TIGHAR
Amelia Earhart Search Forum => Radio Reflections => Topic started by: Mark Petersen on January 17, 2011, 02:53:39 PM
-
I was curious if there is more information about the 3015 Donut (http://tighar.org/TTracks/2008Vol_24/1008.pdf) that Tighar has researched. In particular I was wondering if the antenna peculiarity described in the link that was responsible for the 80-210 donut around the plane would also apply to line-of-sight. Also any information on the nature of the antenna peculiarity would be of interest as well.
As mentioned in the link, this info can be used to establish how close FN and AE came to Howland. From what I've read and Ric has posted, FN was able to navigate to the LOP with reasonable accuracy (~10 miles), but getting to a specific place on that line (Howland) has a comparatively large degree of error because of the long duration of the flight and the dead reckoning course changes that were done at night (with cloudy weather). It's not unreasonable to assume then that the accuracy in hitting a specific place on the line is 8-21x greater than the accuracy in getting to the line itself (note that 8-21x corresponds to the 80-210 mile region postulated in the link vs the 10 miles to get to the LOP). So it certainly seems logical that FN & AE were able to navigate to the LOP SE of Howland. After failing to find Howland it also seems logical that they would follow the line SE to the only other known landfall.
But playing "what if" for a moment. Is it possible that they were somehow able to get closer to Howland and within line of sight? Given how difficult it would have been to spot Howland when looking into the rising sun (as shown in the Waitt video), it's not surprising that in this scenario they were unable to visually spot Howland, but they would have been in line of sight from a radio propagation perspective. Does the donut propogation apply to line of sight or was the model constructed assuming that the radio propogation was done "over the horizon"?
-
Perhaps someone might point me to how, exactly, this "donut" was calculated. I've evidently overlooked it trying to find it. Thanks!
kenb
-
Perhaps someone might point me to how, exactly, this "donut" was calculated. I've evidently overlooked it trying to find it. Thanks!
Summary with link to original article. (http://tighar.org/wiki/Radio_propagation#3105_Donut)
-
Many thanks, Marty, but I'm looking for more technical info behind this hypothesis, such as what the antenna anomaly is, how the "donut" was calculated, what software was used, etc.
Any pointers to where I might find it?
Thanks,
kenb
-
Hi Ken,
I'm busy with another TIGHAR project, but I just happened to check in and saw your question.
The "donut" was computed by the ICEPAC propagation model, using the dorsal antenna gain pattern computed by 4NEC2. The SNR at the Itasca was computed versus Electra distance in 20-mile increments along the LOP, from 20 miles to 340 miles.
At short distances, the antenna gain is sharply reduced due to the radiation pattern. You can visualize the pattern by cutting an apple in half at its "equator", and letting the distance from the the apple's original center point (which is now in the "equator" plane) to the skin represent the gain. The dimple where the stem attached to the apple is a good illustration of the antenna's performance at high takeoff angles. When the Electra was close to Itasca, the SNR was low enough to preclude clear reception. As the distance increased, the SNR increased, until the distance reached the point where the SNR fell off again due to propagation loss. Hence the notion of "donut"
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.
Anyway, it's a safe bet that Earhart wasn't within the horizon of Itasca, since the ship would have been visible against the horizon at that distance -- contrary to the common assumption that the visibility of Howland was the controlling factor. There also seems to be a common assumption that Howland was obscured by the sun. The only plausible reason for Earhart to be at 1,000 is that there was a cloud deck obscuring the surface from 8,000 feet. Going down to 1,000 feet put her in the clear, with the cloud deck between her and the sun. but also limited her search horizon. The Itasca -- even hull down on the horizon -- would be visible against the cloud deck background. Moreover, Itasca reported making smoke, which would blow along the surface with the wind [coming from about east], and which would be visible as a dark mark on the surface. I have serious doubts about the feasibility of laying down black smoke for very long, since doing so carried a non-trivial risk of damaging the ship's boiler(s). But even without the smoke, Earhart and/or Noonan [who had extensive seagoing experience and would know about sighting ships on the horizon] could have seen the Itasca if they were within the visual horizon.
