TIGHAR
Amelia Earhart Search Forum => Radio Reflections => Topic started by: Chuck Varney on January 04, 2011, 10:56:33 AM
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Mike Everette's research report on NR16020's radio gear gives the length of the dorsal V antenna for the second world flight as 54 feet (two 27-foot legs). Do drawings exist that show the legs that long?
In 2008 I used two Harney drawings in Amelia's Shoes to estimate lengths of the dorsal antenna component wires. While trying to err on the long side, I settled on leg lengths of 25.7 feet. Even allowing for the inconsistency between the side and planform views, I can't make the legs longer than 26 feet.
I realize this is of no interest to anyone who isn't trying to analyze the antenna, but the leg length matter is a lead-in to comments I have regarding what Mike E. says about both the original and the lengthened dorsal antennas. This forum may not be the place to do it. Off-line perhaps?
Chuck Varney
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Mike Everette's research report on NR16020's radio gear gives the length of the dorsal V antenna for the second world flight as 54 feet (two 27-foot legs). Do drawings exist that show the legs that long?
Not drawings. All of the information about antenna variations, known and surmised, are in text.
I've tried to summarize the variations in "NR16020 Antennas" (http://tighar.org/wiki/Antennas). Let me know if the article needs correcting or extending.
I realize this is of no interest to anyone who isn't trying to analyze the antenna, but the leg length matter is a lead-in to comments I have regarding what Mike E. says about both the original and the lengthened dorsal antennas. This forum may not be the place to do it. Off-line perhaps?
It seems to me to be a perfect topic for the Forum. If folks don't like it, they don't have to read it. We could also create a new board just for radio discussions, if you'd like.
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Thank you, Marty.
I wasn't prepared for a "text only" response to my dimension query. So the hard data exists in the form of statements like "moved the mast from Sta. 176 to Sta. 129.5" ? And Bill Harney then used them as guidance for his drawings?
My opening comment to the page you linked relates to the third sentence under Bell Labs installation: dorsal Vee, which reads: "This length did not include the lead-in wire which exited the aft fuselage through a feed-through insulator and connected to one leg of the Vee at a point a few feet from one of the vertical stabilizers."
I think there's considerable photographic evidence that shows the lead-in wire exited the starboard fuselage through a feed-through insulator in the cabin roof near station 239 and went forward to connect to the apex of the Vee. Two photos in the Purdue collection (b10f7i22 and b12f9i2) show the connection of the lead-in wire to the Vee quite clearly. (I have links to them, but they're each three lines long and I hesitate to dump them into this post--and I don't know what html tags are supported in these posts that would allow me to make them shorter).
It's the second sentence, coupled with the fourth sentence, that prompt my primary comments to Mike Everette's dorsal antenna write-up. These are: "The antenna was, therefore, a total of 46 feet, doubled back onto itself." and "This antenna was, then, already 15% longer than optimum;. . ." He makes like statements about the revised antenna in his research report, substituting 54 feet for 46 feet.
In each case, Mike is comparing the total length of the horizontal V portion of the antenna to a reference antenna length of 40 feet. The problem with that is twofold:
1) The V portion is just that--a portion of the antenna, not "the" antenna". The antenna also includes the lead-in wire (runs from the V to the feed-through insulator), the interior wire from the lead-in to the transmitter, and the wire from the transmitter to the airframe. These latter wires cannot be disregarded.
2) The effective length of this antenna configuration can be much shorter than one might think. To a first approximation, it equals the length of wire that runs from the airframe to the farthest point on the V. As an example, suppose you have two 27-foot legs and a 7.5-foot "feed-line" that runs from the airframe, through the transmitter, and connects to the V at a point 9.75 feet aft of the apex. The approximate effective length is 7.5 + 9.75 + 27 = 44.25 feet (not 54 feet or 54 + 7.5 feet). In this ideal case 17.25 feet of one leg is inactive and can be ignored.
