|
colinear representation in NEC
a) | b) | | | | | | | | | | | | | | | | | -------| | -------| ||| | ||| | ||| | ||| | ||| | ||| | --- S S --------------- ----------------- Fig a) above is an attempt to portray a colinear vertical over infinite ground with a source at "S". The configuration is easy enough to model in NEC with sensible results. The common explanation for operation of a) is that the U shaped section is a quarter wave s/c stub, that it is responsible for delivering direct in-phase drive to the upper section, and that it plays no part itself in radiation ie, that the common mode current on the pair of conductors is zero at all points. Notwithstanding the conventional wisdom, it seems unlikely that there is no common mode current on that section, and NEC models suggest that there is, and that it accounts for some small asymmetric distortion of the pattern. Fig b) above is an attempt to represent a coaxial arrangement of tubes where the lower end of the tubes are connected together, and that is fed at S against an infinite ground. My questions a 1. To what extent is b) equivalent to a)? 2. How is b) modelled in NEC? Thanks Owen |
colinear representation in NEC
Owen Duffy wrote: a) | b) | | | | | | | | | | | | | | | | | -------| | -------| ||| | ||| | ||| | ||| | ||| | ||| | --- S S --------------- ----------------- Fig a) above is an attempt to portray a colinear vertical over infinite ground with a source at "S". The configuration is easy enough to model in NEC with sensible results. The common explanation for operation of a) is that the U shaped section is a quarter wave s/c stub, that it is responsible for delivering direct in-phase drive to the upper section, and that it plays no part itself in radiation ie, that the common mode current on the pair of conductors is zero at all points. Notwithstanding the conventional wisdom, it seems unlikely that there is no common mode current on that section, and NEC models suggest that there is, and that it accounts for some small asymmetric distortion of the pattern. Fig b) above is an attempt to represent a coaxial arrangement of tubes where the lower end of the tubes are connected together, and that is fed at S against an infinite ground. My questions a 1. To what extent is b) equivalent to a)? I can't answer that question right off, except that at first glance they look quite similar in operation. I'd build both models with EZNEC, then take a look at the reported currents in the View Antenna display. You can get the same information from tabular NEC results, but most people find the graphical display quicker and easier to interpret. You can see the significance of the seemingly small common mode current on the a) model stub by replacing it with a transmission line model stub which of course has zero common mode current. The results are quite different than for the wire model stub. 2. How is b) modelled in NEC? A coaxial line can be modeled as a combination of a transmission line (for the inside of the coax) and a wire (for the outside of the coax). Download the EZNEC demo program and look in the manual index under Coaxial Cable, Modeling. It'll direct you to one of the furnished example files which illustrates how. Then you can do the same thing with NEC if you're so inclined. Roy Lewallen, W7EL |
colinear representation in NEC
On Sat, 14 Mar 2009 21:33:15 GMT, Owen Duffy wrote:
My questions a 1. To what extent is b) equivalent to a)? Hi Owen, To no extent as far as I can tell without modeling. I don't think the phases are going to equivalent. 2. How is b) modelled in NEC? This takes some presuming of your intent, and my presumption, given the symmetry of the two smaller elements (why two otherwise?), is you are attempting to portray a skeletal sleeve with them. Two is insufficient by my standards, six are barely worth chasing the numbers and I typically use 16. One such example, complete with a link to the design can be found at: http://home.comcast.net/~kb7qhc/ante.../Cage/cage.htm It deviates only by a small degree, but could prove a useful boost in adding the longer element after opening the top end of the thick radiator. 73's Richard Clark, KB7QHC |
colinear representation in NEC
Roy Lewallen wrote in
treetonline: Hello Roy, Thanks for the response. .... My questions a 1. To what extent is b) equivalent to a)? I can't answer that question right off, except that at first glance they look quite similar in operation. I'd build both models with EZNEC, then take a look at the reported currents in the View Antenna display. You can get the same information from tabular NEC results, but most people find the graphical display quicker and easier to interpret. Ok, here is the model I constructed of b) (the coaxial tubes construction). For simplicity, the upper and lower outer tubes are the same diameter, the same wire in this model. CM CE GW 10 1 0 -2 2 0 -2 2.1 0.005 GW 1 47 0 0 0 0 0 15 0.005 GE 1 GN 1 EK EX 6 1 1 1 0 TL 10 1 1 16 50 5 1e+99 1e+99 0.0001 FR 0 0 0 0 15 0 EN I have a 3/4 wave vertical over perfect ground, and I have inserted a quarter wave s/c transmission line into the vertical at 1/3 height. I have shunted the TL with 10k ohm to represent some loss in the stub. The currents report shows the currents in the top half wave to be approximately 180° out of phase with the bottom quarter wave. The question is whether such a construction yields three quarter waves in phase, or whether the NEC model is correct that they are not in phase. You can see the significance of the seemingly small common mode current on the a) model stub by replacing it with a transmission line model stub which of course has zero common mode current. The results are quite different than for the wire model stub. My initial feeling is that the wire model of a) is correct. I have not yet done as you suggest in the previous par. 2. How is b) modelled in NEC? A coaxial line can be modeled as a combination of a transmission line (for the inside of the coax) and a wire (for the outside of the coax). Download the EZNEC demo program and look in the manual index under Coaxial Cable, Modeling. It'll direct you to one of the furnished example files which illustrates how. Then you can do the same thing with NEC if you're so inclined. Is my model above what you suggest? Appreciate your comments Roy, thanks. Owen |
colinear representation in NEC
Hello Richard,
Richard Clark wrote in : On Sat, 14 Mar 2009 21:33:15 GMT, Owen Duffy wrote: My questions a 1. To what extent is b) equivalent to a)? Hi Owen, To no extent as far as I can tell without modeling. I don't think the phases are going to equivalent. Ok. Your view is contrary to common explanation... but of course that doesn't make it wrong. NEC models of the wire construction at a) show in phase operation, but a small distortion of the pattern due to common mode current on the stub... so they support the common explanation in the phase aspect, but not in respect of the stub causing phase change with no other effects. The explanation of b) as a) where the stub is relocated coaxially sounds appealing, but that explanation might be wrong. 2. How is b) modelled in NEC? This takes some presuming of your intent, and my presumption, given the symmetry of the two smaller elements (why two otherwise?), is you are attempting to portray a skeletal sleeve with them. Two is You seem to have mininterpreted my ASCII art, and that would be easy to do. I am describing at b), two coaxial tubes, the lowest tube is 1/4 wave, the longer tube is 3/4 wave. The lower tube ends are connected, and fed between ground and the bottom of the tube assembly. See the model that I have posted in response to Roy. Thanks. Owen |
colinear representation in NEC
On Mar 14, 2:33*pm, Owen Duffy wrote:
