I did answer by describing the 1/2 wave sleeve dipole as a center fed antenna. With regards to the error in EZNEC being a simple case of reversing the direction of the wires, I'm highly skeptical since it does affect much more than just the way the phase is indicated in the program. I see severe errors being calculated in collinear models and that shows the original model is not being displayed properly. If you have any evidence that you're able to correct these phase issues in the program this easily, many of us would like to see this.
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Question on the Vector
- Thread starter The DB
- Start date
I did answer by describing the 1/2 wave sleeve dipole as a center fed antenna. With regards to the error in EZNEC being a simple case of reversing the direction of the wires, I'm highly skeptical since it does affect much more than just the way the phase is indicated in the program. I see severe errors being calculated in collinear models and that shows the original model is not being displayed properly. If you have any evidence that you're able to correct these phase issues in the program this easily, many of us would like to see this.
Donald I did not mean this information as a fix to anything except that it did seem to fix the way that Eznec reported the currents in the tabular current report. My point was that if one sees the current results from the first model with two end 1's connecting the dipole...one might figure the software was in error without knowing why.
Did you read page 97, of the Manual I attached below concerning Currents? It explains the situation better than I can. I know you can understand what that rule is all about.
If you still hold that Eznec is in error due to your testing the collinear idea you tell us about...then so be it. I didn't expect my information to change your mind, but I think it does answer the question we discussed back in 2012, and that information was right there in the manual all the time. I just did not recall the words when I saw something strange going on.
Regarding the sleeved dipole that some consider similar to the way the S4/NV4K works in the cone area, and that the cone is in fact a form of a sleeve. We understand that the sleeve carries CMC that terminate in a high impedance point at the bottom end of the 1/4 wave sleeve.
Are you then suggesting that the CMC's on the S4/NV4k emanates where the coax terminates at the feed point, instead of these currents flowing down the feed line, like they do to some degree with all other setups that lack balance at the feed point. But instead, that the currents just happen to flow up the radials in a constructive phase with the radiator above?
You've referred to this S4 type cone as a sleeve right here in this thread: http://www.worldwidedx.com/threads/question-on-the-vector.174038/#post-494731
I remember someone of the ham forum where Booty Monster presented his homemade Vector. You or someone else was claiming the idea that the Vector cone was a sleeve, and this ham member was arguing, saying the sleeve was the opposite idea of the cone on the Vector, and the feed points were at opposite ends.
DB, the Eznec dipole model I posted below shows how the currents should look when done correctly, and when not. That said you will note that all other performance data remains the same.
I posted these attachments and some words describing them last night and it was saved to the forum, I checked. This morning I see that work's disappeared.
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Marconi: We have had this discussion before. I am well aware of how the NEC2 engine handles currents.
Shockwave: I've done some research and thinking, specifically on the style of of the antenna you suggested uses the same mechanics.
The Russian Woodpecker array is made up of a lot of what I am going to call widened dipoles. For those that are confused by that terminology it works like this, the antennas are fed at a point, then widened, in the case of this antenna with a cone. I am aware of Cebik referring to antennas of such design, although in his case multiple wires and a spreaders are used instead of a cone.
You say that because of the design the antenna acts like a faraday cage, in such a case that currents again only flow on the outside of the antenna (in this case). After some playing around with modeling and some thinking on the subject I am not sure that is necessarily true.
First I built a reference model, I used a center fed half wavelength model for this.
Next I built several models of varying width widened dipoles. The radiation pattern was exactly the same as the reference dipole to within 0.01 dB. So far so good, everything was as I expected it would be. However, this neither proves nor disproves the faraday cage effect that you are referring to, so I took my efforts a step further.