I want to emphasize the use of approximation in 2)--but it's an approximation that may have been used by a technician tasked with sizing an antenna in the late 1930's. I've found two variants of the approximation from the early 1940's. They each differ slightly from the wording in 2), and make for a bit shorter estimate. One measured from the transmitter output terminal; the other from the feed-through insulator. Applying the latter method to the dimensions I'd derived from drawings in Amelia's Shoes gave 40 feet as the estimated length. This may be just an interesting coincidence. (I have no great confidence in my dimensions, a situation I'm hoping to improve via this forum.)
An upshot of all this is that, sadly, the dorsal V as used on AE's Electra cannot be computer-modeled simply as a horizontal V with the source positioned in one leg of the V. Doing that defines a totally different antenna.
Chuck Varney
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chuck, and all..
the whole airframe needs to be modeled as part of the antenna--not trivial to do.
kenb
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For a more in-depth discussion of the radio propagation issues see Bob Brandenburg's "The Post-Loss Radio Signals: Technical Analysis" (http://tighar.org/Projects/Earhart/Archives/Research/ResearchPapers/Brandenburg/PLRSTechnical.html)
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chuck, and all..
the whole airframe needs to be modeled as part of the antenna--not trivial to do.
kenb
Ken,
Roger that on both counts. After a couple months of daily attacks on the problem in 2008 I concluded that short of having a $50K antenna modeling package and a super computer to run it on--plus an aircraft installation to use for validation--I couldn't really be confident in any of the results. I found that for the antenna on a simple wire frame model of the Electra, I could move, add, or remove one wire from the airframe and markedly change the calculated input impedance.
Chuck
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For a more in-depth discussion of the radio propagation issues see Bob Brandenburg's "The Post-Loss Radio Signals: Technical Analysis" (http://tighar.org/Projects/Earhart/Archives/Research/ResearchPapers/Brandenburg/PLRSTechnical.html)
Ric,
I have indeed read that document, and all Bob's other papers. In addition I have recently re-read all his archived posts that even remotely relate to antenna modeling--from his first on 1 April 1999 up through 2009.
Chuck
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Thank you, Marty.
You're welcome.
I wasn't prepared for a "text only" response to my dimension query. So the hard data exists in the form of statements like "moved the mast from Sta. 176 to Sta. 129.5"? And Bill Harney then used them as guidance for his drawings?
I actually don't know what Bill's full sources were, so I have to revise my statement: I personally have only seen and collected text about the variations in the antennas.
My opening comment to the page you linked relates to the third sentence under Bell Labs installation: dorsal Vee, which reads: "This length did not include the lead-in wire which exited the aft fuselage through a feed-through insulator and connected to one leg of the Vee at a point a few feet from one of the vertical stabilizers."
I think there's considerable photographic evidence that shows the lead-in wire exited the starboard fuselage through a feed-through insulator in the cabin roof near station 239 and went forward to connect to the apex of the Vee. Two photos in the Purdue collection (b10f7i22 and b12f9i2) show the connection of the lead-in wire to the Vee quite clearly. (I have links to them, but they're each three lines long and I hesitate to dump them into this post--and I don't know what html tags are supported in these posts that would allow me to make them shorter).
"How to insert images into posts." (https://tighar.org/smf/index.php/topic,128.0.html)
"How to insert links into posts." (https://tighar.org/smf/index.php/topic,127.0.html)
I'm going to stop replying there for the moment. Paw through those two posts and put up the links to the photos, if you would be so kind. I've just come back from N.C. and am only spending a few minutes assessing e-mail and the like. When you give me the links to the photos, I'll edit the offending line and indicate that there is a difference of opinion.
I'll also open a "Radio Roundup" board and move this thread over there.
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Chuck,
Just curious; what modeling program are you using?
I know you said that removing a wire from the wire frame changed the impedance, but did that alter the pattern appreciably?
In a practical sense, it's probably sufficient to model a representative wire antenna over ground (using the appropriate ground conductivity figure) at the approximate height. Not as thorough, for sure, but this was the peak of that solar cycle and the Electra was at the edge (or at times over, possibly) salt water. These two variables make up for a lot of deficiencies! :)
I think the point is to see if there's a workable path at the right times with the correct power level.. I think there was. It's another data point, but if the modeling shows no path, that doesn't necessarily mean that one did not exist. The ionosphere doesn't always behave as modeled and then there are other propagation modes that can't be easily predicted such as ducting or skewed, and others that might have been in play.
