use our analogy, when you first shine the flashlight straight up it is very dim because the
batteries are almost dead and when you deflect it slightly from straight up, the reflected spot of
light is not bright enough to illuminate your toes. So you replace the batteries and now the light
is much brighter and aiming the light at the mirrored ceiling at the lower angle, you can now
see your toes, the footboard and the end table across the room, but dimmer over by the end
table since the light had to travel up to the ceiling and then all the way across the room to the
end table.

There is nothing wrong with this analysis of the skywave propagation of Earhart’s radio.

But there is something wrong with the “donut hole” theory. Brandenburg ignored the “direct
wave” propagation of Earhart’s radio which is in addition to the skywave propagation.
Brandenburg admitted that he did not consider this part of the propagation:

-------------------------------------------------------------------

“It's possible that there was direct path propagation at short distances, due to excitation of the
airframe, but ICEPAC only calculates path loss for an ionospheric path.   However, at 1,000
feet altitude (where Earhart said she was flying then), the horizon distance is about 38 miles.
So outside about 40  miles, there wouldn't be any direct path, and skywave would govern. “

See: https://tighar.org/smf/index.php/topic,285.msg2537.html#msg2537


------------------------------------------------------------------------------


Direct wave propagation requires that the transmitter and the receiver be able to see each other,
that there be “line of sight” between the stations. This distance varies with altitude of the plane
and also with the height of the receiving antenna. Brandenburg calculates this as being 38
nautical miles but he then ignores this direct wave propagation! (Using 1,000 feet for the plane
and 60 feet for the Itasca antenna actually makes the distance 47 NM.) Varney calculated
that the strongest signal was sent horizontally which is contrary to
Brandenburg’s speculation of only a “possibility” of a direct wave. Look at my analogy of
shining the flashlight directly on the end table to see the missing cellphone. The direct light is
much brighter than any reflected light off the ceiling and the direct signal from Earhart to the
Itasca would have been much stronger than any reflected skywave signal since it had to travel
only 38 NM (using Brandenburg’s number) instead of 322 NM using skywave to arrive at the
Itasca if it was 38 NM away. So even using Brandenburg’s number, there would be no “hole”
within 38 NM!

So what about beyond that 38 NM and the “donut” at 80 NM? First, it would be filled in by skywave, only
slightly diminished in strength.

Second, the TIGHAR theory, of course, places the plane south of the Itasca where the air was clear enabling
Earhart to climb to higher altitude to facilitate a better visual search for Howland. To reach 80
NM with a direct wave required the plane to climb to only 3300 feet from which a direct wave
would reach the Itasca much stronger than a skywave covering the same radius since the direct
wave would only have traveled 80 NM while the skywave would have gone 330 NM. (And this
ignores the fact that the horizontal direct wave signal was transmitted much stronger than the
skywave signals (according to Varney's table of antenna signal pattern) so the direct wave signal
would have been much stronger as received by the Itasca  than the skywave “donut” signal.) So
both of these types of signals would fill in the donut hole out to 80 nm and then the skywave continues
to supply a strong signal beyond that distance.

So my advice about “donut holes,” just buy them at your local bakery.

--------------------------------------------------------

As long as we are talking about skywave propagation, this is a good time to get rid of the idea
that there was a “skip zone” for Earhart’s signals that prevented nearby stations from hearing
her while stations further away could. At the frequencies Earhart was using there was no skip
zone because even signals sent straight up would be reflected back down to earth. Skip
involves higher frequency transmissions that would pass up through the ionosphere and not
return to earth if the signals were sent up at a high angle. At some lower angle, however,  these
higher frequency signals are reflected back to earth where they can be heard some distance
away but this leaves a gap near the transmitter, this gap is the “skip zone.”