So the donut is our best guess of where the Electra could have been when Itasca was hearing Earhart.
Bob
Bob
-
Hi Bob,
Thanks for taking the time to write up this very thorough post. It certainly seems to preclude that a line of sight proximity was ever achieved. What you also said about it being easier to spot a ship on the horizon is interesting, because in fact, the Waitt video shows exactly this. It was easier in the video to spot the ship anchored off of Howland, than Howland itself. I've also wondered about the effectiveness of the black smoke that the Itasca generated, because the Itasca is a small ship, with small boilers, and it seems as though its ability to generate enough smoke to make a difference would be limited. At any rate, thanks for chiming in and helping the rest of us learn more about your interesting research on the 3105 donut and also your thoughts about AE & FN being able to achieve line-of-sight.
LTM,
Mark
-
Bob,
Thanks for the info. At some point, when you have time, it would be interesting to see the output of this work posted; e.g., az/el/3D pattern and a pictorial of your Electra/antenna model.
Best,
kenb
-
Hi Ken,
It's on my to-do list.
Best,
Bob
-
... the radiation pattern. You can visualize the pattern by cutting an apple in half at its "equator", and letting the distance from the the apple's original center point (which is now in the "equator" plane) to the skin represent the gain. The dimple where the stem attached to the apple is a good illustration of the antenna's performance at high takeoff angles.
OK for the apple model, but I would not assume the apple axis to be vertical. I would rather think the emission diagram of the antenna to have its symmetry axis given by the general direction of the antenna. And the antenna we are speaking about, if it is the wire extending externally above the fuselage, is actually very far from the vertical orientation of an antenna set as a pole on land, much closer to a horizontal position. You can still imagine its emission diagram as a donut, but the donut is not lying flat on the table. It is more like a standing or a rolling donut, the axis of which will vary according to the heading of the plane.
Additional effects probably have to be taken into account (e.g. reflections by the fuselage and the sea surface). The remark does not contradict anyway the idea that AE may not have been heard when she was at her closest position to Howland. Actually, if there is a zero of emission in a direction parallel to the fuselage, we may even think that AE could not be heard as long as she was heading precisely towards Itasca, and became clearly audible only when she turned sidewards. The same dependence of the emission diagram on the fuselage orientation may explain why Betty heard Amelia only at the very last moment (supposing she was listening the days before, was she ?), when the plane had moved sidewards and began slipping to the deep.
May be this is too speculative again, but could antenna specialists tell us at least why this apple-donut would have to stand vertically ?
Note added in the evening : sorry I had not checked for the actual shape of the dorsal vee antenna (cf. http://tighar.org/wiki/Dorsal_Vee). With such a V-shape, especially if fed via one of it side ends (on one of the stabilizers), it is not so clear that one gets a minimum of emission along the forward-backward axis. But I still do not understand how such a horizontal antenna would produce a minimum of emission in the vertical direction and a cylindrically symmetric emission diagram around this vertical axis (hence a range that does not depend on the orientation of the plane). So the question remains : does the donut-calculation actually take the shape and orientation of the antenna into account ?
Christophe Blondel (who is no radio specialist, but tries to put together what he remembers of his old lessons on electromagnetism)
-
OK for the apple model, but I would not assume the apple axis to be vertical. I would rather think the emission diagram of the antenna to have its symmetry axis given by the general direction of the antenna.
Christophe,
The "apple" axis is vertical--if you model the dorsal V antenna over perfect ground, and if you model it so that the excitation source is not within a leg of the V. You will then see that at 3105 kHz and at 6210 kHz the radiation field of the dorsal V is predominantly vertically polarized. The radiation pattern exhibits a zenith "dimple" similar to that of a vertical monopole, but shallower. The magnitude of the horizontal component of the field increases with frequency and serves to fill in the dimple in the pattern as the frequency is raised. The dimple is nearly nonexistent by 8 MHz. At 10 MHz and higher, the dimple in the pattern is gone and the net polarization of the radiated field is horizontal.