As an aside, I've read Bob's papers and found them informative, but if I recall correctly he gives his antenna gain figures in dB (I'm thinking about the gain of Betty's antenna, here) and I've been wondering whether they're dBi or dBd? Probably, I missed this somewhere...
kenb
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Paw through those two posts and put up the links to the photos, if you would be so kind.
Marty,
Thank you for showing me the link and image insertion methods.
Here are links to the two Purdue collection photos that show the feed connection at the V apex for the first world flight dorsal antenna configuration (bottom wire in each photo goes back to the feed-through in the cabin roof near Sta. 239):
Photo b10f7i22 (http://earchives.lib.purdue.edu/cdm4/item_viewer.php?CISOROOT=%2Fearhart&CISOPTR=243&DMSCALE=100&DMWIDTH=600&DMHEIGHT=600&DMMODE=viewer&DMFULL=0&DMX=3460&DMY=1172&DMTEXT=%2520b10f7i22&DMTHUMB=1&REC=1&DMROTATE=0&x=368&y=346)
Photo b12f9i2 (http://earchives.lib.purdue.edu/cdm4/item_viewer.php?CISOROOT=%2Fearhart&CISOPTR=802&DMSCALE=25&DMWIDTH=600&DMHEIGHT=600&DMMODE=viewer&DMFULL=0&DMX=438&DMY=227&DMTEXT=%2520b12f9i2&DMTHUMB=1&REC=1&DMROTATE=0&x=384&y=256)
Chuck
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Thank you for showing me the link and image insertion methods.
Thank you for the links!
I've added them to my synopsis:
"NR16020 Antennas." (http://tighar.org/wiki/NR16020_antennas#Bell_Labs_installation:_dorsal_Vee)
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Just curious; what modeling program are you using?
Ken,
I've primarily used MMANA 2.03, which is MININEC 3-based. To see a NEC-2 result, I've used NEC-2 for MMANA. a program that accepts a MMANA file as input, converts it to NEC format, then runs a NEC-2 engine on it.
I know you said that removing a wire from the wire frame changed the impedance, but did that alter the pattern appreciably?
I was primarily fixated on impedance prediction, so I didn't pay a lot of attention to patterns (that was to be the next phase), but I would expect no significant change in pattern in response to removing or adding a single wire from a wire frame comprising several dozen or more wires.
In a practical sense, it's probably sufficient to model a representative wire antenna over ground (using the appropriate ground conductivity figure) at the approximate height.
That may well be so, but I wanted a "flyable" model, one that could be set near ground or in freespace.
As an aside, I've read Bob's papers and found them informative, but if I recall correctly he gives his antenna gain figures in dB (I'm thinking about the gain of Betty's antenna, here) and I've been wondering whether they're dBi or dBd?
I presumed dBi, but Bob will have to be the one to answer your question.
Chuck
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Thanks, Chuck. I appreciate your comments.
I've not used MMANA, though it looks interesting. I use either EZNEC or 4NEC2.
Best,
kenb
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I actually don't know what Bill's full sources were, so I have to revise my statement: I personally have only seen and collected text about the variations in the antennas.
Marty,
Can you tell me what the current consensus of the TIGHAR Team is regarding the location of the second world flight dorsal mast?
Here are some options that come readily to hand:
Sta. 129 Page 16 of TIGHAR Tracks Vol. 11, # 3 (http://tighar.org/TTracks/1995Vol_11/1103.pdf)
Sta. 129.5 Mike Everette's research paper (http://tighar.org/Projects/Earhart/Research/ResearchPapers/ElectraRadios/ElectraRadios.htm#8)
Sta. 125 Notation on Harney drawing of NR16020, port side (http://tighar.org/aw/mediawiki/images/1/10/Port.gif)
Chuck
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Can you tell me what the current consensus of the TIGHAR Team is regarding the location of the second world flight dorsal mast?