The models above reminded me of phased arrays, so instead of a single antenna, I created four antennas in a square orientation and phased at 0 degrees. I used the same distances apart as above for initial testing. After confirming similar results I spread the antennas further apart to see how far apart I could go and maintain the single dipole pattern, and that distance was about 1/4 wavelength separation before noticeable changes in the pattern started to emerge. That distance is significant for this test, and about where I expected such a change to happen. NOTE: Even at that point the change isn't drastic, it is just where it becomes noticeable.
After that I took advantage of the phased array models, which resembled a 4-square array, and changed the phasing with the intent to make the antenna array directional. Even with a very short 0.02 wavelengths of separation between the antennas there is still a noticeable effect from the phasing of the antennas, more than I actually expected.
Next I did some research on the maximum hole size of a faraday cage. What I found is that the holes in a faraday cage (and by extension your microwave example above) don't completely isolate the inside from the outside. They act more like attenuaters, and don't completely block the signal in question... The attenuation amount is based on the frequency and the size of the holes. I have read in multiple places that a hole of 1/20 of the frequencies wavelength will cause about a 10 dB drop in the signal passing through it. That is considered the maximum "safe" hole size in a faraday cage. Further, the effects of the hole size is on a logarithmic scale, not a linear scale, so the larger you make the hole has an exponential effect on how much signal makes it through the hole. The only way to completely block signals is to have no holes.
Unfortunately I have yet to find an exact formula for hole size vs attenuation. If anyone happens to know said formula I would love to know what it is.
Based on the modeling described above I am mostly certain that the antenna you referred me to, the "Russian Woodpecker" doesn't need the faraday cage effect to function as it does as phased antennas function the same way when noticeably further apart than the size of the holes in a faraday cage and maintain the same pattern.
Further, the pattern change from changing the phasing of antennas that are much closer than the "safe" size of a faraday cage hole is evidence that currents that are directly fed to elements of the "faraday cage effect" you mention are not limited by the effect.
Now I am aware of the limitations of the NEC2 engine, and I took care to keep everything within known useable limits. I am not using anything that I am aware of that is anywhere close to a known problem areas of the NEC2 engine. Because of this caution, the results I am looking at should be reasonably accurate. Further, when checking patterns of the types used they match not only ARRL books and sources, but other more advanced books on the topic, and on-line college courses about EM fields and EM engineering that I have previously watched on youtube as well.
The DB
Shockwave: I've done some research and thinking, specifically on the style of of the antenna you suggested uses the same mechanics.
The Russian Woodpecker array is made up of a lot of what I am going to call widened dipoles. For those that are confused by that terminology it works like this, the antennas are fed at a point, then widened, in the case of this antenna with a cone. I am aware of Cebik referring to antennas of such design, although in his case multiple wires and a spreaders are used instead of a cone.
You say that because of the design the antenna acts like a faraday cage, in such a case that currents again only flow on the outside of the antenna (in this case). After some playing around with modeling and some thinking on the subject I am not sure that is necessarily true.
First I built a reference model, I used a center fed half wavelength model for this.
Next I built several models of varying width widened dipoles. The radiation pattern was exactly the same as the reference dipole to within 0.01 dB. So far so good, everything was as I expected it would be. However, this neither proves nor disproves the faraday cage effect that you are referring to, so I took my efforts a step further.
The models above reminded me of phased arrays, so instead of a single antenna, I created four antennas in a square orientation and phased at 0 degrees. I used the same distances apart as above for initial testing. After confirming similar results I spread the antennas further apart to see how far apart I could go and maintain the single dipole pattern, and that distance was about 1/4 wavelength separation before noticeable changes in the pattern started to emerge. That distance is significant for this test, and about where I expected such a change to happen. NOTE: Even at that point the change isn't drastic, it is just where it becomes noticeable.
After that I took advantage of the phased array models, which resembled a 4-square array, and changed the phasing with the intent to make the antenna array directional. Even with a very short 0.02 wavelengths of separation between the antennas there is still a noticeable effect from the phasing of the antennas, more than I actually expected.