* *a) * * * * | * * * * * * * * * * * *b) * * * * * *| * * * * * * * | * * * * * * * * * * * * * * * * * * *| * * * * * * * | * * * * * * * * * * * * * * * * * * *| * * * * * * * | * * * * * * * * * * * * * * * * * * *| * * * * * * * | * * * * * * * * * * * * * * * * * * *| * * * * * * * | * * * * * * * * * * * * * * * * * * *| * * * * * * * | * * * * * * * * * * * * * * * * * * *| * * * * * * * | * * * * * * * * * * * * * * * * * * *| * * * * * * * | * * * * * * * * * * * * * * * * * * *| * * * * * * * -------| * * * * * * * * * * * * * * * | * * * * * * * -------| * * * * * * * * * * * * * * *||| * * * * * * * | * * * * * * * * * * * * * * * * * * ||| * * * * * * * | * * * * * * * * * * * * * * * * * * ||| * * * * * * * | * * * * * * * * * * * * * * * * * * ||| * * * * * * * | * * * * * * * * * * * * * * * * * * ||| * * * * * * * | * * * * * * * * * * * * * * * * * * ||| * * * * * * * | * * * * * * * * * * * * * * * * * * --- * * * * * * * S * * * * * * * * * * * * * * * * * * *S * * * --------------- * * * * * * * * * * * *----------------- Fig a) above is an attempt to portray a colinear vertical over infinite ground with a source at "S". The configuration is easy enough to model in NEC with sensible results. The common explanation for operation of a) is that the U shaped section is a quarter wave s/c stub, that it is responsible for delivering direct in-phase drive to the upper section, and that it plays no part itself in radiation ie, that the common mode current on the pair of conductors is zero at all points. Notwithstanding the conventional wisdom, it seems unlikely that there is no common mode current on that section, and NEC models suggest that there is, and that it accounts for some small asymmetric distortion of the pattern. Fig b) above is an attempt to represent a coaxial arrangement of tubes where the lower end of the tubes are connected together, and that is fed at S against an infinite ground. My questions a 1. To what extent is b) equivalent to a)? 2. How is b) modelled in NEC? Thanks Owen Hi Owen, I suppose that R.W.P. King disagrees with the "common explanation." He makes it quite clear that there is interaction of the antenna field with the stub perpendicular to the axis of the antenna wire, and that the coaxial stub does not interact in the same way and the antenna performance is therefore different. (Antennas chapter of Transmission Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I like using a feedline to guarantee the phasing. It can be done by driving collinear dipoles with equal lengths of transmission line, or by using an arrangement like the "coaxial collinear," where the radiating elements are outer conductors of coaxial transmission lines used to insure that the multiple feedpoints are at least fed in-phase voltages (and you have to consider that the currents are not exactly in phase). Cheers, Tom |
colinear representation in NEC
Hi Tom,
K7ITM wrote in : .... I suppose that R.W.P. King disagrees with the "common explanation." He makes it quite clear that there is interaction of the antenna field with the stub perpendicular to the axis of the antenna wire, and that the coaxial stub does not interact in the same way and the antenna performance is therefore different. (Antennas chapter of Transmission Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I like using a feedline to guarantee the phasing. It can be done by driving collinear dipoles with equal lengths of transmission line, or by using an arrangement like the "coaxial collinear," where the radiating elements are outer conductors of coaxial transmission lines used to insure that the multiple feedpoints are at least fed in-phase voltages (and you have to consider that the currents are not exactly in phase). That it interesting that Prof King declares that there is more than just a transmission line action with the external style of stub. An NEC model of a) works well, showing in phase operation and a nice pattern. I have played around with two stubs of shorter length on opposite sides of the vertical and stacked on top of each other, and they worked fine (ie in phase current distribution with zero near the stubs) at about 0.15+ wavelenths each... which doesn't fit with a propagation delay around the conductor path explanation. Interesting! I am trying to support the common explanation of the coaxial colinear in my diagram b) using NEC, but I haven't yet been sucessful. Owen |
colinear representation in NEC
Owen Duffy wrote:
Ok, here is the model I constructed of b) (the coaxial tubes construction). For simplicity, the upper and lower outer tubes are the same diameter, the same wire in this model. CM CE GW 10 1 0 -2 2 0 -2 2.1 0.005 GW 1 47 0 0 0 0 0 15 0.005 GE 1 GN 1 EK EX 6 1 1 1 0 TL 10 1 1 16 50 5 1e+99 1e+99 0.0001 FR 0 0 0 0 15 0 EN . . . Is my model above what you suggest? No. But I did take the time to see what would be necessary to actually model it. And what I ended up with is identical to a) except that the wire stub is replaced by the shorted transmission line model, and the lower wire has become the outside of the coaxial structure so is increased in diameter. So those are the two differences between a) and b). As Tom mentioned and I alluded to, there's some interaction between the wire stub and the antenna which doesn't exist between the ideal transmission line and the antenna, so performance is different. You might as well leave your source open circuited as to connect it to the shorted end of the transmission line stub. The current into one transmission line conductor always equals the current out of the other, so if the two are shorted, no more current can go into or out of the shorted end. Therefore, any external connection to it looks like an open circuit since no current will flow through the external connection. What's a type 6 source (EX 6)? The NEC-2 and NEC-4 documentation I have defines only types 1 - 5. Roy Lewallen, W7EL |
colinear representation in NEC
Hi Owen, I suppose that R.W.P. King disagrees with the "common explanation." He makes it quite clear that there is interaction of the antenna field with the stub perpendicular to the axis of the antenna wire, and that the coaxial stub does not interact in the same way and the antenna performance is therefore different. (Antennas chapter of Transmission Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I like using a feedline to guarantee the phasing. It can be done by driving collinear dipoles with equal lengths of transmission line, or by using an arrangement like the "coaxial collinear," where the radiating elements are outer conductors of coaxial transmission lines used to insure that the multiple feedpoints are at least fed in-phase voltages (and you have to consider that the currents are not exactly in phase). Cheers, Tom In most phased arrays, the objective is to get the fields from the elements to be in some particular ratio. Driving them with currents in that same ratio doesn't always accomplish the desired field ratio, though, when elements have different current distributions as they often do. (See http://eznec.com/Amateur/Articles/Current_Dist.pdf.) The difference between field ratio and feedpoint current ratio is particularly great when base feeding half wave elements. As it turns out, you'll often get better field ratios by feeding with voltages having the desired magnitude ratio and phase difference than feeding with properly ratioed currents, when dealing with end fed half wave elements. The coaxial collinear requires a pretty delicate balance of outer and inner velocity factors as well as the effects of mutual coupling, particularly when there are more than a couple of elements. So I suspect that the current distribution can either help or hinder depending on how the factors are traded off. I wouldn't be surprised, though, if ratioing the voltages rather than currents is actually helpful. As an illustration, open the EZNEC example file Cardioid.EZ. Change the number of segments to 10 per wire for better accuracy. (It can still be run with the demo program.) Click FF Plot and note the nice cardioid pattern. Then change the Z coordinates of End 2 of the two wires to 0.47 m to make them nearly anti-resonant, and click FF Plot again. The pattern deterioration is due to the elements having different current distributions. Finally, change the source types from I to V. This will force the voltages, rather than currents, at the antenna bases to be in the desired ratio. Run FF Plot again. You still won't have the nice cardioid back, but it's quite an improvement over the pattern with "correctly" ratioed base currents. The bottom line is that the element currents are more closely related to the base voltages than the base currents, when the elements are near anti-resonance (parallel, or half wave, resonance). Roy Lewallen, W7EL |
