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Antenna Theory
Antenna Theory:
There are many textbooks telling that an antenna to be effcient needs "much wire in the air". Now the time has come to correct this view. This new view is based on many practical tests on all HF bands over a year. As en example read the email below. The antenna used for this contact was 3 meters long and 1.5 m above ground. (= RoomCap Antenna) The contact was on the 160m band over a distance of 600 Km. Date: Thu, 31 Aug 2006 22:57:42 +0200 From: Heino Held To: hb9abx (at) tiscali.ch Subject: Mobilantenne oder Festantenne Hallo Felix, nach unserem tollen QSO auf 160m waren Peter (DL1BLD) und ich (Heino, DJ5ER) total erstaunt mit welch einer tollen Feldstaerke Du in Bremen angekommen bist. Wie man eine solche Antenne bauen kann moechten wir gerne wissen (auch fuer die anderen KW-Baender). 73 aus Bremen von Peter (DL1BLD) und Heino - DJ5ER --- --- Translation of text: Hello Felix, after our fantastic QSO on 160m we (Peter,DL1BLD) and I (Heino, DJ5ER) we were completely surprised with what a strong field strength you arrived in Bremen. We would like to know how such an antenna can be built (also for the other HF bands). 73 from Bremen .... --- --- Heino will receive the construction guide. Best regards Felix HB9ABX |
Antenna Theory
On Fri, 1 Sep 2006 22:01:21 +0000, Felix
wrote: There are many textbooks telling that an antenna to be effcient needs "much wire in the air". Were they written by Joe Miller? |
Antenna Theory
Felix, dude, repeat after me:
"We believe in the one way, the addition of fields, radiated from wires, combined spatially in phase to give gain, or loss, or fire in the case of too much loss. We believe in the fundemental difficulty of delivering power efficiently to physically small radiating structures, and we understand the limitations it places on such structures as compared to their full-size counterparts. We believe in the QSO, and that any antenna can provide many enjoyable ones. We believe in your puny 160m signal, that we dug from the noise with our phased Beverages, and replied in kind with our legal limit and transmitting four-square, and you received us well, yea and verily, despite your use of a dummy-load. We believe in the gain, 20dB or more, that an antenna may exhibit when it or its plans are meant to be sold. We believe in the measurement, scientific and clear, showing clear quantitative comparisons to a suitable reference aerial; let it now be done, or may you fall silent." 73, Dan |
Antenna Theory
Hello Felix,
Regarding antennas that are very small with respect to wavelength. With regards to your example, no correction to antenna theory is necessary. Small antennas can be efficient radiators, but the smaller the antenna, the more difficult to achieve reasonable efficiency. Antenna theory is well established. When you know the current distribution in a structure, one can calculate (mathematically or numerically) how much power is radiated (and also how much power is dissipated in the structure). This approach is used by virtually all Antenna Design software packages. All electrically small antennas have one thing in common, the usable bandwidth is small. The Q-factor of a lossless electrically small structure is proportional to about 0.05*lambda^3/Volume. For example a small loop of thin material with infinite conductivity has higher Q-factor than a loop of same diameter, but made of very wide strip (with infinite conductivity). The last one occupies more volume and therefore has lower natural Q. The Q-factor of practically small antennas can be that high that the bandwidth may by just a few kHz. The problem is in the matching. Matching, for example 0.8 Ohms in series with a reactance of 2800 Ohm (Q=3500), to 50 ohms is not easy. The Q-factor of the components is not high enough, some or most power is dissipated in the additional components, or even the antenna wire itself. So in the end your antenna may have an overall efficiency of 5% (-13 dB). Also voltages can be that high that power is lost by corona effects. Often, due to the high local E- and H-fields, power is lost in nearby constructions. About the practical use of small antennas. In many cases received signal levels are in the S9+20 dB range. As the noise level is far below this, loosing 13 dB (so your signal level will be S9+7 dB), is acceptable. If not, you may increase the input power. In the end, the small inefficient antenna is at least better then no antenna. So in my believe, a nice QSO over 600 km with a small antenna, doesn't prove that antenna theory has to be revised. I'm very curious to see the construction details. Best regards, Wim PA3DJS. |
Antenna Theory
Felix wrote:
Antenna Theory: There are many textbooks telling that an antenna to be effcient needs "much wire in the air". Now the time has come to correct this view. What textbooks say that? EZNEC says that a 3 meter tall monopole has a maximum gain of 1.5 dBi on 160m while a 38 meter 1/4WL monopole has a maximum gain of 1.36 dBi. In other words, they radiate approximately equally well both with high radiation efficiencies. The problem is not with the antenna's ability to radiate. The problem is in getting the RF energy into the antenna. The problem is not with losses in the antenna. The problem is finding an efficient j2250 inductor for the matching network. If we knew what your matching network looks like, we could estimate its efficiency. -- 73, Cecil http://www.qsl.net/w5dxp |