Next I did some research on the maximum hole size of a faraday cage. What I found is that the holes in a faraday cage (and by extension your microwave example above) don't completely isolate the inside from the outside. They act more like attenuaters, and don't completely block the signal in question... The attenuation amount is based on the frequency and the size of the holes. I have read in multiple places that a hole of 1/20 of the frequencies wavelength will cause about a 10 dB drop in the signal passing through it. That is considered the maximum "safe" hole size in a faraday cage. Further, the effects of the hole size is on a logarithmic scale, not a linear scale, so the larger you make the hole has an exponential effect on how much signal makes it through the hole. The only way to completely block signals is to have no holes.
Unfortunately I have yet to find an exact formula for hole size vs attenuation. If anyone happens to know said formula I would love to know what it is.
Based on the modeling described above I am mostly certain that the antenna you referred me to, the "Russian Woodpecker" doesn't need the faraday cage effect to function as it does as phased antennas function the same way when noticeably further apart than the size of the holes in a faraday cage and maintain the same pattern.
Further, the pattern change from changing the phasing of antennas that are much closer than the "safe" size of a faraday cage hole is evidence that currents that are directly fed to elements of the "faraday cage effect" you mention are not limited by the effect.
Now I am aware of the limitations of the NEC2 engine, and I took care to keep everything within known useable limits. I am not using anything that I am aware of that is anywhere close to a known problem areas of the NEC2 engine. Because of this caution, the results I am looking at should be reasonably accurate. Further, when checking patterns of the types used they match not only ARRL books and sources, but other more advanced books on the topic, and on-line college courses about EM fields and EM engineering that I have previously watched on youtube as well.
The DB
DB, I have a good recollection of discussing currents with Donald several years ago. I remember talking to Henry also, but I have no recollections at all of discussing currents with you.
In passing I may have mentioned my concerns, understandings, or the lack about Eznec currents to you, or asked you what you see regarding currents using 4Nec2...but I just don't remember what you said.
Could you post an example of the images your program produces for a simple antenna like I did above for a dipole? I would like to see the detail currents data you get too. You suggest 4Nec2 produces currents and phase that look different somehow.
I don't know that we have talked about this specific antenna, but I'm sure we have had a discussion on currents in NEC2. I know in the past there have been several times you asked about seeing the currents data like I am posting below, and I know that a version of the color chart has been posted for your benefit more than once... In any case, I have definitely read all of your posts on this topic with Donald and Henry.
Anyway...
This is an example of my preferred view.
This is another view that I can use, likely looks more familiar to you.
When it comes to the detail current data in text form, it looks very similar to the detail current data you post on this board sometimes. The only real difference is I have to know where to find it in the file. Unfortunately the raw text doesn't paste well onto the forum, so here is an example text file. I stopped at 20 lines of data, but there are 499 segments in the model that I used. That is something else about my models, they tend to be very segment heavy...
The DB
Marconi, Thank you for taking the time to explain. I now understand the point you were trying to make in identifying which end of the cone would be the source for the CMC energy. I think we agree that the source of the CMC will begin at the end of the coax or in this case, the top of the cone loop that serves as the end of the transmission line to the top 1/2 wave. I think you're suggesting that because the bottom of the cone may represent a low impedance to the CMC that the current could not flow along this 1/4 wavelength resonant path.
There are cases where CMC is handled exactly this way. If you have an antenna with lots of CMC on the transmission line, you can connect the shield back to the feedpoint exactly 1/4 wavelength down the cable to suppress the current from traveling further down the line. You'll notice the bottom end of this 1/4 wavelength cone is connected back to the shield at the main feedpoint the same way. I also do not suggest that there are changes in the phase of this CMC or the normal radial currents we would see on a typical groundplane. The 90 degree phase shift from the source takes place on the vertical inside the cone. That should be obvious since the top 1/2 wave is now an electrical 1/4 wave away from the source.