Here are some options that come readily to hand:
Sta. 129 Page 16 of TIGHAR Tracks Vol. 11, # 3 (http://tighar.org/TTracks/1995Vol_11/1103.pdf)
Sta. 129.5 Mike Everette's research paper (http://tighar.org/Projects/Earhart/Research/ResearchPapers/ElectraRadios/ElectraRadios.htm#8)
Sta. 125 Notation on Harney drawing of NR16020, port side (http://tighar.org/aw/mediawiki/images/1/10/Port.gif)
I don't know what the consensus is. I wasn't aware that there was a discrepancy. :-\
I've added your question to the article on "NR16020 Antennas." (http://tighar.org/wiki/NR16020_antennas#Location_of_dorsal_mast)
I've also found that the same article already had Ric's answer to your other question about the feed point. (http://tighar.org/wiki/NR16020_antennas#Antenna_feed_point)
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I don't know what the consensus is. I wasn't aware that there was a discrepancy. :-\
I'm sorry to hear that.
I've added your question to the article on "NR16020 Antennas." (http://tighar.org/wiki/NR16020_antennas#Location_of_dorsal_mast)
Thank you, Marty.
I've also found that the same article already had Ric's answer to your other question about the feed point. (http://tighar.org/wiki/NR16020_antennas#Antenna_feed_point)
I didn't ask a feed point question, but I did point out that the way Mike E. described the wire connection to the first dorsal V was not borne out by photographs.
Perhaps I should set questions aside and make a few more comments to the aforementioned article (though I fear I'm driving you nuts).
Refer to this section:
Calculating final length of the dorsal Vee
"There are lots and lots of good photos of the airplane after it left Miami and the insulators on the dorsal vee are easy to see. There are two insulators right up close to the forward mast and others right up close to the attach points on the vertical fins. There are no insulators elsewhere on the wire."
The title leads the reader to think he's going to learn something about how the length of the second flight dorsal antenna was determined. Instead, there's an extract from a discussion where one side said the antenna could have been shortened by inserting insulators in the legs to create inactive portions, and the second side said that photos revealed the installation of no such insulators. While what the second side said is true, they could have added: the antenna configuration in and of itself has a significant weakly active portion, so its effective length is less than the sum of all the wire used to construct it.
You could improve the article by deleting all of the Calculating final length of the dorsal Vee section.
The same goes for the last three sentences in the section before it, Gurr's Modifications. I'm referring specifically to these:
It was a terrible compromise that provided no meaningful capability to transmit on 500 Kcs while greatly complicating the problem of putting out a decent signal on 3105 and 6210. There appears to have been, however, another consequence to lengthening the vee. The new length made an excellent antenna for the unintended harmonic frequencies."
If it can be shown that the WE 13C couldn't be loaded into the new antenna on 3105 kHz and 6210 kHz without modification or great difficulty then retain the first sentence; otherwise, delete it. If it can be shown somehow that the new length was a significantly better radiator of harmonics than the original, then retain the final two sentences--but first you need to know what the two lengths actually were, electrically. Otherwise, delete both of them as well.
Chuck
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Perhaps I should set questions aside and make a few more comments to the aforementioned article (though I fear I'm driving you nuts).
Comments and corrections are always welcome. I'd like the wiki to be as accurate as possible, all things considered.
Refer to this section:
Calculating final length of the dorsal Vee ...
You could improve the article by deleting all of the Calculating final length of the dorsal Vee section.
OK.
The same goes for the last three sentences in the section before it, Gurr's Modifications. I'm referring specifically to these:
It was a terrible compromise that provided no meaningful capability to transmit on 500 Kcs while greatly complicating the problem of putting out a decent signal on 3105 and 6210. There appears to have been, however, another consequence to lengthening the vee. The new length made an excellent antenna for the unintended harmonic frequencies."
If it can be shown that the WE 13C couldn't be loaded into the new antenna on 3105 kHz and 6210 kHz without modification or great difficulty then retain the first sentence; otherwise, delete it. If it can be shown somehow that the new length was a significantly better radiator of harmonics than the original, then retain the final two sentences--but first you need to know what the two lengths actually were, electrically. Otherwise, delete both of them as well.
