| Page 2 of 2 | 1 | 2 |
|
|
|
wrote:
Look at Roy, he admitted he knows nothing about the subject which when he next argues with the like of Cecil and others I will now have to think twice instead of accepting his typical riposte that he supplies. In the immortal words of the warden from "Cool Hand Luke", "Whut we haave heah is ahh faiyuah tuh cahmmunicate." I don't think it's anything personal. We-all just don't speak the same language. Ah think Ah'm gonna amble over yonder directly. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Lewallen" wrote in message
... wrote: Oh come on Wes look at your last posting where you poked fun at the idea of a polygon phasor array. And look at the other postings where it was obvious that many were not familiar with the same and needed more direction. Look at Roy, he admitted he knows nothing about the subject. . . What subject is that? Phased arrays? Phasors? Vectors? Polygons? I made no admission of the kind! Can you quote what I said and when I said it? Roy you have no further to look than this thread and it appeared un der your name I learned about vectors in high school physics class, and phasors on my own while in high school over 40 years ago now. Both were, of course, nearly daily fare throughout my college EE curriculum, and frequently used during my career as an electronics design engineer. As for phased arrays, have you ever looked at Chapter 8 of the ARRL Antenna Book (of the last 15 years or so), or my article in Vol. 2 of the ARRL Antenna Compendium (1989), "The Simplest Phased Array Feed System -- That Works"? The techniques I describe in both those publications do, incidentally, work as claimed, and have been shown to do so many times. Yes I am aware of that and I expressed amazement at your lack of knoweledge as expressed in this thread. At the same time I see you are referring to stuff that you wrote when you were younger and things change as you get older. A case in point is the ELZEC program which frankly does not match up with todays technology or competitive programs yet maintains a high price presumably based on your past achievements. But when you express your knoweledge as you did this week and showed complete lack of knoweledge regarding the subject at hand then it may well be a sign of the times as it were and you resorted to attack, not the underpinnings of what I stated but me as an individual. If you were a profesional you would have attacked the polygon example given at the onset of this thread but then you expressed lack of knoweledge of the subject and I commended you for that but the passing of time places no mercy on any of us mortals as we age despite our personally perceived station in life. Or do you mean I know nothing about the subject of voodoo science? If so, I'm guilty as charged. If the subject of vector arrays is voodoo then I agree you are guilty as charged Its a real shame that you are taking this attitude but if all you want is a piece of me then I am not going to go away Regards Art......KB9MZ......XG Roy Lewallen, W7EL |
Yes I have those reference books but I do not have access to IRE procedings.
My quest was not an easy one and I reflected long as to why the NEC model did not reflect an absolute zero Front to back. On reflection I realised that a straight element in an array is not necessarily the most efficient radiator. Then you have the position that a deformed radiator must have a definite coupling on other elements as shown by Moxon to have a resistive impedance, he used the bending of elements to pursue this. And there are other things to be concerned about such as element diameter change as we move away from the center as well as the element structure that is tubular and not solid which would portray a different aproach with respect to skin resistance. One thing I did look at was the difference in F/B when I went for maximum gain and the change that occured when I went for maximum F/B and I was surprised to see the F/R increase at a large rate and reach its maximum of more than 50 percent improvement ( actually 100 % improvement for the low TOA )at the loss of less than one half db loss in gain because the range of maximum gain was reduced. In retrospect this is not surprising as the frontal lobe became larger in diameter at a lesser percentage rate of what was taken from the rear Hopefully the weather will change soon so I can see exactly what is happening with a full scale array Regards Art "Jerry Martes" wrote in message news:Mf0Yd.43497$uc.34067@trnddc01... Art If your question is "is there any written work that pertains to how gain and sidelobes are related", the answer is Yes. I dont know where back issues of the IRE Proceedings can be found. But, the Proceedings of the Professional Group on Antennas will have so much information on current distribution on a planer array that you may not have enough time left to read it all. The current distribution across an antenna aperature has been studdied very seriously. I am not qualified to discuss phased arrays. I am convinced that max gain will not be acheived with the same current distribution as for minimum side lobes. I realize that you write only "back lobes". But, thats a side lobe at that special angle I am rather simple minded when it comes to phased arrays. I use Referance Data For Radio Engineers as a referance book. It has alot of information on phased arrays. I suspect all the information I have has already been concidered by you. Jerry " wrote in message news:2w_Xd.52445$Ze3.8223@attbi_s51... O.K..O..K Seems like everybody has forgotton the basics of the polygon of forces and other uses of vector so I will go over the basics. At the age of 14 yearsI entered the School of Engineering and Navigation where they hashed things from first principles, Since I had little schooling during those war years it gave me an accelerated course on what I had missed during those schoolless years which meant a lot of homework and I had to work like hell. From the name of the school it was evident that I would get a quick introduction on vectors for forces and navigation . This went as follows: When you swim across a swimming pool then you can swim point to point. If you swim across a river and tried to swim point to point you finish up on the other side but down, stream thus to get to the original point of the endeavor you must swim upstream. If you are a ship or a plane it is obvious that you must have enought fuel to get from point to point so this becomes very important. Thus going back to the river swim you can draw a vector or line that follows the path you took first to cross the river. Since you have units such as time and distance you can draw that line in scalar form. Then you add on to the tail end of the line the journey upstream again in scalar form which will be something less than a 90 degree angle. If you then look back at the point that you started from it becomes obvious that when you swim across stream the angle you must follow is the angle which is shown from the beginning of the triangle to the point that you finished up. Next time you are on a plane look downwards and pick up the flight pattern of small private planes and you will see that their flight path is different from the angle projected by the fuselage All this is in accordance with Newton's law that 'every action has an equal and opposite reaction.' Now look below at my original post to what I said and you will see that I applied a scalar drawing that consisted of many scalae directions in the same way a sailing ship would do if it had to keep changing direction to get to shore. The first vector drawn for an element with known phase and current was drawn which happened to be a vertical line of known length. The next line was then added at the end to reflect the current and phase of the next element chosen and then onto the next element chosen. But this element presented a phase and current that was equal and opposite to the one previously drawn which meant that I was back to the tail end of my first vector chosen ! Thes two elements are termed destructive In fact this happened several times where vectors cancelled each other so we are just left with a singe vector in our scalar drawing .This meant to get back to the point of origin and remembering Newton's law previously alluded to the scalar drawing it represents a vector that is equal to the starting vector drawn, THE SAME PHASE and same CURRENT. Thus the polygon reflects an array where the phase is constant but the currents are ADDITIVE This represents the radiation pattern of a figure eight EXCEPT all the radiation is now to one side of the feed point and comprising of a single and larger circle. All of this reflects exactly what I stated below except I assumed that the pologon phase drawing was already known to all, for which I apologise. With NEC I constructed a model that closely followed this format though the real world did not make elements exactly equal but when I rehashed in my mind the basic priciples the polygon aproach verified that this aproach does give extraordinary front to back/rear figures that gave rise to mistrust of the softwear being used where you may remember that I commented on a model that I made and where the response was that the f/b was to high a point that had troubled me for many a month. Sorry for the long winded response which reflects what I have gone thru with my postings which apparently projected me as a total fool that gave rise to dirisive comments. Now I ask again, is there any written work that pertains to max gain and f to b/rear being on the same frequency? Best regards to all, no hard feelings Art KB9MZ................XG " wrote in message news:dySVd.30807$r55.174@attbi_s52... I have just come to realise that if one drew a polygon of element phases in a array and all elements were 180 degrees to its companion element and excluding the driven element, the max gain and max front to back will occur at the SAME frequency! Until now I was of the understanding that these two max figures could not occur at the same frequency. Is there anything written about this possibility? Regards Art |
On Mon, 07 Mar 2005 01:02:18 GMT, "
wrote: It would appear that I have come across something new. "Kraus forgive them, For they know not where they've polygon wrong" 73's Richard Clark, KB7QHC |
Richard Clark wrote: On Mon, 07 Mar 2005 01:02:18 GMT, " wrote: It would appear that I have come across something new. "Kraus forgive them, For they know not where they've polygon wrong" You said it...This is hilarious...Better than the comedy channel.. Poor ole Art just doesn't get it...At all... It wouldn't matter what we said, if it does not align with his thinking, we are all ignorant, or we are trying to crucify him to a rohn 45 tower...It doesn't matter that the problem has nothing to do with polygons, phased arrays, or anything else under the sun. The problem is he wants to keep his "device" a secret, so he can claim it a new invention. So he's afraid to give any details at all. In doing this, he fails to realize no one has a clue to what he is trying to describe 92% of the time...Myself, I think this is just another round the world attempt to justify the validity of that Rube Goldburg looking antenna he has been trying to plug since whenever....Years it would seem... MK |
|
O.K. Jim you have my respect so I will go over things again.