Evidence suggests that TIGHAR researchers Mike Everette, and Bob Brandenburg--like you--viewed the Electra's dorsal V to be a horizontal antenna fed by a single-wire transmission line. A modeling option with this view is to consider the transmission line to be lossless and to put the excitiation source in a leg of the V. If you do this you are modeling a horizontal antenna and will see no dimple in the upper part of the pattern.
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). To see if there was a possibility of a MININEC3-based modeling program failing to show the dimple when properly modeled, I modeled the antenna at 1 foot above perfect ground at 3105 kHz with an interpreted Basic version of MININEC3 (NOSC CODE 822, JCL CHANGE 9, dated 11-26-86).
Here is the result for a vertical slice through the pattern so obtained (ANGLE is elevation angle, in degrees):
ANGLE VERT (DBI) HORIZ (DBI) TOTAL (DBI)
0 4.74 -120 4.74
10 4.61 -33.5 4.61
20 4.21 -27.6 4.21
30 3.51 -24.3 3.52
40 2.46 -22.1 2.47
50 .961 -20.6 .992
60 -1.18 -19.5 -1.11
70 -4.36 -18.8 -4.21
80 -9.72 -18.4 -9.17
90 -17.9 -18.2 -15.1
100 -9.72 -18.4 -9.17
110 -4.36 -18.8 -4.21
120 -1.18 -19.5 -1.11
130 .961 -20.6 .991
140 2.46 -22.1 2.47
150 3.51 -24.4 3.52
160 4.21 -27.6 4.21
170 4.61 -33.5 4.61
180 4.74 -135 4.74
At 90 degrees the vertical component is down 22.6 dB relative to 0 degrees, and the sum of vertical and horizontal contributions (total) is down 19.8 dB. These values indicate very significant dimples in the vertical and total patterns, and show that failure to see a dimple in the pattern of the dorsal V antenna at 3105 kHz cannot be attributed to using MININEC-based software to model it very close to ground.
Chuck Varney
-
ChVar. The transmitter PA layout gives the impression to have a pi-filter configuration , this being a low-pass filter there is very low risk of harmonics radiation from the aerial , especially if the circuit tuned for low standing wave ratio which gives at the same time high effectiveness on the fundamental , the antenna length , being part of the phase loop , playing no great role for such case.
-
ChVar. The transmitter PA layout gives the impression to have a pi-filter configuration , this being a low-pass filter there is very low risk of harmonics radiation from the aerial, especially if the circuit tuned for low standing wave ratio which gives at the same time high effectiveness on the fundamental , the antenna length , being part of the phase loop , playing no great role for such case.
h.a.c. van asten,
I think your comment belongs in a "Could Betty Have Heard. . .?" thread, not this one, but in response to it--and assuming you're referring to the Western Electric 13C transmitter--you have misinterpreted the schematic. There is no pi network, or other low-pass filter provision, in the transmitter's output circuit.
Chuck
-
Ch.Var. I considered to see coil and 2 condensers , but will look sharper next time.
-
B.Brdbg. From alt. 1000 ft / 305 m the horizon distance is 38 st.mls / 61 km. At a distance of 16 mls , 26 km (EJN , July 2008) thence , island and ship Itasca were between A/c and horizon. However , for the 16 mls distance the optical angle for island and ship Itasca was 1 arcmin , which is the minimum resolution for the human eye. At 1912 GMT the crew evidently considered having the island head on below whereas it´s true coordinates were 16 mls on the port bow : for the unarmed eye and against the sun glare the visual range was below limits. When A/c went for repair after the 1st attempt accident , 6 x 30 binoculars were on board , it is not known if these were present on July 2 or , left behind due to weight saving measures.
-
OK for the apple model, but I would not assume the apple axis to be vertical. I would rather think the emission diagram of the antenna to have its symmetry axis given by the general direction of the antenna.