Return to our analogy, only this time paint a black circle on the mirror centered directly above
the flashlight on the bed. Shining the light straight up, or nearly so, will create no reflected
beam since the light is absorbed by the black paint, the same as the blackness of outer space.
Now deflect the beam slowly lower, like you did before, and you eventually reach a point
where the beam hits the mirror outside the black circle and the mirror then
reflects the beam back down, say the beam now lights up the
footboard but it didn’t light up your toes even though your toes were nearer to the flashlight.
The space between the flashlight and the footboard (in this example) is the “skip zone” and no
reflected light beam could be seen in this area. From the footboard outward the flashlight beam
can be reflected and seen.

gl
« Last Edit: December 11, 2011, 03:53:51 AM by Gary LaPook »
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Gary LaPook

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Re: Factors influencing radio propagation
« Reply #26 on: December 11, 2011, 04:10:23 AM »

Can someone please explain what "A peculiarity in the antenna’s transmission pattern" or what "propagation pattern of the aircraft’s transmitting antenna" means?
See links from article on--of all things!--"Radio propagation."

Thank you for the link.

That is interesting but it does not offer the fundamental questions regarding the "transmission patterns" or "propagation patterns".

I remain skeptical of the doughnut hole propagation pattern theory.
You should be skeptical, see: https://tighar.org/smf/index.php/topic,285.msg5763.html#msg5763

gl

Bob Brandenburg has not yet published his paper on the donut hole.  I've kept him too busy working on underwater search technology.
The donut hole is the result of an anomaly in the propagation pattern created by the Electra's transmitting antenna.  Bob discovered it when applying an updated and more precise version of ICEPAC (Ionospheric Communications Enhanced Profile Analysis & Circuit) to the computer wire grid antenna model.  In explaining it to me, Bob wrote:

"Out to 80 nmi. the 50% and 90% probability signals from the wire grid version are about the same as in the previous model.  That's because of the steepness of the radiation pattern "skirt" at low radiation angles, and the "dimple" at the center of the pattern -- it's similar to the dimple in the upper half of an apple.  But at greater distances the wire grid signal strength is considerably higher because the "skirts" of the pattern are steeper than in the previous version -- in which the pattern begins to curve inward at lower radiation angles.  The difference is analogous to the difference between an apple with more vertical sides -- like a Washington Delicious --  and one with more roundish sides, like a Macintosh.   The new mean value rises above the threshold at about 150 nmi, and stays above until 280 nmi.  The 10%, and lower, probability curves stay above the threshold all the way out to 340 nmi.   So there was a 50% chance Itasca could have heard Amelia when she was about 140 nmi away, a 10% chance of hearing her at 80 nmi, a 5% chance of hearing her at 60 nmi, and a 1% chance of hearing her at 40 nmi.

It's doubtful she was within 40 nmi of Howland, since that would put her within visual range of Baker Island if she was on the LOP.   As for the maximum likely distance, it was possible -- at 10% or less probability -- that Itasca could have heard her even as she laid eyes on Niku."

In case you missed it in all that, Itasca had the best chance of hearing Earhart when she was between 150 and 280 nautical miles away.  Obviously, this is a huge game-changer in any speculation about where she was at the time of the last in-flight transmission heard by Itasca.  It knocks the Crashed & Sankers calculations of where they should be searching for a sunken aircraft into a cocked hat.  Millions of dollars utterly wasted - even if the Electra crashed & sank at sea.  If the airplane was south of Howland on the LOP, it was probably much closer to Gardner than has previously been thought likely.  That puts it on the reef at Gardner much earlier and with more fuel remaining than previously thought possible. That, in turn, influences the credibility of the post-loss radio signals which required power from batteries recharged by running an engine.

Once Bob has had a chance to get his paper written and published, any skeptic with the ability and inclination to buy the software and check his calculations will be able to do so.  Replication of results is the essence of scientific investigation.
Your donut is symmetrical  so the plane could have been north, south, east or west and any direction in between so it doesn't help get them closer to Gardner as you hoped.