I commented that I always thought that it was impossible to have max gain at the same frequency that one obtained max F/B F/R. Presumably you read that. I then stated a theoretical situation where elements worked in pairs but equal and opposite except the driven element You read that I assume So to draw a "polygon" of the array element phases and current we start with the driven element, a vertical line of scalar length and then move on to add a scaler length and phase to the end of the director "length. Hopefully you are still with me But remember I stated that all other elements were equal and opposite in a twosome form to another element, thus even tho we we have gone thru the normal routine we still arrive at the end of the driven element scalar line. Hopefully you are still with me So to close the scalar diagram we have a line that represents an element that is in phase with the director i.e additive. This diagram does not show that the element pairs are doing nothing and therefore of no use, those elements still radiate but they oppose each other with the final result that to close the diagram an element is required that has the same phase and magnitude as the fed element Still with me I hope With a single dipole over ground we get a figure 8 radiation pattern but we have just shown how an array can be theoreticaly formed that results in a unidirectional form where one part of the figure 8 pattern has been cancelled and at the same time we have two radiation patterns on the same side of the feed point in additive fashion in the form of a perfect circle which is larger than either of the circles formed in the figure 8 pattern. Yes a lot of steps in this thought process but stick with me Looking at the final large circle we can say that the demise of the rear pattern equates with maximum gain and where the lobe width has become larger instead of the normal narrowing effect that we get with a Yagi Still with me? We can also see that using such an array can avoid the manufacture of side lobes whether they are frontal or otherwise as our "Polygon" is symetrical where one made for a yagi is not such that errant reflective rays are created. To wrap things up: the thread was created because I had created such an array using NEC with 300 segments per half wave which produced awesome front to rear figures which some readers questioned the feasability. I also questioned the results of the model for many months ,UNTIL I came up with the cited analogy The model matched the analogy tho the pairs of elements were not exactly equal but very close and the resulting pattern matched the analogy in that it became LARGER. With two strikes in agreement I then sort for a final crunch mode and that was to make what is a mechanical difficult array to build as well as expensive for something that still had lingering questions. The winter has been harsh but with a little time spent each day during the last six months I now need one half day of good weather without wind to place this new fangled array at the tower top and without the need of the heavy rotor ( prop pitch) which was needed for its equivalent long boom yagi.( I do this without help and I am not as strong as I used to be now that I am past 70) I don't understand the derisve comments regarding my beliefs and the ensueing experimentation and building except there is a prevailing thought in the U.S. that it is impossible to discover anything new as every thing possible was already known, but else where in the world the average ham still experiments to pursue new knoweledge. The world is really seen as out of step with the U.S. in more that one way and this thread portrays just one more thing to add to the list. Regards Art...... KB9MZ,,,,,, XG \ I then illustrated where such an array could be drawn polygon fashion You read that I assume "Jim Kelley" wrote in message ... wrote: Yes I am aware of that and I expressed amazement at your lack of knoweledge as expressed in this thread. At the same time I see you are referring to stuff that you wrote when you were younger and things change as you get older. A case in point is the ELZEC program which frankly does not match up with todays technology or competitive programs yet maintains a high price presumably based on your past achievements. But when you express your knoweledge as you did this week and showed complete lack of knoweledge regarding the subject at hand then it may well be a sign of the times as it were and you resorted to attack, not the underpinnings of what I stated but me as an individual. If you were a profesional you would have attacked the polygon example given at the onset of this thread but then you expressed lack of knoweledge of the subject and I commended you for that but the passing of time places no mercy on any of us mortals as we age despite our personally perceived station in life. What you described would be at best be described in contemporary venacular as a polyline. As has been explained, a polygon is a closed shape. A polyline could form a polygon in the special case where the phase angles sum to 360 and the line segments are of proper length and in a particular order. I think most of us here know that Roy of all people has no trouble at all with such a concept. I also have been following the thread, and what you didn't describe is how any of that relates to the antenna question you asked. Hence the mix-up. 73, ac6xg |
On Thu, 10 Mar 2005 22:56:37 GMT, "
wrote: Oh come on Wes look at your last posting where you poked fun at the idea of a polygon phasor array. And look at the other postings where it was obvious that many were not familiar with the same and needed more direction. Look at Roy, he admitted he knows nothing about the subject He did no such thing. Not only are you having difficulty expressing yourself, you have similar difficulty understanding what others are trying to tell you. I'm not trying to be cruel or harsh, but that's just the way it is. which when he next argues with the like of Cecil and others I will now have to think twice instead of accepting his typical riposte that he supplies. But I give Roy credit for being honest in the face of personal embarassment regarding his lack of knoweledge You say it was not necesary to provide a long convoluted pseudo treatise on vectors but many asked for it and you made a joke of the idea, Regarding front to rear occuring at the same frequency. An operator wants as much gain as possible when communicating so he does not need to resort to more power than needed. For best communication it is nice to block of interference to the rear and thus he needs best front to rear at the frequency of communication even tho it is of interest that he had better rejection at a lower frequency. The fact of the matter is that it is not the frequency being used, he has to live with a lesser value of rejection, your opinion may well be different. So if I understand you correctly ( a *really* dubious proposition) I would have much better success with my 20 meter antenna if I embraced your philosophy. My current antenna is of my design, a three-element monoband Yagi-Uda parasitic array. You can see it in the picture on qrz.com. It is an honest to goodness actual antenna. I have 310 countries confirmed on 20 meters most of them (The hard ones) worked with this antenna. All at the "too-low" height (according to you) of 45' above ground. I would be delighted to send you an EZNEC, Multinec, or NEC file that describes the antenna. The model accounts for boom to mast connection, element taper, etc. (per Leeson, "Physical Design of Yagi Antennas."), includes the stub matching feed system and appears to accurately describe the antenna to the best of my limited capability to measure it. Over the band of interest, 14.0 to 14.25 MHz., the modeled free-space FB exceeds 20 dB and the gain varies from ~ 7.9 to 8.15 dBi. The FB peaks at ~ 14.12 MHz and the gain is maximum at 8.15 dBi at 14.25 MHz. Pray tell, what operational advantage am I giving up because the gain at 14.12 MHz (the FB peak) is *only* about 8.0 dBi instead of 8.15 dBi? Now you also remarked that you do not want explanations, just the meat. I gave what you call a "treatise" that explained the theoretical underpinnings of what I have stated. It would be unwise at this point to declare success without not only having a NEC model to confirm it but also a 20 meter antenna and not say a 144 meg equivalent. Today we had snow, wind and rain so I could not complete the job.If by chance the antenna gives a third aproval i.e.Nec model then polygon discussion plus the antenna then I will forward it to RADCOM for peer review. It is at that time you can vent your displeasure that you rejected my offer to share the actual mathematical and physical findings. If you were looking for a way to undermine what I had stated then my " treatise" now arms you with the knoweledge to disprove what I have stated as it is one factor that convinces me of my origonal findings. If you need more information regarding vectors I will be happy to aid you in your quest No, I don't needed any more of your help with vectors. I wish you every success with your RADCOM paper. |