Christophe,
The "apple" axis is vertical--if you model the dorsal V antenna over perfect ground, and if you model it so that the excitation source is not within a leg of the V. You will then see that at 3105 kHz and at 6210 kHz the radiated pattern of the dorsal V is predominantly vertically polarized. It exhibits a zenith "dimple" similar to that of a vertical monopole, but shallower due to the contribution of a horizontally polarized component. The horizontal component magnitude increases with frequency and serves to fill in the dimple as the frequency is raised. The dimple is nearly nonexistent by 8 MHz. At 10 MHz and higher, the dimple is gone and the net polarization of the pattern is horizontal.
Evidence suggests that TIGHAR researchers Mike Everette, and Bob Brandenburg--like you--viewed the Electra's dorsal V to be a horizontal antenna fed by a single-wire transmission line. A modeling option with this view is to consider the transmission line to be lossless and to put the excitiation source in a leg of the V. If you do this you are modeling a horizontal antenna and will see no dimple in the upper part of the pattern.
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). To see if there was a possibility of a MININEC3-based modeling program failing to show the dimple when properly modeled, I modeled the antenna at 1 foot above perfect ground at 3105 kHz with an interpreted Basic version of MININEC3 (NOSC CODE 822, JCL CHANGE 9, dated 11-26-86).
Here is the result for a vertical slice through the pattern so obtained (ANGLE is elevation angle, in degrees):
ANGLE VERT (DBI) HORIZ (DBI) TOTAL (DBI)
0 4.74 -120 4.74
10 4.61 -33.5 4.61
20 4.21 -27.6 4.21
30 3.51 -24.3 3.52
40 2.46 -22.1 2.47
50 .961 -20.6 .992
60 -1.18 -19.5 -1.11
70 -4.36 -18.8 -4.21
80 -9.72 -18.4 -9.17
90 -17.9 -18.2 -15.1
100 -9.72 -18.4 -9.17
110 -4.36 -18.8 -4.21
120 -1.18 -19.5 -1.11
130 .961 -20.6 .991
140 2.46 -22.1 2.47
150 3.51 -24.4 3.52
160 4.21 -27.6 4.21
170 4.61 -33.5 4.61
180 4.74 -135 4.74
At 90 degrees the vertical signal is down 22.6 dB relative to 0 degrees, and the sum of vertical and horizontal contributions (total) is down 19.8 dB. These values indicate very significant dimples in the vertical and total patterns, and show that failure to see a dimple in the pattern of the dorsal V antenna at 3105 kHz cannot be attributed to using MININEC-based software to model it very close to ground.
Chuck Varney
-------------------------------
Brandenburg never posted his calculations. Varney states that the vertical signal was down 22.6 db but that is not the angle that is important. Brandenberg claims that the inner diameter of the donut is 80 NM (148 KM) for which the radiation angle was 76.1 degrees. This is based on standard near vertical incidence skywave propagation models. Based on Varney's table, the signal for the inner diameter of the donut would be down only about 13 db (somewhere between the values for 70 and 80 degrees.) If we do the calculation for 50 NM (92.5 KM) we find the radiation angle 81.2 degrees so the signal would be down about 15 db, only 2 db less than the signal at 80 NM. At 40 NM the takeoff angle is 83.0 degrees so the antenna output would be down about 17 db, only about 4 db less than at the donut. For a listener to even notice a difference in signal strength, the signal must change by 3 db so, if the signal could be heard from 80 NM away it should have been heard at 50 NM and it would be only slightly weaker at 40 NM. To provide further perspective on this, since these signals have to travel up in the ionosphere for a little bit over 300 KM and then back down, a bit over 300 KM, the path loss for all of these signals is about 110 db. When you factor in the antenna losses, at 80 NM the signal is down 123 db and at 50 NM it is down 125 db and at 40 NM it is down 127 db, not big differences. Brandenberg states that the direct wave propagation would reach out 40 NM from an altitude of 1,000 feet, (and the plane could have climbed higher if south of Howland where the sky was clear,) so most likely the direct wave would reach out 50 NM so filling in the donut hole and eliminating any gap in the radio reception from Earhart.
gl
-
Gary,
How did you do your calculations? By hand, or were you using a propagation model?