I've temporized here and just added your comments to the article (http://tighar.org/wiki/NR16020_antennas#Gurr.27s_modifications) to indicate that there is disagreement. Ric has worked with Mike Everette and Bob Brandenburg for many years and has, in my view, earned the right to summarize their discussions. IF there is any validity to the post-loss radio study, which has not yet been published in full, then some oddities in the antenna configuration may have played a role in producing audible transmissions on harmonics of 3105 or 6210 kcs.
The beauty of the wiki is that it is designed to be updated, so if the discussion here produces consensus, I'll keep on revising the relevant articles.
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Can you tell me what the current consensus of the TIGHAR Team is regarding the location of the second world flight dorsal mast?
Here are some options that come readily to hand:
They're all wrong. There is no Sta. 125 or 129 or 129.5. The mast was located at Sta. 129 5/8.
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Can you tell me what the current consensus of the TIGHAR Team is regarding the location of the second world flight dorsal mast?
They're all wrong. There is no Sta. 125 or 129 or 129.5. The mast was located at Sta. 129 5/8.
Thank you very much, Ric. It appears that in one succinct reply you've provided an answer, a lesson in the use of the term "Station", and an explanation for some of the variability in numbers.
To see if I'm on the right track, let me rephrase the question: What was the distance, in inches, between the Electra's nose and the center of the mast?
I'm interested in both the distance and the source of the information, if you have that at hand.
Chuck
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What was the distance, in inches, between the Electra's nose and the center of the mast?
Lockheed assigned stations numbers based on the number of inches measured aft from the tip of the nose. Only places where there was a circumferential internal structure (formers and bulkheads) were given a station number. Sta. 129 5/8 was a fuselage former located 129 5/8 inches back from the tip of the nose. The position of the exact center of the mast depends on exactly where the mast was installed. The Harney drawing is based on the available photographs and shows the mast mounted a few inches forward of the double rivet line that marks the station. So, I was wrong and I was right and I was wrong. Technically there is no "Sta. 125" but the center of the mast appears to have been - near as dammit - 125 inches aft from the tip of the nose.
I'm interested in both the distance and the source of the information, if you have that at hand.
The position of the antenna mast was derived from numerous photos of the aircraft. The information about stations on the Lockheed Model 10 is in "Catalog of Parts - Lockheed 'Electra' - Lockheed Aircraft Corporation".
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What was the distance, in inches, between the Electra's nose and the center of the mast?
. . . Only places where there was a circumferential internal structure (formers and bulkheads) were given a station number. Sta. 129 5/8 was a fuselage former located 129 5/8 inches back from the tip of the nose. . . . The Harney drawing is based on the available photographs and shows the mast mounted a few inches forward of the double rivet line that marks the station. . . the center of the mast appears to have been . . . 125 inches aft from the tip of the nose.
Excellent! Thank you, Ric.
I'm interested in both the distance and the source of the information, if you have that at hand.
The position of the antenna mast was derived from numerous photos of the aircraft. The information about stations on the Lockheed Model 10 is in "Catalog of Parts - Lockheed 'Electra' - Lockheed Aircraft Corporation".
Again, thank you.
Chuck
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I closed Reply #3 in this thread with: ". . .the dorsal V as used on AE's Electra cannot be computer-modeled simply as a horizontal V with the source positioned in one leg of the V. Doing that defines a totally different antenna."
Bob Brandenburg's papers, and forum posts between 1999 and 2009, are quiet on this point, but there are a number of hints in them that he did model the dorsal antenna as a V with the source in one leg.
Did he?
Chuck Varney
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Bob Brandenburg's papers, and forum posts between 1999 and 2009, are quiet on this point, but there are a number of hints in them that he did model the dorsal antenna as a V with the source in one leg.
Did he?
I'll ask him.
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Chuck,
I modeled the dorsal antenna as it existed, not as a variant of any generic antenna. The model uses the entire feed wire, including the run from the transmitter to the fuselage penetration point. The Electra installation had three insulators, one at the apex support point and one each at the tail fin suspension points.
The antenna model also includes the airframe, using a wire frame model -- comprising about 1,000 segments -- built in 4NEC2.
All dimensions were taken from the Harney drawings.
The antenna gain values are dBi.
Bob
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Thanks, Bob.
kenb
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Bob,
Thank you for taking time out from your current project to answer my question about modeling (and Ken's about dB's).