Wes you are off on a tangent again. Go back the the initial posting on this
thread. It says nothing about your poor antenna or even my antenna as I do not have one. It refers to gain and front to rear at the same frequency It does not refer to an actual physical antenna. None of your postings refer in any sense can be seen as a technical response that directly refutes the accuracy of what I stated. In your responces I see nothing but snide remarkes or deviation from the posed subject by introducing your personal antenna exploits with your own particular antenna which I say is at an incorrect height. There is nothing in the initiating post that refers to an actual antenna or even ones that are at an incorrect height. The true facts behind that initial posting that I had a NEC derived model with extra ordinary back to front figures which made other people suspicious including me when I posted that info on another thread. .. I was looking for answers to my findings. This post consisted solely of presenting a theoretical analysis that reflected the possibility of max gain and max front to rear could appear at the same frequency. Nobody even tried to challenge the analogy becauuse everybody believes all is known about antennas so there was no point in even to attempt to understand the given analogy. Instead snide remarks were made and diversionary tactics in changing the subject to actual antennas such as yours. The subject and question of this thread was made in the hope that technically the analogy made was incorrect and thus allows me personally to discount the accurracy of my modelling or on the other hand state they can find no fault with my analogy. What did I get ? Knee jerk reaction and mirth at the possibility that something new had possibly been found when all had taken the position that it is safe to debunk because every thing is known. Not one person itemised a particular line that gave them particular problems electing to pursue different issues that they have. and not to respond to the initial request such as you bring up your own personal anternna. And remember, in an effort to circumvent this "roast" aproach I offered to share everything I had with you but you preferred to continue with The "roast " aproach. Your antenna and your personal achievements are the same as what Roy did when he introduced his past achievements which again does nothing to vote pro or con to my theoretical analogy in a technical way. Don't you realise that other silent viewers around the world are wondering why the few are having difficulty with the question. I sure hope that there are many hams reading how the so called gurus are dismissing everything other say. Is this the new way America feels about those outside of these borders? Regards Art....KB9MZ.......XG "Wes Stewart" wrote in message ... On Thu, 10 Mar 2005 22:56:37 GMT, " wrote: Oh come on Wes look at your last posting where you poked fun at the idea of a polygon phasor array. And look at the other postings where it was obvious that many were not familiar with the same and needed more direction. Look at Roy, he admitted he knows nothing about the subject He did no such thing. Not only are you having difficulty expressing yourself, you have similar difficulty understanding what others are trying to tell you. I'm not trying to be cruel or harsh, but that's just the way it is. which when he next argues with the like of Cecil and others I will now have to think twice instead of accepting his typical riposte that he supplies. But I give Roy credit for being honest in the face of personal embarassment regarding his lack of knoweledge You say it was not necesary to provide a long convoluted pseudo treatise on vectors but many asked for it and you made a joke of the idea, Regarding front to rear occuring at the same frequency. An operator wants as much gain as possible when communicating so he does not need to resort to more power than needed. For best communication it is nice to block of interference to the rear and thus he needs best front to rear at the frequency of communication even tho it is of interest that he had better rejection at a lower frequency. The fact of the matter is that it is not the frequency being used, he has to live with a lesser value of rejection, your opinion may well be different. So if I understand you correctly ( a *really* dubious proposition) I would have much better success with my 20 meter antenna if I embraced your philosophy. My current antenna is of my design, a three-element monoband Yagi-Uda parasitic array. You can see it in the picture on qrz.com. It is an honest to goodness actual antenna. I have 310 countries confirmed on 20 meters most of them (The hard ones) worked with this antenna. All at the "too-low" height (according to you) of 45' above ground. I would be delighted to send you an EZNEC, Multinec, or NEC file that describes the antenna. The model accounts for boom to mast connection, element taper, etc. (per Leeson, "Physical Design of Yagi Antennas."), includes the stub matching feed system and appears to accurately describe the antenna to the best of my limited capability to measure it. Over the band of interest, 14.0 to 14.25 MHz., the modeled free-space FB exceeds 20 dB and the gain varies from ~ 7.9 to 8.15 dBi. The FB peaks at ~ 14.12 MHz and the gain is maximum at 8.15 dBi at 14.25 MHz. Pray tell, what operational advantage am I giving up because the gain at 14.12 MHz (the FB peak) is *only* about 8.0 dBi instead of 8.15 dBi? Now you also remarked that you do not want explanations, just the meat. I gave what you call a "treatise" that explained the theoretical underpinnings of what I have stated. It would be unwise at this point to declare success without not only having a NEC model to confirm it but also a 20 meter antenna and not say a 144 meg equivalent. Today we had snow, wind and rain so I could not complete the job.If by chance the antenna gives a third aproval i.e.Nec model then polygon discussion plus the antenna then I will forward it to RADCOM for peer review. It is at that time you can vent your displeasure that you rejected my offer to share the actual mathematical and physical findings. If you were looking for a way to undermine what I had stated then my " treatise" now arms you with the knoweledge to disprove what I have stated as it is one factor that convinces me of my origonal findings. If you need more information regarding vectors I will be happy to aid you in your quest No, I don't needed any more of your help with vectors. I wish you every success with your RADCOM paper. |
Art Unwin wrote:
"I have just come to realise that if one drew a polygon of element phases in an array and all elements were 180 degrees to its companion element and excluding the driven elemment, the max gain and max front to back will occur at the same frequency!" Would an antenna made entirely of pairs of identical out of phase elements be a good antenna? A "polygon of element phases" must refer to the resultant current in each element and their combined effect at a point (P) for example in the far field. Art must have resolved and composed vectors or phasors at some time. The resultant of any number of vectors can all add to zero or to some other number and direction. A zero sum often happens in physics when systems are in equiblirium. Newton said that any action results in an equal and opposite reaction. Application of a new force often causes no loss in equilibrium, just a corresponding added reaction. The reflected wave from an antenna may change in magnitude in proportion to an incidebt wave yet be nearly exactly equal in magnitude and 180-degrees out of phase with the incident wave, if the reflection is perfect. A polygon is a closed plane bounded by straight sides. It can represenht forces. Art asked if there were anything written about complete front to back cancellation in two radiators carrying oppositely directed signals if I understood the question. Indeed Kraus of W8JK fame has a lot to say about the possibility. Kraus writes about an "Array of Two Driven 1/2-wavelength El;ements. General Case with Equal Currents of Any Phase Relation." in his 1950 edition of "Antennas". It includes on page 294, field patterns for physical spacings and feed phasings. For example, at a spacing of 1/8-wavelength and a phasing of 135-degrees, there is complete cancellation in one direction while there is maximum radiation in the opposite direction. That`s the good news. Now the bad. On page 297 Kraus says: "However, in the flat-top (an advantage placing all elements at maximum height) antenna such losses may have considerable effect on the gain (as the feedpoint resistance is very low). Therefore, the question of losses and of radiating efficiency will be treated in this section in connection with a discussion of arrays of two closely spaced, out-of-phase elements. The term "closely-spaced" will be taken to mean that the elements are spaced 1/4 wavelength or less." Then, Kraus shows another fly in the ointment on page 300: "Hence the Q for 1/8 wavelength spacing is about four times the Q for 1/4 wavelength spacing. Very large Q indicates a large amount of stored energy near the antenna in proportion to the energy radiated per cycle. This also means that the antenna acts like a sharply tuned circuit." So much for bandwidth! Best regards, Richard Harrison, KB5WZI |
Richard
So the principle is good tho in Kraus case he used two feeds instead of one. Doing it his way with just 2 elements makes it very sensirive to frequency which obviously not a good thing as you point out. Your post does have my interest as I would like to see what gain he arrived at by placig all radiation to the front. In my case the gain just fell short of 16dbi I will look it up in the antenna books, thanks for pointing that one out Best regards Art KB9MZ......XG "Richard Harrison" wrote in message ... Art Unwin wrote: "I have just come to realise that if one drew a polygon of element phases in an array and all elements were 180 degrees to its companion element and excluding the driven elemment, the max gain and max front to back will occur at the same frequency!" Would an antenna made entirely of pairs of identical out of phase elements be a good antenna? A "polygon of element phases" must refer to the resultant current in each element and their combined effect at a point (P) for example in the far field. Art must have resolved and composed vectors or phasors at some time. The resultant of any number of vectors can all add to zero or to some other number and direction. A zero sum often happens in physics when systems are in equiblirium. Newton said that any action results in an equal and opposite reaction. Application of a new force often causes no loss in equilibrium, just a corresponding added reaction. The reflected wave from an antenna may change in magnitude in proportion to an incidebt wave yet be nearly exactly equal in magnitude and 180-degrees out of phase with the incident wave, if the reflection is perfect. A polygon is a closed plane bounded by straight sides. It can represenht forces. Art asked if there were anything written about complete front to back cancellation in two radiators carrying oppositely directed signals if I understood the question. Indeed Kraus of W8JK fame has a lot to say about the possibility. Kraus writes about an "Array of Two Driven 1/2-wavelength El;ements. General Case with Equal Currents of Any Phase Relation." in his 1950 edition of "Antennas". It includes on page 294, field patterns for physical spacings and feed phasings. For example, at a spacing of 1/8-wavelength and a phasing of 135-degrees, there is complete cancellation in one direction while there is maximum radiation in the opposite direction. That`s the good news. Now the bad. On page 297 Kraus says: "However, in the flat-top (an advantage placing all elements at maximum height) antenna such losses may have considerable effect on the gain (as the feedpoint resistance is very low). Therefore, the question of losses and of radiating efficiency will be treated in this section in connection with a discussion of arrays of two closely spaced, out-of-phase elements. The term "closely-spaced" will be taken to mean that the elements are spaced 1/4 wavelength or less." Then, Kraus shows another fly in the ointment on page 300: "Hence the Q for 1/8 wavelength spacing is about four times the Q for 1/4 wavelength spacing. Very large Q indicates a large amount of stored energy near the antenna in proportion to the energy radiated per cycle. This also means that the antenna acts like a sharply tuned circuit." So much for bandwidth! Best regards, Richard Harrison, KB5WZI |
Richard
I looked thru The ARRL antenna book and I cannot find any antenna let alone the two element phased array that showed ZERO radiation to the rear of the feed point. Is it possible you are pointing to F/B for minimum radiation where I am refering to zero front to rear: Regards Art "Richard Harrison" wrote in message ... Art Unwin wrote: "I have just come to realise that if one drew a polygon of element phases in an array and all elements were 180 degrees to its companion element and excluding the driven elemment, the max gain and max front to back will occur at the same frequency!" Would an antenna made entirely of pairs of identical out of phase elements be a good antenna? A "polygon of element phases" must refer to the resultant current in each element and their combined effect at a point (P) for example in the far field. Art must have resolved and composed vectors or phasors at some time. The resultant of any number of vectors can all add to zero or to some other number and direction. A zero sum often happens in physics when systems are in equiblirium. Newton said that any action results in an equal and opposite reaction. Application of a new force often causes no loss in equilibrium, just a corresponding added reaction. The reflected wave from an antenna may change in magnitude in proportion to an incidebt wave yet be nearly exactly equal in magnitude and 180-degrees out of phase with the incident wave, if the reflection is perfect. A polygon is a closed plane bounded by straight sides. It can represenht forces. Art asked if there were anything written about complete front to back cancellation in two radiators carrying oppositely directed signals if I understood the question. Indeed Kraus of W8JK fame has a lot to say about the possibility. Kraus writes about an "Array of Two Driven 1/2-wavelength El;ements. General Case with Equal Currents of Any Phase Relation." in his 1950 edition of "Antennas". It includes on page 294, field patterns for physical spacings and feed phasings. For example, at a spacing of 1/8-wavelength and a phasing of 135-degrees, there is complete cancellation in one direction while there is maximum radiation in the opposite direction. That`s the good news. Now the bad. On page 297 Kraus says: "However, in the flat-top (an advantage placing all elements at maximum height) antenna such losses may have considerable effect on the gain (as the feedpoint resistance is very low). Therefore, the question of losses and of radiating efficiency will be treated in this section in connection with a discussion of arrays of two closely spaced, out-of-phase elements. The term "closely-spaced" will be taken to mean that the elements are spaced 1/4 wavelength or less." Then, Kraus shows another fly in the ointment on page 300: "Hence the Q for 1/8 wavelength spacing is about four times the Q for 1/4 wavelength spacing. Very large Q indicates a large amount of stored energy near the antenna in proportion to the energy radiated per cycle. This also means that the antenna acts like a sharply tuned circuit." So much for bandwidth! Best regards, Richard Harrison, KB5WZI |