BTW, a couple of points re Chuck's previous remarks: an antenna radiation pattern has no polarization -- the radiated field is polarized, but not the radiation pattern. I suspect Chuck meant to say the pattern is vertically oriented, i.e. that its central axis is vertical. Also, the "back story" of the antenna model change is not related to the pattern dimple -- both models show a dimple, that was never an issue -- but rather that NEC2 much more accurately models an antenna close to ground. Before 4NEC2, the NEC2 input/output process FORTRAN punch-card format and was unwieldy. 4NEC2 added a GUI that greatly facilitates using the model.
Bob
-
BTW, a couple of points re Chuck's previous remarks: an antenna radiation pattern has no polarization -- the radiated field is polarized, but not the radiation pattern. I suspect Chuck meant to say the pattern is vertically oriented, i.e. that its central axis is vertical.
Bob,
Thank you for keeping me on my technical term toes. I have edited the first paragraph in the subject post (reply #10, this thread) to clarify what was intended.
Also, the "back story" of the antenna model change is not related to the pattern dimple -- both models show a dimple, that was never an issue -- but rather that NEC2 much more accurately models an antenna close to ground.
Thank you, too, for clarifying what I referred to as the "back story". I can't recall all that motivated my supposition, but in part it included Ric's words in a 27 Jan 09 (http://tighar.org/Projects/Earhart/Archives/Forum/Forum_Archives/200901.txt) forum post (italics are mine):
"As explained in the most recent TIGHAR Tracks (Oct. 2008, "The 3105 Donut"), computer modeling software that has become available since 2001 has revealed an anomaly in NR16020's transmission propagation pattern which changes the whole ball game about where the Electra could have been at 1912 and 2055 (the time of the last transmission heard by Itasca)."
That was coupled with having read how you viewed, and appeared to have modeled, the dorsal V antenna for so long. Take, for example, these words of yours from a 23 Oct 01 (http://tighar.org/Projects/Earhart/Archives/Forum/Forum_Archives/200110.txt) forum post:
"In order for there to be a vertical electrostatic field component, and hence a ground wave, it is necessary that the radiating antenna be vertical. If the antenna is horizontal, the electrostatic field is parallel to the earth's surface, and is immediately shorted out, causing the entire wave front to collapse, thus preventing ground wave propagation. And there's the rub. The Electra's antenna was horizontal, hence there was no groundwave."
The last sentence is incorrect at 3105 and 6210 kHz, of course, as the electric field component at those frequencies was predominantly vertical. (Because electrostatic fields are pretty much reluctant to radiate, I suspect you meant to use the term electric field.)
Gary, How did you do your calculations? By hand, or were you using a propagation model?
You asked Gary how he did his calculations. I wonder the same for you. In reply #4 in this thread you told Ken Brookner that you used ICEPAC. My question is How did you use it for an aircraft in flight? Do you have a version that will accept an elevation input?
Chuck
-
The last sentence is incorrect at 3105 and 6210 kHz, of course, as the electric field component at those frequencies was predominantly vertical. (Because electrostatic fields are pretty much reluctant to radiate, I suspect you meant to use the term electric field.)
I did mean the electric field. And why was the electric field component vertical at those frequencies?
You asked Gary how he did his calculations. I wonder the same for you. In reply #4 in this thread you told Ken Brookner that you used ICEPAC. My question is How did you use it for an aircraft in flight? Do you have a version that will accept an elevation input?
There is no version that accepts elevation input, nor is there a need for one. For skywave propagation, an aircraft's altitude is insignificant in the context of the ionospheric refraction geometry
Bob
-
I did mean the electric field. And why was the electric field component vertical at those frequencies?
Because at those frequencies the currents in the generally vertical wires contributed more to the radiated field than those in the generally horizontal wires, making the net polarization of the field vertical.