Antenna Theory
"Cecil Moore"
What textbooks say that? EZNEC says that a 3 meter tall monopole has a maximum gain of 1.5 dBi on 160m while a 38 meter 1/4WL monopole has a maximum gain of 1.36 dBi. In other words, they radiate approximately equally well both with high radiation efficiencies. The problem is not with the antenna's ability to radiate. The problem is in getting the RF energy into the antenna. The problem is not with losses in the antenna. The problem is finding an efficient j2250 inductor for the matching network. _____________ Radiation resistance, and the r-f resistance in the path for induced ground currents back into a practical antenna system have a larger effect, though. Here are some numbers for two vertical monopoles of about the same scale of height to width. If in both cases the matching network has a 2 ohm loss, there is a 10 ohm loss in the r-f ground connection, and both antenna systems present a 1:1 match to the tx, then with equal tx power output the peak power actually radiated by a 1/4-wave monopole will be about 65X (18 dB) greater than by a 3-meter monopole on 160 meters. RF |
Antenna Theory
Richard Fry wrote:
If in both cases the matching network has a 2 ohm loss, there is a 10 ohm loss in the r-f ground connection, ... Thanks, I should have said the problem is getting the RF energy into the radiation resistance. -- 73, Cecil http://www.qsl.net/w5dxp |
Quote:
Thanks to all, for your comments and opinions. I expected this type of arguments! Let me comment them: N3OX, Dan, wrote his confession in antennas... The fundamentel part :"We believe in the fundemental difficulty of delivering power efficiently to physically small radiating structures, and we understand the limitations it places on such structures as compared to their full-size counterparts." The difficulty is here, and most antennas of traditional design are suffering under this, and therefore are inefficient, if the relation l/lamdba is small. That's different in my design: The Varylink is part of the antenna and permits to feed the available input power with over 90% efficiency into the radiation resistance Rr of the antenna. Furthermore, the relation Rr / Rloss is very high, which ensures that the power is radiated, and not burned. Wim, PA3DJS, is addressing a very similar problem. He calculates an overall efficiency of 5% (-13db) under his assumptions. He is also basing his calculation on data available from traditional constructions. Im my design X/r is between 28 and 50, depending upon frequency and version, and at the same time the total series loss resistance is about 10 times smaller than the radiation resistance. These characterists ensure, that the efficiency (radiated power/accepted power) is in the order of 90 percent. Cecil, W5DXP, is referring to EZNEC, calculating that the small antenna might achieve practically the same radiation as 1/4 lambda monopole. His problem again is, how to get the power into the radiation resistance of the antenna. This is done as said above. RF, Richard Fry, is addressing the losses in the matching, and most important, in the ground loss. The efficient matching is answered above. The ground loss is very important for efficient radiation of the energy into space and not into the ground! This ground loss is not addressed in the IEEE formula for calculation of antenna efficiency, as it says: efficency = radiated-power/antenna-accepted-power. This does not take care of the fact, that in most real antennas, a large amount of the radiated power is lost in the ground as induced ground currents! To minimise these losses I am using a special grounding concept, based on a differential, floating feeding system, ensuring that ground losses are minimal, and that the radiated RF energy reaches the space towards the ionosphere. All this ensures the high efficiency of the small antenna, so far not reached by other designs. Felix Meyer, HB9ABX |
Antenna Theory
Felix,
I call Bull****. Show us REAL data on the efficiency of this antenna. Show us field strength vs. a full size 1/4 wave vertical with a good ground system. If you've invented something new and innovative, awesome, but the world will never know about it. Why? Because you won't do real tests. You want people to buy your plans. You make outrageous claims of efficiency with no data to back it up other than signal reports from closeby EU stations on 160m. As far as I'm concerned, at best, you're a optimist blinded by excitement who's built a pretty good low band mobile antenna and doesn't want to let himself down by measuring anything. At worst, you're an outright scammer. Actually, you know what, how much do you want for the plan again? Maybe we can take up a collection so we can build one and prove that you're wrong. But you know , I bet if we build it and it doesn't work as advertised, you'll make some claim about construction tolerances or sensitivity to nearby objects, or some such and still claim that YOUR antenna is 90% efficient. Such is the way of a good pseudoscientist. Dan |