wrote:
Richard I looked thru The ARRL antenna book and I cannot find any antenna let alone the two element phased array that showed ZERO radiation to the rear of the feed point. In my ARRL Antenna Book, 15th edition, page 8-6: Two phased verticals with 1/8WL spacing and phased at 135 degrees shows a perfect cardioid with zero radiation in a direction 180 degrees from the direction of maximum gain, i.e. directly to the rear. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Yes Cecil, a cardioid pattern ,which cannot produce zero radiation at the
rear 180 degrees of the feed point. Remember we started of with a figure 8 or two balloon pattern so for total reversal of radiation the front lobe must finish up as a perfect circle . Said another way, the two ballons are merged thus making a larger single balloon Best Regards Art "Cecil Moore" wrote in message ... wrote: Richard I looked thru The ARRL antenna book and I cannot find any antenna let alone the two element phased array that showed ZERO radiation to the rear of the feed point. In my ARRL Antenna Book, 15th edition, page 8-6: Two phased verticals with 1/8WL spacing and phased at 135 degrees shows a perfect cardioid with zero radiation in a direction 180 degrees from the direction of maximum gain, i.e. directly to the rear. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Art, KB9MZ wrote:
"---I would like to see the gain he arrives at---." Kraus gives the W8JK close-spaced beam (spacing:1/8-wavelength) a gain of 5.8 dBi on page 552 in his 2002 3rd edition of "Antennas". A dipole in free-space has a gain of 2.14 dB over an isotropic, so the W8JK has a gain of 3.66 dBd. This makes the W8JLK comparable in gain to other 2-element beams in Arnold B. Bailey`s "TV and Other Receiving Antennas" catalog. Best regards, Richard Harrison, KB5WZI |
Art, KB9MZ wrote:
"I looked through the ARRL book and I cannot find any antenna let alone the two element phased array that showed zero radiation to the rear of the feed point." I was quoting the patterns in Kraus` 1950 edition of "Antennas" on page 294. However, there is a measured by D.C. Cleckner of Ohio State University pattern of a 3-element Yagi-Uda with more than 7 dB gain and almost zero radiation in the direction of 180-degrees from the maximum. It is on page 246. This has 0.1-wavelength spacing between elements. Best regards, Richard Harrison, KB5WZI |
Art,
Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. A detailed description of all of this is in Kraus' Antennas, 2nd Ed., in Chapter 11. He uses equations, and he does not mention coordination of balloon patterns, so perhaps you have another new invention. 73, Gene W4SZ wrote: Yes Cecil, a cardioid pattern ,which cannot produce zero radiation at the rear 180 degrees of the feed point. Remember we started of with a figure 8 or two balloon pattern so for total reversal of radiation the front lobe must finish up as a perfect circle . Said another way, the two ballons are merged thus making a larger single balloon Best Regards Art "Cecil Moore" wrote in message ... wrote: Richard I looked thru The ARRL antenna book and I cannot find any antenna let alone the two element phased array that showed ZERO radiation to the rear of the feed point. In my ARRL Antenna Book, 15th edition, page 8-6: Two phased verticals with 1/8WL spacing and phased at 135 degrees shows a perfect cardioid with zero radiation in a direction 180 degrees from the direction of maximum gain, i.e. directly to the rear. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
A cardioid pattern has radiation in the 180 degree portion behind the feed
point Regards Art "Gene Fuller" wrote in message ... Art, Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. A detailed description of all of this is in Kraus' Antennas, 2nd Ed., in Chapter 11. He uses equations, and he does not mention coordination of balloon patterns, so perhaps you have another new invention. 73, Gene W4SZ wrote: Yes Cecil, a cardioid pattern ,which cannot produce zero radiation at the rear 180 degrees of the feed point. Remember we started of with a figure 8 or two balloon pattern so for total reversal of radiation the front lobe must finish up as a perfect circle . Said another way, the two ballons are merged thus making a larger single balloon Best Regards Art "Cecil Moore" wrote in message ... wrote: Richard I looked thru The ARRL antenna book and I cannot find any antenna let alone the two element phased array that showed ZERO radiation to the rear of the feed point. In my ARRL Antenna Book, 15th edition, page 8-6: Two phased verticals with 1/8WL spacing and phased at 135 degrees shows a perfect cardioid with zero radiation in a direction 180 degrees from the direction of maximum gain, i.e. directly to the rear. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
The formula for z which you gave would give me nightmares. Are you absolutely sure that what you have written is correct? Perhaps first principles was overstated n my posting Art "Richard Clark" wrote in message ... On Fri, 11 Mar 2005 23:05:28 GMT, " wrote: A cardioid pattern has radiation in the 180 degree portion behind the feed point first described by Johann Castillon made a Fellow of the Royal Society of London in 1753: r = 2a(1 + cos(theta)) where theta = 180 such that: r = 2a(1 + cos(180)) r = 2a(1 + -1) r = 0 Hi Art, Understandably, the term you are so unfamiliar with, insofar as no rearward radiation (not the same as no radiation to 180 degrees) is Lambertian. As may be expected, the term is derived from the work of mathematician Johann Heinrich Lambert (1728-1777). It is a distribution curve derived from reflections off of a "diffuse surface" (note, not the same thing as reflections off of a specular antenna element): (x²+y²+z²)² = z At 300 years+ both are pretty old works that each easily qualify within the purview of "first principles" if one is serious about radiation. 73's Richard Clark, KB7QHC |
I wrote:
"However, there is a measured by D.C. Glockner of Ohio State University pattern of the 3-element Yagi-Uda with more than 7 dB gain and almost zero radiation from the direction of 180-degrees from the maximum. It is on page 246." No correction required but I should add that the book is the 3rd edition (2002) of "Antennas" by John D. Kraus et al. Best regards, Richard Harrison, KB5WZI |
I did rotate it and placed it on the ground, the gain dropped
by 4db and the circular lobe pointed straight up. Some time I will look at same at 1 WL Art "Cecil Moore" wrote in message ... wrote: If the phases and and magnitudes of the paired elements are exactly the same, then radiation to the rear is zero. If you rotate the elements by 90 degrees, can you make the radiation toward the ground zero? -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
On Sat, 12 Mar 2005 00:38:31 GMT, "
wrote: The formula for z which you gave would give me nightmares. Hi Art, That is the Cartesian form. A 2D polar form would follow: I = I0 · A · cos (phi) · cos (theta) / Pi where I is the intensity at a point with elevation theta A is the area of the surface I0 is the radiation directed normal to a diffuse surface phi is the angle of incidence (all angles being considered) A variant for conforming radiators is found in: I = I0 · cos (theta) Observation will reveal why this is called Lambert's Cosine Law. Simple draughting techniques will reveal the circular distribution curve. A simple example of the last equation is found in the common, unlensed LED. Insofar as radios go, I expect the same response would follow from placing an isotropic source above a diffuse reflector (you would then have to use the first equation). 73's Richard Clark, KB7QHC |
wrote:
A cardioid pattern has radiation in the 180 degree portion behind the feed point You want zero radiation in an entire hemisphere? Arecibo probably meets that specification. :-) -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Yup.