It's not the upward radials that have experienced some magical change in the phase of their currents. It's the simple idea that the top 1/2 wave has its signal effectively delayed by a 90 degree phase shift that must occur since it's that distance from the source. That is the only phase correction required in order to form the "non apparent collinear" since that's all that is required to bring the top 1/2 wave into a constructive phase with the cone. Whatever radial currents that are not canceled on this unbalanced design can now radiate constructively from the cone along with the CMC on the outside of the cone.
I thought you may have found the problem with EZNEC when you first pointed out the direction of the wires in the program. You confirmed the corrections did nothing to make the antenna look any different than a half wave using this software. I then provided several drawings with explanations on how to add four simple wires to your model in order to test the programs ability to accurately predict the phase after your correction with no response. That information is quickly revealed by the length of the added phase shift wires since EZNEC predicts they should be 100% too long. Not some minor amount attributed to velocity factor or some other small issue. Enough to make all models of the Sigma done in the program useless.
DB, Your 3-D work on the currents is a new approach to me and I appreciate this insight. I suppose it is better to take a deeper look than to just consider the cones elements as one. I agree the cone does not provide a perfect Faraday cadge effect. I also wish that CST provided a written theory of operation in its data. In the absence of that, I have to rely on both my field testing and the analysis of the CST data provided by the engineer who made the model. The engineer described the action taking place inside the cone as "confining" the radiation of the lower 1/4 wave within the cone which suggest more shielding or the Faraday explanation. Without wrapping the cone around the base of the radiator, we cannot achieve gain in the field either. The pattern also begins to look more distorted like the J-Pole.
I've built all these models in copper for field testing and found improvements going from 3 to 4 upward radials. Adding another did nothing and wrapping it in wire mesh barely made an improvement that could be measured on test equipment. That was somewhat inconclusive due to pattern distortion around the gamma and wire mesh. The Sigma cone does share some other properties with the broadband dipole example in that it increases the bandwidth of this antenna too. When an element flares out and is terminated with a curved conductor, bandwidth is further expanded. There is no doubt that there are individual currents on these radials however, they are equally sharing the currents as one bigger element around the outside of the vertical to produce a uniform effect.
To follow up here is an example of single conductors used to form "coax" carrying a megawatt at lower frequencies: http://en.wikipedia.org/wiki/File:Solec_Kujawski_longwave_antenna_feeder.jpg With relatively few wires this method forms a very efficient and well shielded transmission line. The Russian OTH radar also used similar coax well into the HF band. I totally understand at higher frequencies it would require closer shielding wires to provide the same degree of confinement but then again we don't need 100% shielding of the signal inside to achieve noticeable gain from this antenna. I point this out to show the "skeleton" design can be used as both a transmission line and a broadband radiator since both appear to be happening with the Sigma IV cone.
There are cases where CMC is handled exactly this way. If you have an antenna with lots of CMC on the transmission line, you can connect the shield back to the feedpoint exactly 1/4 wavelength down the cable to suppress the current from traveling further down the line. You'll notice the bottom end of this 1/4 wavelength cone is connected back to the shield at the main feedpoint the same way. I also do not suggest that there are changes in the phase of this CMC or the normal radial currents we would see on a typical groundplane. The 90 degree phase shift from the source takes place on the vertical inside the cone. That should be obvious since the top 1/2 wave is now an electrical 1/4 wave away from the source.
It's not the upward radials that have experienced some magical change in the phase of their currents. It's the simple idea that the top 1/2 wave has its signal effectively delayed by a 90 degree phase shift that must occur since it's that distance from the source. That is the only phase correction required in order to form the "non apparent collinear" since that's all that is required to bring the top 1/2 wave into a constructive phase with the cone. Whatever radial currents that are not canceled on this unbalanced design can now radiate constructively from the cone along with the CMC on the outside of the cone.