Quote:
You are criticising by error, or by ignorance. I did the measuring of the efficiency by feeding the RoomCap antenna with 1 KW HF, havinga VSWR below 1,1 and the result: Less than 70 W heat is produced by the antenna, and, as energy can not be destroyed, 930 W was radiated by the antenna. The definition of efficiency by the IEEE is: "The radiation efficiency of an antenna is the ratio of the total power radiated by the antenna to the net power accepted by the antenna at its terminals." You can measure these data your own, using your instruments in your laboratory. The construction plan is available. You get it a a very moderate cost, for 30 Euros, as contribuiton to the development costs. This is less than you pay here in a restaurant for dining with a drink. And you call that "business". Please keep realistic and study before you accuse me of what you did. Regarding the tests on 160m you commented: I was testing between 22h and 24h local time, during the last weeks. At this time no oversea stations were reachable due to the present conditions. Even the biggest european stations were calling CQ DX, using Kilowatts, without any reply from DX. When calling on 1933 Khz, a pile-up of UK stations appeared. They all wanted to contact me - and gave very good reports. Ask John, G3WWM and Eric, G3IMX who are very familiar with the 160m band with decades of experience. 73s Felix HB9ABX |
Antenna Theory
Hello Felix,
Reading your text, I also get a somewhat unpleasant feeling. As I mentioned in my first contribution, A good GSO is not a representative figure for efficiency, because the difference between 10% and 100% radiated power is 10 dB. I am very curious to know your test setup for assessing efficiency. For me efficiency is ratio between radiated output and input. Matching networks are considered part of the antenna. At low frequency, determining efficiency is difficult. You probably must hire a helicopter to determine the 3D radiation pattern, or you must rely on "traditional antenna knowledge". In some cases the efficiency may appear to be higher (based on loss measurements). If your antenna is close to a structure that has reasonable coupling to your antenna, It may act as a re-radiator (or absorber). I like people that don't follow straight paths; many times it resulted in better products or better understanding. However, when you claim a certain efficiency, you should fully state how you measured efficiency and under what circumstances to enable review by others. I am a little bit skeptical to efficiency claims, especially when I have to pay in advance. I was professionally involved in measurements of very small UHF antennas with wide band, high efficiency characteristics. In most cases the measurements against standard antennas did not show the characteristics claimed. I believe you should give more details about your antenna (mechanical and electrical). Best Regards, Wim PA3DJS |
Antenna Theory
On Wed, 13 Sep 2006 11:57:28 +0000, Felix
wrote: Dan, N3OX, please no flames. That is not the correct style here ... You are criticising by error, or by ignorance. Hi Felix, Given your statements that follow, it appears that Dan IS criticizing about error and about ignorance: I did the measuring of the efficiency by feeding the RoomCap antenna with 1 KW HF, havinga VSWR below 1,1 Your claims are based on very thin technical knowledge - and your lack of experience shows. VSWR is not an indication of efficiency - far from it. I seriously doubt you know how to measure the V of VSWR. Your meter measures power, not V. Your understanding of the SWR versus efficiency relationship also reveals a lack of basic understanding. For small antennas, low SWR can be solid proof of high inefficiency. The simple fact of the matter is that no small antenna presents a load that is remotely close to any standard transmitter's output Z, nor any commercial transmission line. I fully expect you will attempt to claim matching solves this. When you do attempt that, we will clear up your lack of experience there too. and the result: Less than 70 W heat is produced by the antenna, and, as energy can not be destroyed, 930 W was radiated by the antenna. In fact, you do not prove you measured 10W heat, nor 20W heat, nor 40W heat, nor "less than 70W heat." You cannot even prove you radiated 930W watts. There are methods to "prove" these claims, and you don't show any knowledge of those basic principles. Relying on one definition (poorly extracted from a text) is not sufficient. It may qualify for sales, but this is not a sales group and you are not going to find customers here with your poor quality of discussion. This leaves us with one question: "What do you expect to achieve here?" 73's Richard Clark, KB7QHC |
Antenna Theory
In article , Richard Clark