The model uses the entire feed wire, including the run from the transmitter to the fuselage penetration point. . . . The antenna model also includes the airframe, using a wire frame model -- comprising about 1,000 segments -- built in 4NEC2.
That marks a significant change from the way you appeared to perceive the antenna and model it a decade ago.
Does your wire frame model comprise about 1,000 segments, or about 1,000 wires? I ask because my own 84-wire minimalist wire frame model is divided into thousands of segments, the number depending upon, and increasing with, frequency.
And I know you intended to say The Electra installation had six insulators, a pair at the apex support point and at each tail fin suspension point.
Chuck Varney
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Chuck,
Yes, there has been a significant change since a decade ago. In those days, I was using NEC4WIN95, which is based on MiniNEC-3. Although NEC2 gave better results with wires close to ground, its card-format input scheme and lack of a convenient data editing feature made it unwieldy for quick turnaround modeling, and its antenna gain output was not directly usable in ICEPAC. I switched to 4NEC2, published about 6 years ago, because it added a 3-D graphical user interface and a spreadsheet geometry editor to NEC2, and generates 3-D antenna gain tables for direct use in ICEPAC. I mentioned the transition to 4NEC2 in my 2006 paper "Harmony and Power".
The wire frame model comprises 1069 wires which, at 3105 kHz, happens to be the number of segments. Of course, as you noted, the number of segments increases with frequency.
As for insulators, my intention was merely to convey that there were insulators at those three locations, the point being that the wire section aft of the wire junction on the starboard side was not inactive.
Bob
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Thank you, Bob.
I mentioned the transition to 4NEC2 in my 2006 paper "Harmony and Power".
I noted the change in modeling software, but from a 2007 forum post it appeared that you retained the method used originally with NEC4WIN95, and described in 2000. Specifically,
With 4NEC2, 28 Apr 2007 (13:25:40): (http://tighar.org/Projects/Earhart/forum/Forum_Archives/200704.txt)
". . .1). You need the details of the 3-dimensional antenna configuration: segment lengths and distances from the fuselage (ground). You can get these by careful measurements on scale drawings of the Electra, and from photographs of the Electra interior. . . ."
and earlier with NEC4WIN95, 2 Nov 2000 (20:09:30 EST): (http://tighar.org/Projects/Earhart/forum/Forum_Archives/200011.txt)
". . .The antenna model I use (see the 8th Edition) does a nice job of computing the 3-dimensional gain pattern for any antenna. The model does, however, assume a flat ground plane. To accommodate the model, I measured (using TIGHAR scale drawings) the 3-D distance from the antenna wire to the closest point on the fuselage at 2-foot intervals along the longitudinal axis of the airplane and computed the root-mean-square value over all distances, which I then used as antenna virtual height over a plane ground in the model. . . ."
The wire frame model comprises 1069 wires which, at 3105 kHz, happens to be the number of segments.
What a project that must have been. I'd love to see what it looks like. Can you tell me what your model predicts for antenna input impedances at 3105 kHz and 6210 kHz in freespace?
As for insulators, my intention was merely to convey that there were insulators at those three locations, the point being that the wire section aft of the wire junction on the starboard side was not inactive.
Thanks for clarifying that, as I missed the intended point altogether. As you say, the aft section of the starboard leg was not inactive, but the average current magnitudes in the forward section were enough greater (more than 3 times at 3105 kHz and 7 times at 6210 kHz in one of my models) that one can see how the practical result of it might give birth to the length rule of thumb I mentioned in reply #12. (Modeling the antenna with and without the aft starboard section in a few examples showed that removal of the aft section made the reactive component of the input impedance more capacitive and increased the resistance.)
Chuck Varney
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Chuck,
Sorry about the lag in documenting the antenna model evolution. The good news is that it's stable now. When time permits, I'll write up a research paper describing the process and the results.
The project was far from trivial, but it was greatly facilitated by the excellent tools in 4NEC2, and the detailed dimensions in the Harney drawings.
The antenna impedance at 3105 kHz is 2.13 - j2.29 ohms. This is the value obtained with the antenna loading coil set to 19 uH, and a fixed 4,000 pF capacitor in series with the coil per the transmitter specifications.