They have now upgraded the mountain road so you don't have to take a spare rear axle with you now. Art "Cecil Moore" wrote in message ... wrote: A cardioid pattern has radiation in the 180 degree portion behind the feed point You want zero radiation in an entire hemisphere? Arecibo probably meets that specification. :-) -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
But that formula for z does not appear to be correct!
Art "Richard Clark" wrote in message ... On Sat, 12 Mar 2005 00:38:31 GMT, " wrote: The formula for z which you gave would give me nightmares. Hi Art, That is the Cartesian form. A 2D polar form would follow: I = I0 · A · cos (phi) · cos (theta) / Pi where I is the intensity at a point with elevation theta A is the area of the surface I0 is the radiation directed normal to a diffuse surface phi is the angle of incidence (all angles being considered) A variant for conforming radiators is found in: I = I0 · cos (theta) Observation will reveal why this is called Lambert's Cosine Law. Simple draughting techniques will reveal the circular distribution curve. A simple example of the last equation is found in the common, unlensed LED. Insofar as radios go, I expect the same response would follow from placing an isotropic source above a diffuse reflector (you would then have to use the first equation). 73's Richard Clark, KB7QHC |
On Sat, 12 Mar 2005 03:15:42 GMT, "
wrote: But that formula for z does not appear to be correct! Hi Art, As I said, you are not working in Cartesians in the first place with "polygons." It is a formula for computer generation of the surface. If you absolutely need to understand the formula, read through the code that is used to construct the distribution curve: #define sz 2000000 #define bc 19 static double fr() {return rand()/(double)~(131);} static double sq(double x){return x*x;} int hi[bc]; int out=0; int main(){ {int j=bc; while(j--) hi[j]=0;} {int j=sz; while(j--) { double x = fr()*1.4-.7, y=fr()*1.4-.7, z=fr(); if(sq(x*x+y*y+z*z) z) { ++hi[(int)(z/sqrt(x*x+y*y+z*z)*bc)];} else ++out;}} {int j; for(j=0; jbc; ++j) printf("%d %d %8.2f\n", j, hi[j], (sz-out)*(j+.5)/(bc*bc/2));} printf ("%d out of box.\n", out); } If you still don't understand, the polar coordinate formulas are just as useful, simpler, and take very little work to construct a "polygon" that obtains complete closure. It is, after all, a construction much like any of a number of classic curves. You need only conform to the requirements of a Lambertian surface or emitter to obtain the curve you describe. 73's Richard Clark, KB7QHC |
Put the antenna half way up and a small part broke!
Have put the antenna to one side and I will pick it up again during the summer Regards Art " wrote in message news:dySVd.30807$r55.174@attbi_s52... I have just come to realise that if one drew a polygon of element phases in a array and all elements were 180 degrees to its companion element and excluding the driven element, the max gain and max front to back will occur at the SAME frequency! Until now I was of the understanding that these two max figures could not occur at the same frequency. Is there anything written about this possibility? Regards Art |
Gene Fuller wrote:
Art, Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. . . . Actually, this isn't quite true. If you manage to get perfectly phased and equal magnitude currents in two identical elements where the phase angle equals 180 degrees minus the element spacing (such as the classic 90-degree fed, 90-degree spaced cardioid), you don't get an infinite front-back ratio. In the case of the cardioid with typical diameter quarter wavelength elements, you end up with around a 35 dB front/back ratio. With longer elements, close to a half wavelength, the front/back ratio can deteriorate to less than 10 dB when base currents are identical in magnitude and correctly phased. The reason is that the mutual coupling between elements alters the current distribution on the elements. The mutual coupling from element 1 to element 2 isn't the same as the coupling from element 2 to element 1 (the mutual Z is the same, but the coupled voltage and coupled impedance aren't). The net result is that the two elements have different current distributions, so despite having identical magnitude base currents the two elements don't generate equal magnitude fields. The overall fields from the two elements end up being imperfectly phased, also. This occurs for theoretically perfect and perfectly fed elements, and isn't due to "real world" effects. I published some comments about this effect in "Technical Correspondence" in July 1990 QST ("The Impact of Current Distribution on Array Patterns"). I'm certainly not the first to have observed it -- some papers published as early as the '40s are referenced in my article. But I had never seen its effect on front/back ratio of cardioids mentioned before. Modern versions of the ARRL Antenna Book clearly show the small reverse lobe of a typical antenna with quarter wavelength elements. I stumbled across it when doing some modeling with ELNEC, the predecessor of EZNEC, and originally thought it was an error in the program. You'll see it in a plot from the Cardioid.EZ EZNEC example file (which is also included with the demo program), and a brief explanation in the corresponding Antenna Notes file. A theoretically infinite front/back ratio can be achieved by modification of the base currents. The amount of modification required depends on the length and diameter of the elements. Only a small modification is needed if elements are a quarter wavelength high and small diameter, but in that case, real world effects will probably have at least as much and likely more of an effect on the front/back than the current distribution phenomenon. Rather drastic modification is required of the base currents of elements approaching a half wavelength high, however, as elaborated in the "Technical Correspondence" piece. Roy Lewallen, W7EL |
Hi Roy,
I have read many of your articles, and I have no doubt you are correct. However, in the ideal case, specifically in the limit as the wire diameter goes to zero, the current perturbation from mutual inductance vanishes. (The mutual inductance does not vanish, only its impact on current distribution.) I just spent a few minutes playing around with EZNEC 3, and I was able to achieve a null of -52 dBi (-57 dBmax) for two half-wave elements, with nominal 90 degree spacing and 90 degree phasing. The wire size was as small as possible. This null was in the symmetry plane and directly in the anti-end-fire direction of course. I expect with more computational precision, and perhaps fine tuning frequencies and dimensions this null could be driven farther. The reported current imbalance was a maximum of 0.2%, mid-way between the center and the ends of the wires. The phase imbalance between the wires was a maximum of 0.2 degrees. I am not trying to say this is practical. I was just pointing out the Art's use of polygons and canceling phasors was not particularly unique. We have since learned that what Art is trying to accomplish is to eliminate all radiation in the back hemisphere. The cardioid example is obviously moot for his quest. 