wrote: This leaves us with one question: "What do you expect to achieve here?" 73's Richard Clark, KB7QHC Hello, and if you're a good enough salesman and have at least one university professor to explanation your "interpretation" of electromagnetic theory, you could rename the antenna "Son of CFA" ;-) At some point, however, a prospective customer is going to require some verifiable test data. All kidding aside, the challenge is the use of technique(s) that allows for the direct or indirect measurement of radiation resistance and loss (structure including any earth loss in the vicinity of the feedpoint) resistance over the operating frequency range. A measurement of the real (resistive) part of the antenna feedpoint impedance can only provide the sum of both types of resistance. We know how much power is being dissipated (heat + radiated) but that's all we can know from this one measurement. There is also the shape of the radiation pattern...but that is another matter. Sincerely, and 73s from N4GGO, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
Antenna Theory
Felix,
I apologize for the strong language. I would believe that you've designed and built a quite good mobile antenna that impresses veteran topband operators. However, it's not turning 93% of the power you're feeding it into radiation. I barely believe that it might be taking 7% of the power you're feeding it and turning it into heat in the antenna, but what about the ground return losses in the earth? These are, of course, proportional to the square of the current flowing in ground system. You seemed to suggest that your method of matching to the low radiation resistance is practically lossless. Even if this were to be true, and there were no loss in the matching network or the antenna conductor, you still must have a connection to earth. If this connection is not perfectly conducting, pushing all that current into it will result in high losses. For a ground system with 1 ohm of ground loss, and an antenna radiation resistance of 0.2 ohms, assuming a lossless matching network and radiator, I get an efficiency of 12% over PERFECT earth. Taking the ground reflection losses into account over average ground, it's about 5%. This is assuming PERFECT matching network and a very good (maybe impossibly good?) grounding system. The ground return current has to flow somewhere, the matching network I'm assuming is a black box. It doesn't matter what it is or how novel it is, the ground return current has to flow in your grounding system, and with an antenna as short as yours, that's a LOT of current. You do realize that if you were completely losslessly feeding 1kW into 0.2 ohms, the antenna current would be 70.7A, right? You won't notice a kilowatt's worth of power dissipation in your car and the earth around it. Felix, are you willing to do an experiment? Feed your RoomCap antenna against another one as a dipole, adjust the matching network for a good match, and feed 1kW into it and measure the heat produced in the matching network and antenna, if it survives long enough to do so. And another thing, Felix... even a full size, perfectly conducting 1/4 wavelength monopole with a practically lossless place for the ground return currents to flow doesn't radiate more than about 30% of the power applied to it anyway over average earth. The ground reflection losses in the Fresnel zone dissipate much of the power. This is better over better earth of course, but I doubt your antenna has some sort of control over the soil conductivity and permittivity for tens of wavelengths in every direction. You may wish to revise your claims of 93% efficiency down to 93% efficiency relative to a full size ground mounted 1/4 wavelength monopole; it would be a more convincing untruth. One further comment: I have a hard time believing that these are all innocent mistakes.. It reflects badly on your character to make vague, inaccurate statements about a miraculous antenna and then tell people they need to dish out 30 Euros just to be able to try it. My apologies for thinking you're a big scammer if you are merely a victim of your own optimism.... I could see the argument that only 70W are being dissipated in the antenna as convincing even the innocent experimenter that he was on to something big! However, now you know the truth. If you revise your claims with an eye to the reality of feeding a small antenna against the earth, then I won't be so upset with you. It is counter to the ham spirit to mislead people in this way, if that's what you're doing, and only you know that. We are all trying to learn RF engineering in our spare time, and it's important that the new hams out there take their 38 bucks and apply it to their inverted L project for 160m, or a copy of ON4UN's low band DXing book instead of handing them over to you for the plans to one disappointing antenna. 73, Dan |
Antenna Theory - And ON4UNs book
d an antenna radiation