Does incredulity prompt your asking?
There is no version that accepts elevation input, nor is there a need for one. For skywave propagation, an aircraft's altitude is insignificant in the context of the ionospheric refraction geometry
OK, thanks. You do use the antenna's at-altitude gain pattern for an aircraft's in-flight transmission, don't you?
BTW, are you aware of my 11 July post (http://tighar.org/smf/index.php/topic,418.0.html) to you?
Chuck
-
Not necessarily incredulity, but I am curious. The term " generally vertical wires" suggests one or more such wires other than the one through the starboard cabin wall. Where would it/they be? What are the relative amplitudes of the vertical and horizontal currents?
Re: the at-altitude gain pattern, since the fuselage essentially is ground as far as the antenna is concerned, it doesn't matter how far above geographical ground the plane is.
Re: your 11 July post, I wasn't aware until you just mentioned it. I somehow missed it. I'm open to insights on the harmonic power paper -- or anything else.
Bob
-
Not necessarily incredulity, but I am curious. The term " generally vertical wires" suggests one or more such wires other than the one through the starboard cabin wall. Where would it/they be? What are the relative amplitudes of the vertical and horizontal currents?
I was using "generally vertical" to refer to all wires exclusive of those forming the legs of the V.
I've attached a screen shot (file: MMANA-GAL_screen) from the antenna simulation software I use most often: MMANA (MININEC3-based), or in this specific case, MMANA-GAL, which is the current iteration. It shows a results summary for the dorsal V at 3.105 MHz and its harmonics. Forget the details of what's being modeled for the moment. Note that the far right column gives the polarization of the radiated field, and that it's vertical for 3.105 and 6.21 MHz, but horizontal for higher frequencies. That's the depth to which I typically go on the polarization issue.
I've attached a screen shot (file: 3105_currents) showing an example of the segment currents for the V at 3105 kHz. The current sums give some feel, but it's how their effects combine that tells the story. That's what the simulation software is for (and I've never yearned to learn how it does it). In this case the "generally vertical" currents sum to a value very slightly greater than those of the "generally horizontal". I intended to include a screen shot for the 12.42 MHz case, the first one for which MMANA reports a horizontal polarization, but because I used a constant segment size of wavelength/100 it gave far too many lines to include in a screen shot. It gave a sum for the "generally horizontal" segment currents that was 16 times that of the "generally vertical". Again, a feel, but not the whole story.
We can continue with this until you feel less uncomfortable. If it's going to take a "why" to satisfy, I'll have to work at it, and that will take some motivation and time.
Re: the at-altitude gain pattern, since the fuselage essentially is ground as far as the antenna is concerned, it doesn't matter how far above geographical ground the plane is.
I tend to disagree with that statement in general, but let's look at some specific cases. You indicated in January that you've now modeled the antenna on a wire frame model of the Electra. If you put it at 1000 feet above sea water (I use dielectric constant 80 and conductivity 5 S/m), does it give the same pattern that you get with the fuselage bottom a couple feet above real ground (say 0.6 m and dielectric constant 13 with conductivity 0.005 S/m)? How about above perfect ground at same height? Try it in free space. What does the pattern look like when compared to the other cases?
When I model the antenna over a conductive mat, I get quite different patterns for the three cases.
I've attached a vertical panorama of screen shots (file: dorsal_V_patterns...) of the total, vertical, and horizontal patterns that I got with the dorsal V over a conductive mat 1000 feet above sea water. Do you see anything like this with your model?
Re: your 11 July post, I wasn't aware until you just mentioned it. I somehow missed it. I'm open to insights on the harmonic power paper -- or anything else.
Let's finish this thread first, then move on to that.
Chuck
-
Chuck,
A few questions to help me understand your results:
1) Where on the aircraft are wires 5 and 6, which "are vertical and connect to the lead-in and ground"?
2) Did your test runs involve only the antenna in isolation -- i.e. without the fuselage?