The impedance at 6210 kHz is 5.73 - j0.42 ohms, with the loading coil set to 10.6 uH, and a 50 pF series capacitor.
Bob
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Thanks, Bob.
When time permits, I'll write up a research paper describing the process and the results.
I look forward to reading it.
The antenna impedance at 3105 kHz is 2.13 - j2.29 ohms. This is the value obtained with the antenna loading coil set to 19 uH, and a fixed 4,000 pF capacitor in series with the coil per the transmitter specifications.
The impedance at 6210 kHz is 5.73 - j0.42 ohms, with the loading coil set to 10.6 uH, and a 50 pF series capacitor.
Backing out your series components, assuming them lossless and my arithmetic passable, I have your antenna input impedances as:
2.13 - j 360 ohms at 3105 kHz
and
5.73 + j 98.6 ohms at 6210 kHz
Your resistance values are in the range that I'd hoped to see in my attempts, but haven't. For example, the NEC2-computed freespace impedances for my sparse wire frame model (78 12-ga wires for the "Electra" and 6 18-ga wires for the antenna components), are:
0.86 - j 353 ohms at 3105 kHz (0.19 ohms radiation resistance, 0.67 ohms loss resistance)
and
2.97 + j 32.5 ohms at 6210 kHz (1.62 ohms radiation resistance, 1.35 ohms loss resistance)
I hope to someday learn what's going on with this.
Chuck
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The project was far from trivial, but it was greatly facilitated by the excellent tools in 4NEC2, and the detailed dimensions in the Harney drawings.
Bob,
I'm nearly non-conversant with 4NEC2, but I used its Geometry Builder tool a couple years ago to build some objects to fly the dorsal V on. In the wee hours this morning it occurred to me that you may have used the same tool to build your Electra wire frame. Did you?
The impedance at 6210 kHz is 5.73 - j0.42 ohms, with the loading coil set to 10.6 uH, and a 50 pF series capacitor.
To stay on topic yesterday I didn't ask, but I'm curious as to what motivated the use of 50 pF as the series capacitor for 6210 kHz? Why didn't you use the optional 62.5 pF capacitor cited in Morgan for the WE 13C? Using it and 8 uH would reduce + j 98.6 to + j 0.7 (and 7.98 uH would essentially nullify it).
Chuck
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Bob,
I'm nearly non-conversant with 4NEC2, but I used its Geometry Builder tool a couple years ago to build some objects to fly the dorsal V on. In the wee hours this morning it occurred to me that you may have used the same tool to build your Electra wire frame. Did you?
Yes.
To stay on topic yesterday I didn't ask, but I'm curious as to what motivated the use of 50 pF as the series capacitor for 6210 kHz? Why didn't you use the optional 62.5 pF capacitor cited in Morgan for the WE 13C? Using it and 8 uH would reduce + j 98.6 to + j 0.7 (and 7.98 uH would essentially nullify it).
Chuck
Morgan doesn't specify 62.5 pF. He says that "occasionally it will be found that the series condenser s supplied will not give correct operation, and a 62.5 mmf condenser may be used."
The context of his discussion covers two options: a nose-to-tail 40 foot wire, or a 70 ohm transmission line with tuning unit. Neither of the cases applied to the Electra, and we don't know exactly what capacitors were used for 3105 and 6210 in the Electra installation. So I made a conservative assumption in both cases. Since tuning the transmitter was accomplished by adjusting the loading coil, it follows that the design intent was for the unloaded antenna to be capacitive at the operating frequency. Using 50 pF at 6210, less than the minimum Morgan suggests for the two cases he discusses, seemed a reasonable way to force the unloaded antenna to be capacitive at that frequency.
Bob
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Using 50 pF at 6210, less than the minimum Morgan suggests for the two cases he discusses, seemed a reasonable way to force the unloaded antenna to be capacitive at that frequency.
OK, thank you, Bob. With the reactive term of the antenna input impedance at + j 98.6, a series 250 pF would make it marginally capacitive, so your jumping beyond 125 pF and 62.5 pF to 50 pF prompted the question.
Chuck