73, Gene W4SZ Roy Lewallen wrote: Gene Fuller wrote: Art, Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. . . . Actually, this isn't quite true. If you manage to get perfectly phased and equal magnitude currents in two identical elements where the phase angle equals 180 degrees minus the element spacing (such as the classic 90-degree fed, 90-degree spaced cardioid), you don't get an infinite front-back ratio. In the case of the cardioid with typical diameter quarter wavelength elements, you end up with around a 35 dB front/back ratio. With longer elements, close to a half wavelength, the front/back ratio can deteriorate to less than 10 dB when base currents are identical in magnitude and correctly phased. The reason is that the mutual coupling between elements alters the current distribution on the elements. The mutual coupling from element 1 to element 2 isn't the same as the coupling from element 2 to element 1 (the mutual Z is the same, but the coupled voltage and coupled impedance aren't). The net result is that the two elements have different current distributions, so despite having identical magnitude base currents the two elements don't generate equal magnitude fields. The overall fields from the two elements end up being imperfectly phased, also. This occurs for theoretically perfect and perfectly fed elements, and isn't due to "real world" effects. I published some comments about this effect in "Technical Correspondence" in July 1990 QST ("The Impact of Current Distribution on Array Patterns"). I'm certainly not the first to have observed it -- some papers published as early as the '40s are referenced in my article. But I had never seen its effect on front/back ratio of cardioids mentioned before. Modern versions of the ARRL Antenna Book clearly show the small reverse lobe of a typical antenna with quarter wavelength elements. I stumbled across it when doing some modeling with ELNEC, the predecessor of EZNEC, and originally thought it was an error in the program. You'll see it in a plot from the Cardioid.EZ EZNEC example file (which is also included with the demo program), and a brief explanation in the corresponding Antenna Notes file. A theoretically infinite front/back ratio can be achieved by modification of the base currents. The amount of modification required depends on the length and diameter of the elements. Only a small modification is needed if elements are a quarter wavelength high and small diameter, but in that case, real world effects will probably have at least as much and likely more of an effect on the front/back than the current distribution phenomenon. Rather drastic modification is required of the base currents of elements approaching a half wavelength high, however, as elaborated in the "Technical Correspondence" piece. Roy Lewallen, W7EL |
Gene,
At a matter of interest during the 80s I tried to get to zero radiation at 180 degree point since Lawson stated it was possible. After covering the half acre under the long boom with a ground screen, in fraustration, I finally gave up with the pursuit. On one of my present models the rear radiation never exceed 40db for more than 180 degrees but as Roy pointed out earlier you still have to deal with the higher angles which was the case with my model in that when the angle reached 30 degrees elevation we were back to 20 db.. The center "plume" radiation seems difficult to eradicate. I think I will try your suggetion of radiators with radiators of 0.01 diameter to see what happens Regards Art "Gene Fuller" wrote in message ... Hi Roy, I have read many of your articles, and I have no doubt you are correct. However, in the ideal case, specifically in the limit as the wire diameter goes to zero, the current perturbation from mutual inductance vanishes. (The mutual inductance does not vanish, only its impact on current distribution.) I just spent a few minutes playing around with EZNEC 3, and I was able to achieve a null of -52 dBi (-57 dBmax) for two half-wave elements, with nominal 90 degree spacing and 90 degree phasing. The wire size was as small as possible. This null was in the symmetry plane and directly in the anti-end-fire direction of course. I expect with more computational precision, and perhaps fine tuning frequencies and dimensions this null could be driven farther. The reported current imbalance was a maximum of 0.2%, mid-way between the center and the ends of the wires. The phase imbalance between the wires was a maximum of 0.2 degrees. I am not trying to say this is practical. I was just pointing out the Art's use of polygons and canceling phasors was not particularly unique. We have since learned that what Art is trying to accomplish is to eliminate all radiation in the back hemisphere. The cardioid example is obviously moot for his quest. 73, Gene W4SZ Roy Lewallen wrote: Gene Fuller wrote: Art, Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. . . . Actually, this isn't quite true. If you manage to get perfectly phased and equal magnitude currents in two identical elements where the phase angle equals 180 degrees minus the element spacing (such as the classic 90-degree fed, 90-degree spaced cardioid), you don't get an infinite front-back ratio. In the case of the cardioid with typical diameter quarter wavelength elements, you end up with around a 35 dB front/back ratio. With longer elements, close to a half wavelength, the front/back ratio can deteriorate to less than 10 dB when base currents are identical in magnitude and correctly phased. The reason is that the mutual coupling between elements alters the current distribution on the elements. The mutual coupling from element 1 to element 2 isn't the same as the coupling from element 2 to element 1 (the mutual Z is the same, but the coupled voltage and coupled impedance aren't). The net result is that the two elements have different current distributions, so despite having identical magnitude base currents the two elements don't generate equal magnitude fields. The overall fields from the two elements end up being imperfectly phased, also. This occurs for theoretically perfect and perfectly fed elements, and isn't due to "real world" effects. I published some comments about this effect in "Technical Correspondence" in July 1990 QST ("The Impact of Current Distribution on Array Patterns"). I'm certainly not the first to have observed it -- some papers published as early as the '40s are referenced in my article. But I had never seen its effect on front/back ratio of cardioids mentioned before. Modern versions of the ARRL Antenna Book clearly show the small reverse lobe of a typical antenna with quarter wavelength elements. I stumbled across it when doing some modeling with ELNEC, the predecessor of EZNEC, and originally thought it was an error in the program. You'll see it in a plot from the Cardioid.EZ EZNEC example file (which is also included with the demo program), and a brief explanation in the corresponding Antenna Notes file. A theoretically infinite front/back ratio can be achieved by modification of the base currents. The amount of modification required depends on the length and diameter of the elements. Only a small modification is needed if elements are a quarter wavelength high and small diameter, but in that case, real world effects will probably have at least as much and likely more of an effect on the front/back than the current distribution phenomenon. Rather drastic modification is required of the base currents of elements approaching a half wavelength high, however, as elaborated in the "Technical Correspondence" piece. Roy Lewallen, W7EL |
Yes, as the wire diameter goes to zero, the current distribution