3) Does MMANA-GAL have the same antenna height limitation as MININEC-3, i.e. 0.2 wavelength?
Bob
-
1) Where on the aircraft are wires 5 and 6, which "are vertical and connect to the lead-in and ground"?
Consider wires 5 and 6 together to be one vertical wire, call it wire 5*, that is 29 inches long. (Wire 5* was split into two wires, 5 and 6--each 14.5 inches long, as it afforded a convenient way in MMANA to place the source at five different heights above ground. [A MMANA option is to place the source at the beginning, center, or end of a wire]).
[9/20/11 additional comment: I typically made the lengths of wires 5 and 6 equal, but sometimes used different proportions to place the source in a desired location. I saved such a case in January, and see that I grabbed that one for the examples attached to my 16 Sep 11 post. For that case, the lengths of wires 5 and 6 were 6 inches and 23 inches, respectively.]
Wire 5* connects to the inboard end of the lead-in and drops vertically to "ground", which in the case of the pattern attachment to my previous message, is a conductive mat (a patch, in Geometry Builder) with dimensions as given in the annotation.
2) Did your test runs involve only the antenna in isolation -- i.e. without the fuselage?
That's correct. Nothing beyond the mat that allowed modeling it off the ground. The aforementioned pattern attachment to my previous message shows the modeled antenna configuration (in blue) in each of the four pattern presentations.
3) Does MMANA-GAL have the same antenna height limitation as MININEC-3, i.e. 0.2 wavelength?
If there's a height limitation, I'm ignorant of it. What, specifically, was the 0.2 wavelength limitation for MININEC3--and the effect of ignoring it?
Chuck
-
Wire 5* connects to the inboard end of the lead-in and drops vertically to "ground"
2) Did your test runs involve only the antenna in isolation -- i.e. without the fuselage?
That's correct. Nothing beyond the mat that allowed modeling it off the ground.
3) Does MMANA-GAL have the same antenna height limitation as MININEC-3, i.e. 0.2 wavelength?
If there's a height limitation, I'm ignorant of it. What, specifically, was the 0.2 wavelength limitation for MININEC3--and the effect of ignoring it?
Chuck
It's interesting that you ran the lead-in directly to ground. On the aircraft, the lower end of the lead-in connected to the antenna terminal on the transmitter.
Modeling the antenna in isolation ignores the aircraft structure. The antenna field induces currents in the structure, which generate field components that interact with the antenna field.
The antenna height limitation is discussed briefly in the MMANA-GAL user notes, and more extensively in the web literature.
Bob
-
It's interesting that you ran the lead-in directly to ground.
It's interesting that you didn't note that the path to ground included a source (a "transmitter").
On the aircraft, the lower end of the lead-in connected to the antenna terminal on the transmitter.
And through the transmitter, and on to a connection to the airframe ("ground"). If I had information regarding the actual path, and path length, from the feedthrough insulator to the airframe--and if I thought that incorporating it would materially alter the characterization of the antenna--I would have used it.
What do you have to offer with regard to the path particulars?
Modeling the antenna in isolation ignores the aircraft structure.
Of course.
The antenna field induces currents in the structure, which generate field components that interact with the antenna field.
No argument. I'm interested in hearing some details of what accounting for the interaction revealed to you.
The antenna height limitation is discussed briefly in the MMANA-GAL user notes, and more extensively in the web literature.
Are you referring to differences between NEC-2 and MININEC-3 feed-point impedance and gain results for horizontal antennas modeled close to ground? If so, do you know that the differences are germane to the issue of modeling the dorsal V, say at 7-8 feet above ground at 3105 kHz? If the answer is in the affirmative, how close would the NEC-2 and MININEC-3 impedance and gain results need to be for you to judge them insignificantly different? Perhaps you could try it yourself and we could compare results.
Perhaps you can clarify something for me. I thought the two topics we were pursuing were:
1) The polarization issue (whether the dorsal V, by virtue of its largely horizontal array of wire, could not radiate a ground wave [that is, was not vertically polarized], as you contended).