colinear representation in NEC
Roy Lewallen wrote in
treetonline: Owen Duffy wrote: Ok, here is the model I constructed of b) (the coaxial tubes construction). For simplicity, the upper and lower outer tubes are the same diameter, the same wire in this model. CM CE GW 10 1 0 -2 2 0 -2 2.1 0.005 GW 1 47 0 0 0 0 0 15 0.005 GE 1 GN 1 EK EX 6 1 1 1 0 TL 10 1 1 16 50 5 1e+99 1e+99 0.0001 FR 0 0 0 0 15 0 EN . . . Is my model above what you suggest? No. But I did take the time to see what would be necessary to actually model it. And what I ended up with is identical to a) except that the wire stub is replaced by the shorted transmission line model, and the lower wire has become the outside of the coaxial structure so is increased in diameter. So those are the two differences between a) and b). As Tom mentioned and I alluded to, there's some interaction I think that is what I had done, but I used the same diameter top to bottom. Here is a revised deck with different diameters: CM CE GW 10 1 0 -2 2 0 -2 2.1 0.005 GW 1 15 0 0 0 0 0 5 0.015 GW 2 30 0 0 5 0 0 15 0.005 GE 1 GN 1 EK EX 0 1 1 1 0 TL 10 1 2 1 50 5 1e+99 1e+99 0.0001 FR 0 0 0 0 15 0 EN In the above, the lower conductor is three times the diameter of the upper conductor. The TL is wired into the lowest segment of the upper conductor. Again, I have shunted the TL with 10k R to represent loss in a real TL. This model does not show in phase currents in upper and lower parts of the vertical. between the wire stub and the antenna which doesn't exist between the ideal transmission line and the antenna, so performance is different. You might as well leave your source open circuited as to connect it to the shorted end of the transmission line stub. The current into one I don't think I did that. transmission line conductor always equals the current out of the other, so if the two are shorted, no more current can go into or out of the shorted end. Therefore, any external connection to it looks like an open circuit since no current will flow through the external connection. What's a type 6 source (EX 6)? The NEC-2 and NEC-4 documentation I have defines only types 1 - 5. I have been playing with this in EZNEC and 4NEC2. The deck I offered was from 4NEC2 as my EZNEC files are binaries and couldn't go inline. The EX 6 is an extension for a current source. It is immaterial in this case, and the 6 can be changed to a 0. Thanks. Owen |
colinear representation in NEC
On Mar 16, 12:21*am, Roy Lewallen wrote:
Hi Owen, I suppose that R.W.P. King disagrees with the "common explanation." He makes it quite clear that there is interaction of the antenna field with the stub perpendicular to the axis of the antenna wire, and that the coaxial stub does not interact in the same way and the antenna performance is therefore different. *(Antennas chapter of Transmission Lines, Antennas and Wave Guides, King, Mimno and Wing.) *This is why I like using a feedline to guarantee the phasing. *It can be done by driving collinear dipoles with equal lengths of transmission line, or by using an arrangement like the "coaxial collinear," where the radiating elements are outer conductors of coaxial transmission lines used to insure that the multiple feedpoints are at least fed in-phase voltages (and you have to consider that the currents are not exactly in phase). Cheers, Tom In most phased arrays, the objective is to get the fields from the elements to be in some particular ratio. Driving them with currents in that same ratio doesn't always accomplish the desired field ratio, though, when elements have different current distributions as they often do. (Seehttp://eznec.com/Amateur/Articles/Current_Dist.pdf.) The difference between field ratio and feedpoint current ratio is particularly great when base feeding half wave elements. As it turns out, you'll often get better field ratios by feeding with voltages having the desired magnitude ratio and phase difference than feeding with properly ratioed currents, when dealing with end fed half wave elements. The coaxial collinear requires a pretty delicate balance of outer and inner velocity factors as well as the effects of mutual coupling, particularly when there are more than a couple of elements. So I suspect that the current distribution can either help or hinder depending on how the factors are traded off. I wouldn't be surprised, though, if ratioing the voltages rather than currents is actually helpful.. As an illustration, open the EZNEC example file Cardioid.EZ. Change the number of segments to 10 per wire for better accuracy. (It can still be run with the demo program.) Click FF Plot and note the nice cardioid pattern. Then change the Z coordinates of End 2 of the two wires to 0.47 m to make them nearly anti-resonant, and click FF Plot again. The pattern deterioration is due to the elements having different current distributions. Finally, change the source types from I to V. This will force the voltages, rather than currents, at the antenna bases to be in the desired ratio. Run FF Plot again. You still won't have the nice cardioid back, but it's quite an improvement over the pattern with "correctly" ratioed base currents. The bottom line is that the element currents are more closely related to the base voltages than the base currents, when the elements are near anti-resonance (parallel, or half wave, resonance). Roy Lewallen, W7EL Thanks for the clarifications, Roy. Indeed, with my last slightly cryptic comment about considering that currents might not be in phase, I was wanting to communicate that you always want to check the currents on the elements to make sure they do what you want. That's true no matter how you feed the antenna, though as you say the feed you use may aid in insuring that the currents stay the way you want. I'm a bit surprised about your comment about the coaxial (fed) collinear requiring a "pretty delicate balance" between coax propagation velocity and (presumably) radiating element geometry. What I've found in my simulations is that I could change the coax vf, keeping the elements a transmission-line half wave long so that the feedpoints were all the same in-phase voltage, and the net gain of the antenna for a given physical length was only slightly affected. I'd typically see a couple of the elements in a ten element array with considerably lower current magnitude, but the currents were nearly in- phase on all the elements, and the pattern was always the desired "flat pancake". On the other hand, I wasn't trying for any up or down slope to the pattern, and I can see that things might change in that case. With the propagation velocities I was using, between 0.66 and about 0.9, and the element diameters I was using, I suppose the elements were always shorter than resonance, and the self and mutual impedances were not changing in any dramatic fashion. Or, perhaps my model was all screwed up! ;-) Cheers, Tom |
colinear representation in NEC
Owen Duffy wrote:
I think that is what I had done, but I used the same diameter top to bottom. Sorry, my mistake when looking at the source. Your model is just as I described. I apologize for the error. Here is a revised deck with different diameters: CM CE GW 10 1 0 -2 2 0 -2 2.1 0.005 GW 1 15 0 0 0 0 0 5 0.015 GW 2 30 0 0 5 0 0 15 0.005 GE 1 GN 1 EK EX 0 1 1 1 0 TL 10 1 2 1 50 5 1e+99 1e+99 0.0001 FR 0 0 0 0 15 0 EN In the above, the lower conductor is three times the diameter of the upper conductor. The TL is wired into the lowest segment of the upper conductor. Again, I have shunted the TL with 10k R to represent loss in a real TL. This model does not show in phase currents in upper and lower parts of the vertical. I've been running your model without the loss, and I'm seeing currents in the upper and lower wires which are nearly 180 degrees out of phase. between the wire stub and the antenna which doesn't exist between the ideal transmission line and the antenna, so performance is different. For sure -- maximum gain is about 46 degrees above the horizon. You might as well leave your source open circuited as to connect it to the shorted end of the transmission line stub. The current into one I don't think I did that. You're right, you didn't. My mistake. . . . In playing with the model, I noticed something surprising -- length and Z0 of the transmission line have very little effect on the pattern, even over wide ranges (5 to 5000 ohm Z0, lengths from essentially zero to one wavelength). In fact, try removing the transmission line altogether, leaving the wires connected directly together and look at the pattern. Then try changing one wire end slightly to break the connection between them -- again, very little change in the pattern. The fact is that the junction of the two wires is at a point of very little current, so you can connect or disconnect them with almost no change. Likewise, you can insert just about anything (of zero physical size), including an ideal transmission line of any length, without any real effect. So the transmission line stub doesn't really do anything significant at all. What I don't understand yet is exactly why the wire stub does what it does. It sure doesn't work like the simplified explanations imply. Roy Lewallen, W7EL |
colinear representation in NEC
Hi Roy,
Roy Lewallen wrote in treetonline: .... Thanks, all noted. What I don't understand yet is exactly why the wire stub does what it does. It sure doesn't work like the simplified explanations imply. Returning to my diagram a), below is an expansion of the detail at the junction of the stub and vertical sections. | | | | | | | | A B | ---------------------| --------------------| | C | | D | | | | | | | | It strikes me that if we omit the stub all together, and leave a gap in its place, we have two unconnected resonant elements, the top half wave, and the bottom quarter wave with a driving source. The two elements are field coupled to some extent, and currents will setup in each section out of phase. NEC models support this, and I think they are correct in doing so. Returning now to a) with the stub connected and very close to resonance, and with reference to the diagram above, for A, B, C and D very close to the corners, I(A)=I(B) and I(C)=I(D). If the desired outcome of using the stub is that the upper and lower sections are in phase, then I(A)~=I(D). That implies common mode current in the stub, so to cause I(A)~=I(D), the stub must have common mode current (equal to (I(A)+I(D))/2 per conductor). If that is true, then reduction of the physical stub to a pure differential mode TL element is discarding part of what makes it "work". That implies that replacement of the stub with a two terminal equivalent impedance, eg by insertion of a load in an NEC segment, or insertion of one port of a TL network in an NEC segment is an inadequate model. Am I on the wrong track here? Owen |
colinear representation in NEC
On Mar 16, 2:33*pm, Owen Duffy wrote:
Hi Roy, Roy Lewallen wrote ystreetonline: ... Thanks, all noted. What I don't understand yet is exactly why the wire stub does what it does. It sure doesn't work like the simplified explanations imply. Returning to my diagram a), below is an expansion of the detail at the junction of the stub and vertical sections. * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| A * * * * * * * * * * * *B * * | * * * * ---------------------| * * * * *--------------------| * * * * * * * * * * * * * * *| * * * * * * * * * * * *C * * | * * * * * * * * * * * * * * *| D * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| It strikes me that if we omit the stub all together, and leave a gap in its place, we have two unconnected resonant elements, the top half wave, and the bottom quarter wave with a driving source. The two elements are field coupled to some extent, and currents will setup in each section out of phase. NEC models support this, and I think they are correct in doing so. Returning now to a) with the stub connected and very close to resonance, and with reference to the diagram above, for A, B, C and D very close to the corners, I(A)=I(B) and I(C)=I(D). If the desired outcome of using the stub is that the upper and lower sections are in phase, then I(A)~=I(D). That implies common mode current in the stub, so to cause I(A)~=I(D), the stub must have common mode current (equal to (I(A)+I(D))/2 per conductor). If that is true, then reduction of the physical stub to a pure differential mode TL element is discarding part of what makes it "work". That implies that replacement of the stub with a two terminal equivalent impedance, eg by insertion of a load in an NEC segment, or insertion of one port of a TL network in an NEC segment is an inadequate model. Am I on the wrong track here? Owen For what it's worth, I think you're on exactly the right track, Owen. Some things to ponder: does it make any significant difference if the stub is, say, 2mm wires spaced 20mm apart or 1mm wires spaced 10mm apart (that is, the same impedance line, but different physical size), and does it make any significant difference if the wires are kept in a plane that includes the antenna elements, or if they are twisted near their attachment point so they lie in a plane perpendicular to the antenna wire, or if they are twisted throughout their length? What if they are coiled in a spiral out from the antenna wire, so their shorted end lies much closer than a quarter wave from the axis of the antenna? I don't have any answers to these questions; they just seem like an interesting and reasonable extension of your original question. Cheers, Tom |
colinear representation in NEC
On Mar 16, 2:33*pm, Owen Duffy wrote:
Hi Roy, Roy Lewallen wrote ystreetonline: ... Thanks, all noted. What I don't understand yet is exactly why the wire stub does what it does. It sure doesn't work like the simplified explanations imply. Returning to my diagram a), below is an expansion of the detail at the junction of the stub and vertical sections. * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| A * * * * * * * * * * * *B * * | * * * * ---------------------| * * * * *--------------------| * * * * * * * * * * * * * * *| * * * * * * * * * * * *C * * | * * * * * * * * * * * * * * *| D * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| * * * * * * * * * * * * * * *| It strikes me that if we omit the stub all together, and leave a gap in its place, we have two unconnected resonant elements, the top half wave, and the bottom quarter wave with a driving source. The two elements are field coupled to some extent, and currents will setup in each section out of phase. NEC models support this, and I think they are correct in doing so. Returning now to a) with the stub connected and very close to resonance, and with reference to the diagram above, for A, B, C and D very close to the corners, I(A)=I(B) and I(C)=I(D). If the desired outcome of using the stub is that the upper and lower sections are in phase, then I(A)~=I(D). That implies common mode current in the stub, so to cause I(A)~=I(D), the stub must have common mode current (equal to (I(A)+I(D))/2 per conductor). If that is true, then reduction of the physical stub to a pure differential mode TL element is discarding part of what makes it "work". That implies that replacement of the stub with a two terminal equivalent impedance, eg by insertion of a load in an NEC segment, or insertion of one port of a TL network in an NEC segment is an inadequate model. Am I on the wrong track here? Owen I'm sorry...perhaps I don't understand your notation. Don't you expect that the current at A will be (rather roughly) out of phase with the current at D? If I think about a collinear with three half- wave elements end to end, and drive the center of the center element, if it's going to act like I want, I'll have high current near the middle of each element, and those three will be in-phase. Because of the mutual impedances among the elements, things get a bit funny at the ends. I suppose there is a fairly large voltage across the gap between adjacent elements, and therefore there will be moderately high current near those ends to account for the capacitive current in the air between them. That's what I'm seeing in the EZNEC model I just hacked, and it's as I'd expect. The currents near the ends of the central element are considerably higher than the currents near the open ends of the outer elements. (Now to spend a few minutes playing with changing the length of the stubs through resonance...) Cheers, Tom |
colinear representation in NEC
Hi Tom,
K7ITM wrote in : .... I'm sorry...perhaps I don't understand your notation. Don't you I am taking a convention that the sense of currents in segments is from bottom to top. That means that I defined all segments in order from bottom to top. My notation ~= is to mean approximately equal. Does that clarify things? Cheers Owen |
colinear representation in NEC
On Mar 16, 11:39*pm, Owen Duffy wrote:
Hi Tom, K7ITM wrote : ... I'm sorry...perhaps I don't understand your notation. *Don't you I am taking a convention that the sense of currents in segments is from bottom to top. That means that I defined all segments in order from bottom to top. My notation ~= is to mean approximately equal. Does that clarify things? Cheers Owen Yes--and then if they were exactly equal, would that not imply only transmission line current on the stub? Obviously, they are exactly equal if you simply connect the ends of the elements together...but that isn't what gets us to in-phase currents at the centers of each element (in the case of the symmetrical 3 element design; or the base current in the bottom quarter wave in phase with the center current in the top half wave...), and (nearly) equal currents at those current maxima. To the extent that the currents A and D in your diagram differ, there is common-mode or "antenna" current on the stub. Cheers, Tom |
colinear representation in NEC
K7ITM wrote in
: .... Yes--and then if they were exactly equal, would that not imply only transmission line current on the stub? Obviously, they are exactly Thinking some more about it, my current thinking is that my analysis was flawed. I was using the standing wave currents, when I should be using the travelling wave components. I suspect that when NEC models the conductor arrangement at my fig a), it correctly accounts for propagation delay and the phase relationships compute correctly. If we replace the stub with a TL element, I suspect that NEC reduces the TL to a two port network and loads a segment of the vertical with an equivalent steady state impedance of the s/c stub network. If that is done, the reduction to a lumped load means that there is zero delay to travelling waves, and the computed currents (amplitude and phase) in the vertical will be incorrect. This means that you cannot replace a resonant stub with a high value of resistance, it doesn't work. If that is the case, it suggests that NEC cannot model such phasing schemes using TL elements. Owen |
colinear representation in NEC
Owen Duffy wrote:
Thinking some more about it, my current thinking is that my analysis was flawed. I was using the standing wave currents, when I should be using the travelling wave components. That's exactly the flaw committed by w8ji and w7el when they tried to measure the delay through a 75m loading coil using standing wave current which doesn't appreciably change phase through a loading coil or through the entire 90 degree length of a monopole. Using standing wave current, w8ji measured a 3 nS delay through a 10 inch long coil, a VF of 0.27. http://www.w8ji.com/inductor_current_time_delay.htm W7EL reported: "I found that the difference in current between input and output of the inductor was 3.1% in magnitude and with *no measurable phase shift*, despite the short antenna... The result from the second test was a current difference of 5.4%, again with *no measurable phase shift*." Of course, phase shift is not measurable when one is using standing wave current with its almost unchanging phase. EZNEC supports that assertion. Bench measurements support that assertion. When traveling waves are used to measure the delay through a 75m loading coil, the correct delay through w8ji's 10 inch coil turns out to be about 26 nS (~37 degrees) at 4 MHz with a more believable VF of 0.033. http://www.w5dxp.com/current2.htm -- 73, Cecil http://www.w5dxp.com "Government 'help' to business is just as disastrous as government persecution..." Ayn Rand |
colinear representation in NEC
Owen Duffy wrote:
K7ITM wrote in : ... Yes--and then if they were exactly equal, would that not imply only transmission line current on the stub? Obviously, they are exactly Thinking some more about it, my current thinking is that my analysis was flawed. I was using the standing wave currents, when I should be using the travelling wave components. I suspect that when NEC models the conductor arrangement at my fig a), it correctly accounts for propagation delay and the phase relationships compute correctly. If we replace the stub with a TL element, I suspect that NEC reduces the TL to a two port network and loads a segment of the vertical with an equivalent steady state impedance of the s/c stub network. If that is done, the reduction to a lumped load means that there is zero delay to travelling waves, and the computed currents (amplitude and phase) in the vertical will be incorrect. This means that you cannot replace a resonant stub with a high value of resistance, it doesn't work. If that is the case, it suggests that NEC cannot model such phasing schemes using TL elements. Owen Why would NEC reduce a TL two-port to a lumped load? Two-port parameters can handle transmission line problems quite well without the simplifying assumption that all components are of zero length. 73, Tom Donaly, KA6RUH |
colinear representation in NEC
Cecil Moore wrote:
Owen Duffy wrote: Thinking some more about it, my current thinking is that my analysis was flawed. I was using the standing wave currents, when I should be using the travelling wave components. That's exactly the flaw committed by w8ji and w7el when they tried to measure the delay through a 75m loading coil using standing wave current which doesn't appreciably change phase through a loading coil or through the entire 90 degree length of a monopole. Using standing wave current, w8ji measured a 3 nS delay through a 10 inch long coil, a VF of 0.27. http://www.w8ji.com/inductor_current_time_delay.htm W7EL reported: "I found that the difference in current between input and output of the inductor was 3.1% in magnitude and with *no measurable phase shift*, despite the short antenna... The result from the second test was a current difference of 5.4%, again with *no measurable phase shift*." Of course, phase shift is not measurable when one is using standing wave current with its almost unchanging phase. EZNEC supports that assertion. Bench measurements support that assertion. When traveling waves are used to measure the delay through a 75m loading coil, the correct delay through w8ji's 10 inch coil turns out to be about 26 nS (~37 degrees) at 4 MHz with a more believable VF of 0.033. http://www.w5dxp.com/current2.htm Cecil, if I ever have a dead horse on my hands, I won't let you near it because you'll beat it even deader. 73, Tom Donaly, KA6RUH |