resistance of 0.2 ohms, assuming a lossless matching network and radiator, I get an efficiency of 12% over PERFECT earth. Taking the ground reflection losses into account over average ground, it's about 5%. This is assuming PERFECT matching network and a very good (maybe new hams out there take their 38 bucks and apply it to their inverted L project for 160m, or a copy of ON4UN's low band DXing book Great advice Dan. That book, in my opinion, is the finest book ever written concerning practical information for the HF operator. And I have read them all. You know, one thing that impressed me (and there were many) in that book is where he admitted that previous versions were wrong in telling us there was benefit to sloping the end of a Beverage antenna down to the feedpoint. Simply running it vertically down is the same. To readers of this newsgroup, if you have only read previous editions, you should get the new one. It is completely rewritten. Rick K2XT |
Antenna Theory - And ON4UNs book
Rick wrote:
To readers of this newsgroup, if you have only read previous editions, you should get the new one. It is completely rewritten. In a nutshell, what does he say about the delay/phase-shift through a loading coil in the latest edition? -- 73, Cecil http://www.w5dxp.com |
Antenna Theory - And ON4UNs book
|
[/email] (Rick) wrote:
d an antenna radiation resistance of 0.2 ohms, assuming a lossless matching network and radiator, I get an efficiency of 12% over PERFECT earth. Taking the ground reflection losses into account over average ground, it's about 5%. This is assuming PERFECT matching network and a very good (maybe new hams out there take their 38 bucks and apply it to their inverted L project for 160m, or a copy of ON4UN's low band DXing book ... Rick K2XT I am sorry, that some people oppose so strongly against what I said about my experience I gained with the new HB9ABX Roomcap antenna. They have no idea what they are talking about. They criticize without having seen what they talk about. The referenced antenna books describe what they know about antennas. This is applicable for the traditional antennas, not for the new concept of the RoomCap. I know very good the losses that may occur in antenna systems and especially in ground losses. That is, why the RoomCap antenna has its own new grounding system to prevent such losses. If in the past all new discoveries were treated that way, the world would still be flat and the earth would be the centre of the universe ... --- If you are feeding the antenna with 1 KW real power, and the in total, less than 70 W is converted to heath, then 930 W is radiated by this antenna. And the special grounding system is assuring that the radiated power is not burned by the surrounding ground, and is radiated efficiently into the space. The result is, that I am told frequently: You are the strongest station I hear presently on this band. That happend many time last week on 40m, and was reported by many UK stations. --- And read the comment I received from my 160m tests (with a 3 m long radiator): Hello Felix, after our fantastic QSO on 160m we (Peter,DL1BLD) and I (Heino, DJ5ER) we were completely surprised with what a strong field strength you arrived in Bremen. We would like to know how such an antenna can be built (also for the other HF bands). 73 from Bremen .... (QRB = 600 Km) - and - Hello Felix. Congratulations for your antenna work. You are the first mobile station on 160 meters for me. Even you are stronger as the Germans on 160 meters. I give you on 1.862 Mhz S 9+15 db in the peak!! The background noise level was S 8 during our QSO Hope to meet you agn for next report and test. Piet Schipper / PA0QRS (near Rotterdam) Schipper mobiele Telecom 2931 LH Krimpen a/d Le --- --- --- Would you obtain such reports with a "dummy load" antenna ? 73s Felix HB9ABX |
Antenna Theory
Ah, the perennial cry of the pseudoscientist. I am misunderstood! My
idea will change the WORLD. But then, you offer no real evidence. You're basically giving a supernatural explanation for the operation of the antenna. The RF current flow is a ghost.. it's a haunted antenna. What measurements do you have to show that your grounding system does what you say? A very, very inefficient antenna can get you good signal reports. You're trying to get people to send you cash for your haunted antenna, so you won't do real measurements. You don't really know how much power is being radiated by this antenna, and never will we, unless we send you money. Dan |
Antenna Theory
|
Antenna Theory
wrote in message ups.com... Ah, the perennial cry of the pseudoscientist. I am misunderstood! My idea will change the WORLD. But then, you offer no real evidence. You're basically giving a supernatural explanation for the operation of the antenna. The RF current flow is a ghost.. it's a haunted antenna. What measurements do you have to show that your grounding system does what you say? A very, very inefficient antenna can get you good signal reports. You're trying to get people to send you cash for your haunted antenna, so you won't do real measurements. You don't really know how much power is being radiated by this antenna, and never will we, unless we send you money. Dan Sounds like the junk ads you see on TV where the real profit is in the shipping and handling. |
Antenna Theory