I thought you may have found the problem with EZNEC when you first pointed out the direction of the wires in the program. You confirmed the corrections did nothing to make the antenna look any different than a half wave using this software. I then provided several drawings with explanations on how to add four simple wires to your model in order to test the programs ability to accurately predict the phase after your correction with no response. That information is quickly revealed by the length of the added phase shift wires since EZNEC predicts they should be 100% too long. Not some minor amount attributed to velocity factor or some other small issue. Enough to make all models of the Sigma done in the program useless.
DB, Your 3-D work on the currents is a new approach to me and I appreciate this insight. I suppose it is better to take a deeper look than to just consider the cones elements as one. I agree the cone does not provide a perfect Faraday cadge effect. I also wish that CST provided a written theory of operation in its data. In the absence of that, I have to rely on both my field testing and the analysis of the CST data provided by the engineer who made the model. The engineer described the action taking place inside the cone as "confining" the radiation of the lower 1/4 wave within the cone which suggest more shielding or the Faraday explanation. Without wrapping the cone around the base of the radiator, we cannot achieve gain in the field either. The pattern also begins to look more distorted like the J-Pole.
I've built all these models in copper for field testing and found improvements going from 3 to 4 upward radials. Adding another did nothing and wrapping it in wire mesh barely made an improvement that could be measured on test equipment. That was somewhat inconclusive due to pattern distortion around the gamma and wire mesh. The Sigma cone does share some other properties with the broadband dipole example in that it increases the bandwidth of this antenna too. When an element flares out and is terminated with a curved conductor, bandwidth is further expanded. There is no doubt that there are individual currents on these radials however, they are equally sharing the currents as one bigger element around the outside of the vertical to produce a uniform effect.
To follow up here is an example of single conductors used to form "coax" carrying a megawatt at lower frequencies: http://en.wikipedia.org/wiki/File:Solec_Kujawski_longwave_antenna_feeder.jpg With relatively few wires this method forms a very efficient and well shielded transmission line. The Russian OTH radar also used similar coax well into the HF band. I totally understand at higher frequencies it would require closer shielding wires to provide the same degree of confinement but then again we don't need 100% shielding of the signal inside to achieve noticeable gain from this antenna. I point this out to show the "skeleton" design can be used as both a transmission line and a broadband radiator since both appear to be happening with the Sigma IV cone.
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s
Sorry DB, but your images are of no use to me.
It seem a bit much to me to make a simple single element dipole model with 500 segments. I cannot tell a thing about these currents, with each segment being so small.
Is there some advantage you find in making a single 30 mhz dipole with 500 segments?
This type of cooperation may be why I don't remember ever talking to you about antenna currents specifically with regards to modeling.
I don't know that we have talked about this specific antenna, but I'm sure we have had a discussion on currents in NEC2. I know in the past there have been several times you asked about seeing the currents data like I am posting below, and I know that a version of the color chart has been posted for your benefit more than once... In any case, I have definitely read all of your posts on this topic with Donald and Henry.
Anyway...
This is an example of my preferred view.
This is another view that I can use, likely looks more familiar to you.
When it comes to the detail current data in text form, it looks very similar to the detail current data you post on this board sometimes. The only real difference is I have to know where to find it in the file. Unfortunately the raw text doesn't paste well onto the forum, so here is an example text file. I stopped at 20 lines of data, but there are 499 segments in the model that I used. That is something else about my models, they tend to be very segment heavy...
The DB
Sorry DB, but your images are of no use to me.
It seem a bit much to me to make a simple single element dipole model with 500 segments. I cannot tell a thing about these currents, with each segment being so small.
Is there some advantage you find in making a single 30 mhz dipole with 500 segments?
This type of cooperation may be why I don't remember ever talking to you about antenna currents specifically with regards to modeling.
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I'm curious, did the engineer say anything about the presence of common mode currents on the cone directly? Also, was anything mentioned about the action that confines the RF field of the lower 1/4 wavelength section of the vertical element inside the cone, or was it merely stated that it exists?
