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Author Topic: Working the Flight backwards  (Read 130720 times)

Gary LaPook

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Re: Working the Flight backwards
« Reply #135 on: September 15, 2011, 04:09:57 PM »

If she was hearing 7500, which would be a skywave signal, the horizontal polarization component of the downcoming field could dominate, preventing her from getting a null.   She could hear the signal regardless of what sense mode she selected, but would be unable to get a null.  There's a reason why loop direction finder bands were in the low- and medium-frequency regions  -- signals there are vertically polarized, and tend to be that way when received within groundwave range. 

Another possibility is that the signal duration was too short for her to get a null.

Bob

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Ric points out that there was no sense antenna on the plane. Bob, assume that she was within the 40 NM circle where you said she could get ground wave reception, my question is, what would she have heard in her earphones if she had the RDF set to uni-directional and  same question if she switched it to bi-directional?  No signal received?, equal signal all around the dial? one null? two nulls? one strong null and one weak null?

Any ideas?

gl
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Ric Gillespie

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Re: Working the Flight backwards
« Reply #136 on: September 26, 2011, 09:04:50 AM »

The latest propagation analysis for the best chance of Itasca hearing a "Strength 5" signal from NR16020 on 3105 kHz puts the airplane a whole lot farther away than 5 or 40 miles.  At 40 nautical miles there was a 1% chance.  At 60 nm there was a 5% chance.  At 80 nm a 10% chance.  Itasca had the best chance (50%) of hearing a Strength 5 signal if the airplane was between about 150 and 260 nautical miles away. 
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Gary LaPook

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Re: Working the Flight backwards
« Reply #137 on: September 27, 2011, 01:50:20 AM »

The latest propagation analysis for the best chance of Itasca hearing a "Strength 5" signal from NR16020 on 3105 kHz puts the airplane a whole lot farther away than 5 or 40 miles.  At 40 nautical miles there was a 1% chance.  At 60 nm there was a 5% chance.  At 80 nm a 10% chance.  Itasca had the best chance (50%) of hearing a Strength 5 signal if the airplane was between about 150 and 260 nautical miles away.
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Ric,
Bob wrote on the "3105 donut" thread:


"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. "

So I was asking him about the operation of the RDF within the 40 miles that he said was possible with the direct wave, a signal that would be much stronger than the "near vertical incidence" skywave signals that create the donut. Those sky waves that create the donut travel at least 400 miles and suffer path losses one hundred times greater, 20 db, than the direct wave traveling only 40 miles. Plus, the signal strength of the signal emitted horizontally was much stronger, about 9 db, than at the takeoff angle of the skywaves that form the donut, according to Varney, so the direct signals would be about 29 db stronger than the skywaves in the donut so should have been easily received.

 So I am still curious what Bob thinks would be heard on the RDF.

gl

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Bob Brandenburg

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Re: Working the Flight backwards
« Reply #138 on: September 27, 2011, 08:46:20 AM »

Gary,

In the scenario you describe, who is listening to what on which RDF?

Bob
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Gary LaPook

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Re: Working the Flight backwards
« Reply #139 on: September 27, 2011, 09:16:42 AM »

Gary,

In the scenario you describe, who is listening to what on which RDF?

Bob
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Earhart is listening on her RDF for the 7500 signal from Itasca.

gl
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