approaches the same on all elements. But in some cases (where the element height is in the vicinity of a half wavelength) the wires have to get impossibly thin to achieve good f/b with equal magnitude and correctly phased base currents. I guess you could categorize needing a finite diameter wire as a "real world effect" and a zero diameter wire as "theoretically perfect". As I mentioned, it's not hard to do well at a quarter wavelength height, but much harder at heights approaching a half wavelength. For example, I took the EZNEC Cardioid.ez example file and increased the element heights to 0.4 meter (0.4 wavelength) using 25 segments/element. With wire diameter of 10^-15 mm, the front/back ratio was still 32 dB. With the original wire diameter of about 0.24 mm, the front/back was less than 15 dB. And things get worse yet as the elements get closer to a half wavelength high. But in practice, even at a quarter wavelength height, people using phased towers might encounter an unexpectedly low f/b ratio. For anyone who's interested, I've posted the Technical Correspondence piece on my web site. You can get it at http://eznec.com/Amateur/Articles/Current_Dist.pdf. Roy Lewallen, W7EL Gene Fuller wrote: Hi Roy, I have read many of your articles, and I have no doubt you are correct. However, in the ideal case, specifically in the limit as the wire diameter goes to zero, the current perturbation from mutual inductance vanishes. (The mutual inductance does not vanish, only its impact on current distribution.) I just spent a few minutes playing around with EZNEC 3, and I was able to achieve a null of -52 dBi (-57 dBmax) for two half-wave elements, with nominal 90 degree spacing and 90 degree phasing. The wire size was as small as possible. This null was in the symmetry plane and directly in the anti-end-fire direction of course. I expect with more computational precision, and perhaps fine tuning frequencies and dimensions this null could be driven farther. The reported current imbalance was a maximum of 0.2%, mid-way between the center and the ends of the wires. The phase imbalance between the wires was a maximum of 0.2 degrees. I am not trying to say this is practical. I was just pointing out the Art's use of polygons and canceling phasors was not particularly unique. We have since learned that what Art is trying to accomplish is to eliminate all radiation in the back hemisphere. The cardioid example is obviously moot for his quest. 73, Gene W4SZ Roy Lewallen wrote: Gene Fuller wrote: Art, Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. . . . Actually, this isn't quite true. If you manage to get perfectly phased and equal magnitude currents in two identical elements where the phase angle equals 180 degrees minus the element spacing (such as the classic 90-degree fed, 90-degree spaced cardioid), you don't get an infinite front-back ratio. In the case of the cardioid with typical diameter quarter wavelength elements, you end up with around a 35 dB front/back ratio. With longer elements, close to a half wavelength, the front/back ratio can deteriorate to less than 10 dB when base currents are identical in magnitude and correctly phased. The reason is that the mutual coupling between elements alters the current distribution on the elements. The mutual coupling from element 1 to element 2 isn't the same as the coupling from element 2 to element 1 (the mutual Z is the same, but the coupled voltage and coupled impedance aren't). The net result is that the two elements have different current distributions, so despite having identical magnitude base currents the two elements don't generate equal magnitude fields. The overall fields from the two elements end up being imperfectly phased, also. This occurs for theoretically perfect and perfectly fed elements, and isn't due to "real world" effects. I published some comments about this effect in "Technical Correspondence" in July 1990 QST ("The Impact of Current Distribution on Array Patterns"). I'm certainly not the first to have observed it -- some papers published as early as the '40s are referenced in my article. But I had never seen its effect on front/back ratio of cardioids mentioned before. Modern versions of the ARRL Antenna Book clearly show the small reverse lobe of a typical antenna with quarter wavelength elements. I stumbled across it when doing some modeling with ELNEC, the predecessor of EZNEC, and originally thought it was an error in the program. You'll see it in a plot from the Cardioid.EZ EZNEC example file (which is also included with the demo program), and a brief explanation in the corresponding Antenna Notes file. A theoretically infinite front/back ratio can be achieved by modification of the base currents. The amount of modification required depends on the length and diameter of the elements. Only a small modification is needed if elements are a quarter wavelength high and small diameter, but in that case, real world effects will probably have at least as much and likely more of an effect on the front/back than the current distribution phenomenon. Rather drastic modification is required of the base currents of elements approaching a half wavelength high, however, as elaborated in the "Technical Correspondence" piece. Roy Lewallen, W7EL |
Art Unwin wrote:
"I have just come to realize that if one drew a polygon of element phases in an array and all elements were 180 degrees to its companion element and excluding its driven element, the max gain and max front to back will occur at the dame frequency!" I missed a step or two between the polygon`s resultant and coincidence of maximum gain with maximum front to back ratio. A vector has both magnitude and direction. A scalar quantity has only magnitude. Vectors are represented by arrows whose lengths correspond to their magnitudes. Directions of the arrows correspond to the directions of the vectors. The combined effect of two or more vectors is called a resultant. A resultant can be found by a geometrical rule called vector addition. Vectors are placed head to tail while maintaining their magnitudes and directions. The resultant is a vector drawn from the tail of the first vector to the head of the final vector. Any number of vectors can be added by the head to tail method. These can create a polygon of vectors. If only two vectors are to be added at a time, they can produce a resultant by the parallelogram method. Two sides of the parallelogram are formed by the two vectors connected at their tails. Parallel same length sides are added to form the parallelogram. A diagonal emanating from the junction of the two vectors forms the resultant When one vector (call it c) is the resultant of two vectors (call them and b) we can say that a and b are components of c. Any given vector can be resolved into an infinite number of pairs. Usually we find it convenient to resolve a vector into a pair which are at right angles with each other. Then we can use the Pythagorean theorem (c squared = a squared + b squared) to find the magnitude of the resultant (c). In a right triangle in which two sides are perpendicular, all the trignometric functions are useful in determining the lengths and directions of its sides. Best regards, Richard Harrison, KB5WZI |
"Richard Harrison" wrote in message ... Art Unwin wrote: "I have just come to realize that if one drew a polygon of element phases in an array and all elements were 180 degrees to its companion element and excluding its driven element, the max gain and max front to back will occur at the dame frequency!" I missed a step or two between the polygon`s resultant and coincidence of maximum gain with maximum front to back ratio. snip What exactly did you miss? Was it something I missed or an error or a assumption that I made? What followed the above statement was a treatise regarding the formation of a polygon that had been described earlier, but I did not see any relevance or connection to what you porport that you or I didn't understand or missed! Regards Art .. Best regards, Richard Harrison, KB5WZI |
On Sat, 26 Mar 2005 01:48:44 GMT, "
wrote: I missed a step or two between the polygon`s resultant and coincidence of maximum gain with maximum front to back ratio. snip What exactly did you miss? Hi Art, The answer to your question is found in Richard's question you answered. 73's Richard Clark, KB7QHC |
| All times are GMT +1. The time now is 10:35 AM. |
|
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com