When it comes to your second paragraph above, near the end, weather or not those currents are present isn't what I am exploring at the moment, I am definitely sure they are there. What I'm trying to figure out specifically is the fields those currents are generating, and what those fields are doing, and potentially how they are interacting not only with each other, but also the field from the current on the central vertical element. Maybe I'm over thinking this, but I have the impression that there is more going on than what the currents themselves are telling us.
Essentially they made a coax out of various wires... Neat. At much lower frequencies that we typically worry about such a system would have the loss characteristics of ladder line as the dielectric would be air. That in conjunction with the lower loss characteristics of the lower frequencies in general, I bet you could get several hundred miles with only marginal losses... My only concern with the comparison between that and the Vector basket is the outer wiring is there to merely contain the field, it is not intended to be an active radiating component like the basket on the Vector acts.
When I get some time I am going to see if I can put down what I think I am seeing on paper (or, well, my computer anyway) in an understandable way. I'm busy the next few days so it may be a while. Its frustrating for me to have this thought in my head and not be able to describe it...
The DB
The number of segments has to do with some experiments I have been running using 4NEC2. These experiments would take some data from a model, change some aspect of the model, say length, then take more data, then change the model in the same direction the same amount. I would record the data from all of the models and put it in a spreadsheet to graph out the changes. For accuracy I need to maintain the segment length to get reliable results, 20 or even 50 segments is just not enough for said data. Every change in length has to be accompanied by an equivalent number of additional segments. The changes are so small that starting with the maximum amount of segments some models have, a change might add in one or two more total segments. I might make such measurements over 1/5 of a wavelength, and potentially in the future even more. It is long and tedius work, but there is generally a goal, or something to learn.
That being said there are some downsides to all those segments as well, but that is neither here nor there. That model came from one such experiment.
I can lower the segment count quick and post another file for you, give me a minute. How about 19 segments?
The DB
OK, DB. Could you try a two wire version of this antenna with both wire end 1's connecting in the middle of the antenna and about 10 segments per wire, so we can see if 4Nec2 does the same with the current distribution and phase as Eznec does.
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7.67 dBi gain at 14 degrees, the antenna is a horizontal center fed dipole one wavelength high and has a second higher angle lobe.
I could tell they were pretty close to a match just from the pictures above... There was no reason that they shouldn't have been a match...
So you want a dipole made of two wires, I'm assuming equal length wires? The feedpoint won't be exactly centered so I doubt currents will match exactly, or should I end feed it?
The DB
DB, When asked about the cone, the specific questions you bring up were not mentioned in detail. The part of the email regarding the function of the cone is posted below.
"The presence of the cone (four elements around the whip), are the difference between a standard J-pole. The out of phase radiation is confined inside the cone and outside the cone the radiation is in phase. To understand the better performance of the antenna in comparison of the standard 5/8 Lambda, it is necessary to also consider that the radiation pattern has been maximized to irradiate on the horizon line and does not elevate like a standard 5/8 Lambda. Considering also the installation, the small current that descends on the first part (the most important) of the mast, near the antenna, is in phase (another small increase in gain) That is not true for a standard 5/8 Lambda."
The outer wiring is there to contain the field in both the Vector and the "coax" example. The difference is the load placed at the end of each. That is what will determine if the "shield" will become a radiator on the outside as well as a shield to the currents inside the transmission line. If the load is an end fed vertical as we see at the end of the cone on the Sigma, the shield would be expected to radiated CMC as well. The shield is the 1/4 wave cone and the source of some speculation.
I believe Marconi suggests the current can't travel back down the cone since it does not terminate in a high impedance. Since the source of this CMC is the end fed 1/2 wave at the top of the cone, the current must begin at the top of the cone and fold back over the outside of the cone regardless of the impedance across this path since it is the only path CMC can take from the 1/2 wave load. The fact it is also a resonant 1/4 wave element also makes it an efficient radiator of this CMC.
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