2) The radiation pattern vs. height above ground issue (whether altitude is immaterial to propagation modeling as far as the pattern goes, since it's the same in the air as on the ground, as you contended.)
What is your conclusion regarding 1)?
How about 2)?
What did you find out with regard to the following, which I asked about in Reply #20 four days ago:
You indicated in January that you've now modeled the antenna on a wire frame model of the Electra. If you put it at 1000 feet above sea water (I use dielectric constant 80 and conductivity 5 S/m), does it give the same pattern that you get with the fuselage bottom a couple feet above real ground (say 0.6 m and dielectric constant 13 with conductivity 0.005 S/m)? How about above perfect ground at same height? Try it in free space. What does the pattern look like when compared to the other cases?
Chuck
-
Bob,
This post regards my "3105 Donut back story" ( Reply #9, paragraph three (http://tighar.org/smf/index.php/topic,285.msg2598.html#msg2598)) and your revision to it in Reply #16 (http://tighar.org/smf/index.php/topic,285.msg5766.html#msg5766), where you wrote:
. . .Also, the "back story" of the antenna model change is not related to the pattern dimple -- both models show a dimple, that was never an issue -- but rather that NEC2 much more accurately models an antenna close to ground.
In Reply #17, I accepted your revision to my version and gave two examples of what led me to believe what I had written. There were others that didn't come readily to hand at the time I responded. One of them follows.
These are your words, as appended to a Ric Gillespie forum post from 9 January 2009,12:25:25 (http://tighar.org/Projects/Earhart/forum/Forum_Archives/200901.txt):
. . .With the increased speed available on a modern PC, and with the GUI, NEC4WIN95 was the most practical tool for TIGHAR's purposes at the time. As it turned out, the model missed the "dimple" that was later found by 4NEC2.
In 2009 you said the "dimple" was revealed when 4NEC2 (NEC-2-based antenna modeling software) replaced NEC4WIN95 (MININEC-3-based antenna modeling software).
In 2011 you said the dimple was never the issue--both software programs showed it.
Please explain.
Chuck
-
The phrase "dimple that was missed" should be read in the context of the fact that NEC2 handles antennas close to ground better than NEC4WIN95. The dimple is there in both, but is deeper and steeper in NEC2.
Bob
-
The phrase "dimple that was missed" should be read in the context of the fact that NEC2 handles antennas close to ground better than NEC4WIN95. The dimple is there in both, but is deeper and steeper in NEC2.
Bob,
The following is from your The Post-Loss Radio Signals: Technical Analysis paper (http://tighar.org/Projects/Earhart/Archives/Research/ResearchPapers/Brandenburg/PLRSTechnical.html). You are describing the radiated power patterns you obtained for operation at 3105 kHz and 6210 kHz.
". . .It is interesting to consider the effective radiated power from the NR16020 antenna used in this analysis.
At 3105 kHz, the radiated power is 0.3 watts at takeoff angles less than 15 degrees, with a smooth nonlinear taper up to 6 watts at takeoff angles above 80 degrees. Perpendicular to the main-lobe axis, the radiated power is 0.003 watts at takeoff angles less than 5 degrees, tapering up to 6 watts for takeoff angles greater than 75 degrees.
At 6210 kHz, the radiated power is 0.2 watts at takeoff angles less than 5 degrees, with a smooth nonlinear taper up to 40 watts at takeoff angles greater than 80 degrees. Perpendicular to the main-lobe axis, the radiated power is 0.06 watts at takeoff angles less than 5 degrees and 40 watts at takeoff angles greater than 80 degrees."
These are not descriptions of dimpled patterns.
Please explain.
Chuck
-
That was eleven years ago, Chuck. What's your point?
Bob
-
That was eleven years ago, Chuck. What's your point?
Bob,
My point is that you would have the general reader think that, because you now understand a thing, all the papers you authored reflect that understanding. They don't. The excerpt I gave from your The Post-Loss Radio Signals: Technical Analysis paper is one example.
Chuck