colinear representation in NEC
On Mar 17, 1:31*am, Owen Duffy wrote:
K7ITM wrote : ... Yes--and then if they were exactly equal, would that not imply only transmission line current on the stub? *Obviously, they are exactly Thinking some more about it, my current thinking is that my analysis was flawed. I was using the standing wave currents, when I should be using the travelling wave components. I suspect that when NEC models the conductor arrangement at my fig a), it correctly accounts for propagation delay and the phase relationships compute correctly. If we replace the stub with a TL element, I suspect that NEC reduces the TL to a two port network and loads a segment of the vertical with an equivalent steady state impedance of the s/c stub network. If that is done, the reduction to a lumped load means that there is zero delay to travelling waves, and the computed currents (amplitude and phase) in the vertical will be incorrect. This means that you cannot replace a resonant stub with a high value of resistance, it doesn't work. If that is the case, it suggests that NEC cannot model such phasing schemes using TL elements. Owen Of course, if the TL model doesn't "know about" the antenna field (which I believe is in fact the case), there will be no common-mode current on it because of that field. It's pretty clear to me that the common-mode current is very important to correctly simulating the situations you are interested in. In fact, figure (B) of your original posting puts the stub in a position where it does not see the antenna field, and I would expect it to behave much differently from the perpendicular stub of figure (A). One of the things I did in my simulation playing last night was to delete the stubs, leaving just the three 1/2 wave elements end-to-end with a bit of gap between them. (0.01m gap between 0.5m elements, 1mm diameter, 11 segments each.) I'm sure you know what that pattern and current distribution look like. Then I added sources at the centers of the outer elements. I set all the sources to 1 amp, in-phase. The pattern was somewhat sharper (though just marginally more gain) than the stub-coupled case. What I didn't try, but will as I have a chance, is to put sources at the centers of the outer elements and set them to the values (magnitudes and phases) I see in the stub-coupled collinear, and see how much the current distribution near the ends looks like the stub coupled case. I suppose it will be pretty close, and the antenna pattern will look very similar to the stub coupled pattern. Thanks for bringing this subject up. I'm learning something from it. Cheers, Tom |
colinear representation in NEC
"Tom Donaly" wrote in
: .... Why would NEC reduce a TL two-port to a lumped load? Two-port parameters can handle transmission line problems quite well without the simplifying assumption that all components are of zero length. Hi Tom, I expect that NEC does model the propagation delay from end to end on a transmission line. My comment was that NEC reduces a s/c TL stub to a lumped load for the stub input end which is inserted in the vertical. The problem here perhaps is our viewing the phasing section as a s/c stub of two wire line, when perhaps is it better described as a single wire TL of a half wave length. With that thought in mind, I have constructed a model where the phasing section is configured in a double triangular shape, but with the same conductor length, and NEC suggests in-phase currents. In fact, it has slightly better pattern symmetry than a). CM CE GW 1 15 0 0 0 0 0 5 0.0005 GW 5 30 0 0 5.1 0 0 15 0.0005 GW 10 7 0 0 5 1.47 0 5. 0.0005 GW 11 10 1.47 0 5. 0 1.47 5. 0.0005 GW 12 15 0 1.47 5. 0 -1.47 5.1 0.0005 GW 13 7 0 -1.47 5.1 -1.47 0 5.1 0.0005 GW 14 15 -1.47 0 5.1 0 0 5.1 0.0005 GE 1 GN 1 EK EX 0 1 1 0 1.0 0 FR 0 0 0 0 15 0 EN Owen |
colinear representation in NEC
On Mar 17, 12:41*pm, Owen Duffy wrote:
"Tom Donaly" wrote : ... Why would NEC reduce a TL two-port to a lumped load? Two-port parameters can handle transmission line problems quite well without the simplifying assumption that all components are of zero length. Hi Tom, I expect that NEC does model the propagation delay from end to end on a transmission line. My comment was that NEC reduces a s/c TL stub to a lumped load for the stub input end which is inserted in the vertical. The problem here perhaps is our viewing the phasing section as a s/c stub of two wire line, when perhaps is it better described as a single wire TL of a half wave length. .... I'm not sure why you want to reduce it to something less complex than it is. Transmission lines like this support both even and odd mode propagation, I guess what we'd normally call "transmission line currents" and "antenna currents." It seems perfectly OK to me to let both exist on the line at the same time. It also seems to me there is value in doing that, because I believe there's insight to be gained from understanding how each of those currents contributes to the net performance of the antenna. It's important that the stub be in the field of the antenna so that antenna current is excited on it, and it's also important that the stub be shorted a quarter wave away from where it attaches to the collinear elements, so that the differential transmission line currents do the right thing. On the other hand, you may well discover some insights looking at it in a different way, so I hope my comments won't discourage you from doing that! Cheers, Tom |
colinear representation in NEC
K7ITM wrote in
: On Mar 17, 12:41*pm, Owen Duffy wrote: "Tom Donaly" wrote innews:QoQvl.13889$8_3.3071@f lpi147.ffdc.sbc.com: .... On the other hand, you may well discover some insights looking at it in a different way, so I hope my comments won't discourage you from doing that! My real objective is to model b) in NEC. Trying to understand a) and to deconstruct it is part of an approach to finding a solution to b). The 'stub' in a) cannot simply be replaced by a s/c TL element, so that suggests that a s/c TL element is not a solution for b) either. The last configuration with the triangular / diamond configuration of the phasing line seems to work in an NEC model, and the deconstruction suggests that having a half wave of conductor is fundamental, and that it need not be in the form of a two wire TL. I have also tried removing the 'stub' from a) and using a half wave TL to drive segments each side of the gap from each other. If the segments are close to, but not the last, this does produce a current distribution that is not 180° out of phase, but it does not produce the almost perfect in- phase outcome of modelling the wire structure. Nevertheless, playing with the length of that TL, being very close to half wave in length is essential to overriding the natural tendency to out of phase currents. This hasn't solved the problem of modelling a coaxial configuration, expecially where the coaxial section was coax cable, apart from excluding some approaches as invalid. Owen |
colinear representation in NEC
Owen Duffy wrote:
K7ITM wrote in : ... Yes--and then if they were exactly equal, would that not imply only transmission line current on the stub? Obviously, they are exactly Thinking some more about it, my current thinking is that my analysis was flawed. I was using the standing wave currents, when I should be using the travelling wave components. I suspect that when NEC models the conductor arrangement at my fig a), it correctly accounts for propagation delay and the phase relationships compute correctly. If we replace the stub with a TL element, I suspect that NEC reduces the TL to a two port network and loads a segment of the vertical with an equivalent steady state impedance of the s/c stub network. If that is done, the reduction to a lumped load means that there is zero delay to travelling waves, and the computed currents (amplitude and phase) in the vertical will be incorrect. This means that you cannot replace a resonant stub with a high value of resistance, it doesn't work. If that is the case, it suggests that NEC cannot model such phasing schemes using TL elements. Owen It's easy to reason yourself into traps by dividing currents into "standing wave" and "traveling wave" components. They're different things and don't add or superpose. Results of attempts to make this differentiation can be seen in a vast number of postings on this forum in the past. Rather, I recommend considering a current to be a single value or, at most, made of differential and common mode components which *can* be added to obtain the total current. In a steady state single frequency analysis, which is what NEC performs, there is no such thing as delay. All time relationships can be expressed as phase difference, which can't be tied to a unique delay -- you can't even tell if the phase difference was due to time delay or magical prescience-caused time lead. In a steady state analysis there is no way to distinguish a half wave lossless transmission line from a 1-1/2 wave line; they act exactly the same in all ways. So does a magical -1/2 wavelength line whose output appears a half cycle *before* the input appears. Only in a time-domain analysis will you be able to tell the difference. So yes, NEC models the transmission line as a two port network. It does force the correct voltage and current amplitude and phase relationships between the input and output. And it's indistinguishable in the steady state analysis from an ideal transmission line which effects the phase difference by means of delay. The NEC transmission line model is equivalent to a real (but lossless) transmission line on which the current is purely differential, e.g., a coax line with a large number of ferrite cores on the outside. The model is accurate within the constraints of a steady state analysis. If you're interested in looking at the effects of delay in a transient system, you'll need to use an analysis tool other than NEC. But if you let your transient analysis run until steady state is reached, the results will be the same as NEC. Roy Lewallen, W7EL |
colinear representation in NEC
Tom Donaly wrote:
Cecil, if I ever have a dead horse on my hands, I won't let you near it because you'll beat it even deader. The horse is alive and well - the nonsense that I quoted is still on W8JI's web page. -- 73, Cecil http://www.w5dxp.com "Government 'help' to business is just as disastrous as government persecution..." Ayn Rand |
colinear representation in NEC
Roy Lewallen wrote:
If you're interested in looking at the effects of delay in a transient system, you'll need to use an analysis tool other than NEC. But if you let your transient analysis run until steady state is reached, the results will be the same as NEC. But in NEC, if you load a transmission line with its characteristic impedance, reflections are eliminated and the delay along the wire is proportional to the phase shift *even during steady-state*. -- 73, Cecil http://www.w5dxp.com "Government 'help' to business is just as disastrous as government persecution..." Ayn Rand |
colinear representation in NEC
Cecil Moore wrote:
Owen Duffy wrote: Thinking some more about it, my current thinking is that my analysis was flawed. I was using the standing wave currents, when I should be using the travelling wave components. That's exactly the flaw committed by w8ji and w7el when they tried to measure the delay through a 75m loading coil using standing wave current which doesn't appreciably change phase through a loading coil or through the entire 90 degree length of a monopole. Using standing wave current, w8ji measured a 3 nS delay through a 10 inch long coil, a VF of 0.27. http://www.w8ji.com/inductor_current_time_delay.htm W7EL reported: "I found that the difference in current between input and output of the inductor was 3.1% in magnitude and with *no measurable phase shift*, despite the short antenna... The result from the second test was a current difference of 5.4%, again with *no measurable phase shift*." Of course, phase shift is not measurable when one is using standing wave current with its almost unchanging phase. EZNEC supports that assertion. Bench measurements support that assertion. When traveling waves are used to measure the delay through a 75m loading coil, the correct delay through w8ji's 10 inch coil turns out to be about 26 nS (~37 degrees) at 4 MHz with a more believable VF of 0.033. http://www.w5dxp.com/current2.htm I agree that electromagnetic traveling waves are the kinds of waves that propagate on and cause radiation to emanate from an antenna. But your claims about 'standing waves not changing phase along the antenna' provoke the following questions: 1.) what relation (if any) do you believe the wavelength of the standing wave has to the wavelength of the radio frequency waves traveling on an antenna? And, 2.) what relation (if any) does the phase of a sinusoidal wave have to its amplitude? 73, ac6xg |
colinear representation in NEC
Jim Kelley wrote:
I agree that electromagnetic traveling waves are the kinds of waves that propagate on and cause radiation to emanate from an antenna. But your claims about 'standing waves not changing phase along the antenna' ... Jim, I thought you have EZNEC. Here are the currents at all of the segments along a 20m dipole with 21 segments from end to end. Please note that in a dipole that is 180 degrees long, the phase of the (mostly standing-wave) current varies by less than 3 degrees. How can the current in a 180 degree antenna vary by less than 3 degrees? Quoting my web page: "Standing wave current cannot be used to directly measure either a valid amplitude change or a valid phase shift through a loading coil. All of the reported conclusions based on loading coil measurements using standing-wave current on standing-wave antennas are conceptually flawed." Owen had an epiphany of a sort when he realized that fact of physics. 20m dipole 3/18/2009 5:28:50 PM --------------- CURRENT DATA --------------- Frequency = 14.2 MHz Wire No. 1: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open .0836 -2.75 2 .23595 -2.57 3 .37707 -2.38 4 .50791 -2.17 5 .62692 -1.95 6 .73226 -1.71 7 .82218 -1.44 8 .89511 -1.13 9 .94979 -0.78 10 .98539 -0.37 11 1 0.00 12 .98539 -0.37 13 .94979 -0.78 14 .89511 -1.13 15 .82218 -1.44 16 .73226 -1.71 17 .62691 -1.95 18 .50791 -2.17 19 .37707 -2.38 20 .23595 -2.57 21 Open .0836 -2.75 -- 73, Cecil http://www.w5dxp.com "Government 'help' to business is just as disastrous as government persecution..." Ayn Rand P.S. I posted this reply but it didn't show up on my server. I apologize if it is a duplicate. |
colinear representation in NEC
Cecil Moore wrote:
Jim Kelley wrote: I agree that electromagnetic traveling waves are the kinds of waves that propagate on and cause radiation to emanate from an antenna. But your claims about 'standing waves not changing phase along the antenna' ... Jim, I thought you have EZNEC. Here are the currents at all of the segments along a 20m dipole with 21 segments from end to end. Please note that in a dipole that is 180 degrees long, the phase of the (mostly standing-wave) current varies by less than 3 degrees. How can the current in a 180 degree antenna vary by less than 3 degrees? It seems to me that computers are completely stupid about certain things. Could it be a case of garbage in, garbage out? Quoting my web page: "Standing wave current cannot be used to directly measure either a valid amplitude change or a valid phase shift through a loading coil. All of the reported conclusions based on loading coil measurements using standing-wave current on standing-wave antennas are conceptually flawed." And what more authoritative reference could someone cite than their own web page? :-) I've never actually known what it was that made you believe Roy had measured standing wave current - whatever that means. Or, how his measurements compare with your own measurements of the phenomenon. Owen had an epiphany of a sort when he realized that fact of physics. It may not even be as elusive a fact as one is given to believe around here. 73, ac6xg 20m dipole 3/18/2009 5:28:50 PM --------------- CURRENT DATA --------------- Frequency = 14.2 MHz Wire No. 1: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open .0836 -2.75 2 .23595 -2.57 3 .37707 -2.38 4 .50791 -2.17 5 .62692 -1.95 6 .73226 -1.71 7 .82218 -1.44 8 .89511 -1.13 9 .94979 -0.78 10 .98539 -0.37 11 1 0.00 12 .98539 -0.37 13 .94979 -0.78 14 .89511 -1.13 15 .82218 -1.44 16 .73226 -1.71 17 .62691 -1.95 18 .50791 -2.17 19 .37707 -2.38 20 .23595 -2.57 21 Open .0836 -2.75 |
colinear representation in NEC
Jim Kelley wrote:
I've never actually known what it was that made you believe Roy had measured standing wave current - whatever that means. Good Grief! Could it be that a monopole is a "STANDING WAVE ANTENNA"? -- 73, Cecil http://www.w5dxp.com "Government 'help' to business is just as disastrous as government persecution..." Ayn Rand |
colinear representation in NEC
Cecil Moore wrote:
Could it be that a monopole is a "STANDING WAVE ANTENNA"? Here's an EZNEC simulation of a 1/4WL monopole. It is a 1/4WL stub with the wire resistivity adjusted to simulate monopole radiation. The standing wave current distribution (lack of phase) and feedpoint resistance are similar to a monopole. http://www.w5dxp.com/stub_dip.EZ Add a short at the top and a load of 600 ohms in the shorted segment and observe the traveling wave. http://www.w5dxp.com/stubsht.EZ Turn on the current phase display and observe the traveling wave phase shift. -- 73, Cecil http://www.w5dxp.com "Government 'help' to business is just as disastrous as government persecution..." Ayn Rand |
colinear representation in NEC
Cecil Moore wrote:
Could it be that a monopole is a "STANDING WAVE ANTENNA"? The supposition is true, so the intended implication must be that only standing wave current can be measured on monopole antennas. And Roy therefore would have to have measured standing wave current (whatever that is). I must decline to agree. :-) 73, ac6xg |
colinear representation in NEC
On Fri, 20 Mar 2009 17:35:02 -0800, Jim Kelley
wrote: I must decline to agree. :-) Couldn't you incline to disagree? 73's Richard Clark, KB7QHC |
colinear representation in NEC
Jim Kelley wrote:
Cecil Moore wrote: Could it be that a monopole is a "STANDING WAVE ANTENNA"? The supposition is true, so the intended implication must be that only standing wave current can be measured on monopole antennas. And Roy therefore would have to have measured standing wave current (whatever that is). I must decline to agree. :-) About 90% of the total current on an open-ended 1/4WL monopole is standing wave current with close to unchanging phase. That's why a 1/4WL monopole is called a "standing wave antenna". That is the current that Roy and Tom used so the component traveling wave, accounting for about 10% of the total current where the phase shift actually occurs, was mostly ignored and swamped by the huge component standing wave. This is such a simple concept - I don't see the problem in understanding that a wave with the following equation doesn't change phase with position (x). The phase is the same over 90 degrees of length no matter what fixed x and fixed t are chosen. EZNEC supports that fact of physics. Here's the standing wave equation from "Optics", by Hecht: E(x,t) = 2E01*sin(kx)*cos(wt) quoting "Optics", by Hecht: "[Standing wave phase] "doesn't rotate at all, and the resultant wave it represents doesn't progress through space - its a standing wave." Another interesting thing about the standing wave equation is that the sign of (wt) can be reversed, i.e. standing waves don't move in either direction - they just stand there. EM waves cannot stand still so "EM standing wave" is an oxymoron. Quoting one of my college textbooks, "Electrical Communication", by Albert: "Such a plot of voltage is usually referred to as a *voltage standing wave* or as a *stationary wave*. Neither of these terms is particularly descriptive of the phenomenon. A plot of effective values of voltage, appearing as in Fig. 6(e), *is not a wave* in the usual sense. However, the term "standing wave" is in widespread use." From "College Physics", by Bueche and Hecht: "These ... patterns are called *standing waves*, as compared to the propagating waves considered above. They might better not be called waves at all, since they do not transport energy and momentum." One can use EZNEC's VERT1.EZ to view the essentially unchanging phase on a standing wave monopole. Just look at the difference in phase between the feedpoint and a point 45 degrees up the antenna. In 45 degrees of antenna, the current phase changes by 3.65 degrees. That is the current Roy used to measure phase shift through a coil in order to support w8ji's 3 nS delay "measurements". -- 73, Cecil http://www.w5dxp.com "Government 'help' to business is just as disastrous as government persecution..." Ayn Rand |
colinear representation in NEC
Owen Duffy wrote:
"Tom Donaly" wrote in : ... Why would NEC reduce a TL two-port to a lumped load? Two-port parameters can handle transmission line problems quite well without the simplifying assumption that all components are of zero length. Hi Tom, I expect that NEC does model the propagation delay from end to end on a transmission line. My comment was that NEC reduces a s/c TL stub to a lumped load for the stub input end which is inserted in the vertical. No it doesn't do prop delay. It does a steady state model. The TL is just another two port that gets dumped into a giant matrix which is solved as a system of linear equations. Think of TL as a special case of NT. |
colinear representation in NEC
Jim Lux wrote:
No it doesn't do prop delay. The prop delay is easily calculated by loading the TL with Rload=Z0 and observing the resulting traveling wave phase shift while taking VF into account. In the same manner, the prop delay through a loading coil can be calculated. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com "Government 'help' to business is just as disastrous as government persecution..." Ayn Rand |
colinear representation in NEC
Cecil Moore wrote:
Jim Lux wrote: No it doesn't do prop delay. The prop delay is easily calculated by loading the TL with Rload=Z0 and observing the resulting traveling wave phase shift while taking VF into account. In the same manner, the prop delay through a loading coil can be calculated. What's the Z0 of a loading coil, Cecil? 73, Tom Donaly, KA6RUH |
colinear representation in NEC
Jim Lux wrote:
Owen Duffy wrote: "Tom Donaly" wrote in : ... Why would NEC reduce a TL two-port to a lumped load? Two-port parameters can handle transmission line problems quite well without the simplifying assumption that all components are of zero length. Hi Tom, I expect that NEC does model the propagation delay from end to end on a transmission line. My comment was that NEC reduces a s/c TL stub to a lumped load for the stub input end which is inserted in the vertical. No it doesn't do prop delay. It does a steady state model. The TL is just another two port that gets dumped into a giant matrix which is solved as a system of linear equations. Think of TL as a special case of NT. What kind of two port does NEC use, Jim? What is "just another two port?" 73, Tom Donaly, KA6RUH |
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