You also left out the direct wave from a higher altitude. There is no reason for Earhart to stay down at 1,000 feet all the way to Gardner since the sky was clear south of Howland and climbing gives a better chance of spotting any island.

Brandenburg admits that there is direct wave transmission out to 40 NM from 1,000 feet but he ignores the height of the Itasca's antenna in his calculation which increases it to 47 NM. The direct wave would be much stronger than any skywave signal. So within direct wave range the Itasca would receive the strongest signal. Brandenburg tries to get around this fact by saying that the plane was never this close since the plane did not sight Howland or the Itasca. Problem with this is that the visibility was only reported to be greater than 20 NM and it is not going be much greater than that, very unlikely to exceed 25 nm. Based on the Navy Climatic Atlas, in the vicinity of Howland in July, you can expect visibility less than 25 NM 70% of the time. Visibilities over the ocean are never very great which is why the scales max out at 25 NM in the Climatic Atlas and at 20 NM in the Navy reporting scheme. They may exceed 25 NM but never by much. So if the plane passed 35 NM north of Itasca Earthart would not have seen Howland  or the Itasca but her signal would have come booming in on the direct wave. If the plane climbed to a higher altitude then the direct wave range would have increased beyond the 47 NM.

See: https://tighar.org/smf/index.php/topic,452.msg5875.html#msg5875
gl
« Last Edit: December 11, 2011, 04:21:04 AM by Gary LaPook »
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Ric Gillespie

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Re: Factors influencing radio propagation
« Reply #27 on: December 11, 2011, 08:40:59 AM »

I'll let Bob respond to your treatise on flashlights if he has time.

Your donut is symmetrical  so the plane could have been north, south, east or west and any direction in between so it doesn't help get them closer to Gardner as you hoped.

What gives you the idea that I "hoped" the donut hole would help get them closer to Howland?  We didn't go looking for a donut hole in the propagation pattern.  It was a surprise that Bob discovered only through the use of more precise analytical software.  Yes, it was symmetrical so the plane could have been north, south, east or west and any direction in between, but Earhart said she was "on the line 157 337" and you have frequently assured us that Noonan could accurately place them on that line, so the possibility that she was far to the east or west of Howland seems quite small.  Searches by Itasca and, in recent years, by four different deep-water searches have found no evidence that she was north of Howland.  The discoveries on Gardner Island strongly suggest that she was south of Howland.

You also left out the direct wave from a higher altitude. There is no reason for Earhart to stay down at 1,000 feet all the way to Gardner since the sky was clear south of Howland and climbing gives a better chance of spotting any island.

Tell me how you know what the sky conditions were like 150 nautical miles south of Howland.

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John Ousterhout

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Re: Factors influencing radio propagation
« Reply #28 on: December 11, 2011, 09:10:09 AM »

gl, thanks for the lengthy and detailed "flashlight" analogy and propogation description.  I still don't understand how 3105 might enable a strong signal at less than 47 miles (or further with more altitude), yet 6210 failed completely.  Line of sight is not frequency dependant.  Her successful 2-way contact on 6210 with Darwin on departure indicates that her radio worked properly on 6210, and their report of 200-mile radio range seems consistant with everything else you've described, since she was also at much higher altitudes.
Cheers,
JohnO
 
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Gary LaPook

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Re: Factors influencing radio propagation
« Reply #29 on: December 11, 2011, 04:16:31 PM »

gl, thanks for the lengthy and detailed "flashlight" analogy and propogation description.  I still don't understand how 3105 might enable a strong signal at less than 47 miles (or further with more altitude), yet 6210 failed completely.  Line of sight is not frequency dependant.  Her successful 2-way contact on 6210 with Darwin on departure indicates that her radio worked properly on 6210, and their report of 200-mile radio range seems consistant with everything else you've described, since she was also at much higher altitudes.
There's a saying in science, "It's sad when a beautiful theory is killed by an ugly fact."
gl
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