Felix wrote: Cecil Moore Wrote: Richard Fry wrote:- Felix Meyer, HB9ABX Felix Felix- your confusion is based on the fact that no one will believe you without a basis for a real comparison of your antenna with a reference antenna, done by another person, with publication of the results and a description of the method. NO qso "data" will do this. In that regard, you are just another pusher of an EH or CFA antenna. If you are serious, you will let some independent expert make one to your description, and test it properly. (That's what shot down the EH) In regard to the inability of such programs as EZNEC to properly evaluate your antenna, I have not seen a well described antenna that could not be evaluated honestly by a person aware of antenna theory and the modelling programs. Good luck-Bill |
Antenna Theory
Bill wrote:
I have not seen a well described antenna that could not be evaluated honestly by a person aware of antenna theory and the modelling programs. The Lentine (sp?) antenna, consisting of different lengths of radiating transmission stubs proved impossible for me to model with EZNEC. -- 73, Cecil http://www.w5dxp.com |
Antenna Theory
Cecil Moore wrote: Bill wrote: I have not seen a well described antenna that could not be evaluated honestly by a person aware of antenna theory and the modelling programs. The Lentine (sp?) antenna, consisting of different lengths of radiating transmission stubs proved impossible for me to model with EZNEC. -- 73, Cecil http://www.w5dxp.com Cecil- Obviously, you fit the qualifications I mentioned, and- just as obviously, I did not know of that example. I need to do some homework. Thanks-Bill |
Antenna Theory
EZNEC can model radiating transmission line stubs made from either
parallel wires or coax. To do it, parallel wire lines have to be modeled as wires, not with the non-radiating transmission line model. Radiating coax is modeled with a combination of a non-radiating transmission line model for the inside, and a wire to represent the radiating outside of the coax. This technique is described in the EZNEC manual and illustrated with the DipTL.EZ example file included with EZNEC. There are some types of antennas which aren't possible to model with NEC-based programs. An example is a patch antenna on a dielectric substrate -- NEC and EZNEC have no way to model the dielectric. Likewise, a "loopstick" antenna -- a solenoid wound on a ferrite rod -- isn't possible because of the ferrite and possibly because of the exceptionally small dimensions (for one used at AM broadcast frequencies). But most often when you see an antenna inventor or seller claim that his antenna "can't be modeled" by NEC, EZNEC, or other programs, it just means that modeling fails to show the extraordinary performance he claims for it. That's simply a failure of the program to include the effects of magical properties and wishful thinking in its calculations. I've come to regard such claims as a red flag indicating a probable exaggeration of antenna performance. Roy Lewallen, W7EL Bill wrote: Cecil Moore wrote: Bill wrote: I have not seen a well described antenna that could not be evaluated honestly by a person aware of antenna theory and the modelling programs. The Lentine (sp?) antenna, consisting of different lengths of radiating transmission stubs proved impossible for me to model with EZNEC. -- 73, Cecil http://www.w5dxp.com Cecil- Obviously, you fit the qualifications I mentioned, and- just as obviously, I did not know of that example. I need to do some homework. Thanks-Bill |
Antenna Theory
Roy Lewallen wrote:
But most often when you see an antenna inventor or seller claim that his antenna "can't be modeled" by NEC, EZNEC, or other programs, it just means that modeling fails to show the extraordinary performance he claims for it. That's simply a failure of the program to include the effects of magical properties and wishful thinking in its calculations. I've come to regard such claims as a red flag indicating a probable exaggeration of antenna performance. I wish I could remember the correct spelling for the antenna I tried to model. Something like "Lentine". It is a dipole of sorts made from shorted and open sections of balanced transmission line. I tried modeling it with wires in EZNEC and got all sorts of errors. It looked something like this: +--------+--------+--------FP--------+--------+--------+ +------ +------ +------ ------+ ------+ ------+ Anyone remember the correct spelling for that antenna? -- 73, Cecil http://www.w5dxp.com |
Antenna Theory
Cecil Moore wrote:
Roy Lewallen wrote: But most often when you see an antenna inventor or seller claim that his antenna "can't be modeled" by NEC, EZNEC, or other programs, it just means that modeling fails to show the extraordinary performance he claims for it. That's simply a failure of the program to include the effects of magical properties and wishful thinking in its calculations. I've come to regard such claims as a red flag indicating a probable exaggeration of antenna performance. I wish I could remember the correct spelling for the antenna I tried to model. Something like "Lentine". It is a dipole of sorts made from shorted and open sections of balanced transmission line. I tried modeling it with wires in EZNEC and got all sorts of errors. It looked something like this: +--------+--------+--------FP--------+--------+--------+ +------ +------ +------ ------+ ------+ ------+ Anyone remember the correct spelling for that antenna? Google for "Lattin antenna". (Too many "lentils", Cecil :-) One of the first hits is http://www.g3ycc.karoo.net/lattin.htm which shows a good sketch. The antenna is made from sections of 300-ohm ribbon or tubular feeder, configured as a string of quarter-wave stubs that progressively make the dipole shorter as the frequency increases. The modeling challenge is that the ribbon operates in two different modes at the same time: a radiating common mode with a velocity factor of say 0.95; and a non-radiating "stub" mode with a VF of about 0.8. The problem is to model both modes simultaneously, for the whole string of stubs, without changing the physical dimensions of the real antenna. I'm not sure if NEC can do this, but maybe Roy can comment? -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek |
Antenna Theory
On Tue, 3 Oct 2006 08:43:07 +0100, Ian White GM3SEK
wrote: The modeling challenge is that the ribbon operates in two different modes at the same time: a radiating common mode with a velocity factor of say 0.95; and a non-radiating "stub" mode with a VF of about 0.8. Hi Ian, This "two different modes" is the magic mode factor that has not been designed into EZNEC. One need only look at the Lattin designs that "work" to discover they violate the precepts of "how" they work. Then note those that "should" work result in those don't work. The bottom line is fairly obvious, but there are those who can 'splain how its done (see magic mode factor). 73's Richard Clark, KB7QHC |
Antenna Theory
In article , Ian White GM3SEK
wrote: Google for "Lattin antenna". (Too many "lentils", Cecil :-) One of the first hits is http://www.g3ycc.karoo.net/lattin.htm which shows a good sketch. The antenna is made from sections of 300-ohm ribbon or tubular feeder, configured as a string of quarter-wave stubs that progressively make the dipole shorter as the frequency increases. The modeling challenge is that the ribbon operates in two different modes at the same time: a radiating common mode with a velocity factor of say 0.95; and a non-radiating "stub" mode with a VF of about 0.8. The problem is to model both modes simultaneously, for the whole string of stubs, without changing the physical dimensions of the real antenna. I'm not sure if NEC can do this, but maybe Roy can comment? Hello, and Roy will probably want to weigh in here. What I can say is that if you can create a wire model of the antenna consisting of interconnected segments (ideally about 1/20 wavelength each) then NEC will find the currents in each by considering all the interactions (conductive, capacitive, inductive) between the segments. NEC doesn't care about the geometry or "modes" of the antenna - it just sees a bunch of interconnected segments distributed in 3-D space. There is no magic here as NEC is merely applying text-book electromagnetic theory (you wouldn't want to tackle this with just pencil and paper). Once the individual segment currents are found (the time-consuming part) It is relatively straight-forward for NEC to find the radiation pattern shape, antenna gain and driving point(s) impedances. As with any modelling program the trick is to make sure the wire segment model adequately represents the actual/planned structure. Besides segment length, there are a few other rules imposed by NEC that must also be adhered to in order to obtain the correct results. Roy is absolutely right in a previous post that an antenna vendor is most likely blowing smoke by proclaiming that his/her antenna can't be modelled by a method-of-moments program like NEC. (My favorite antenna "myth busters" using NEC are Drs. John Belrose and Gerald Burke). Sincerely, and 73s from N4GGO, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
Antenna Theory
J. B. Wood wrote:
One of the first hits is http://www.g3ycc.karoo.net/lattin.htm which shows a good sketch. The antenna is made from sections of 300-ohm ribbon or tubular feeder, configured as a string of quarter-wave stubs that progressively make the dipole shorter as the frequency increases. The modeling challenge is that the ribbon operates in two different modes at the same time: a radiating common mode with a velocity factor of say 0.95; and a non-radiating "stub" mode with a VF of about 0.8. The problem is to model both modes simultaneously, for the whole string of stubs, without changing the physical dimensions of the real antenna. I'm not sure if NEC can do this, but maybe Roy can comment? Hello, and Roy will probably want to weigh in here. What I can say is that if you can create a wire model of the antenna consisting of interconnected segments (ideally about 1/20 wavelength each) then NEC will find the currents in each by considering all the interactions (conductive, capacitive, inductive) between the segments. NEC doesn't care about the geometry or "modes" of the antenna - it just sees a bunch of interconnected segments distributed in 3-D space. There is no magic here as NEC is merely applying text-book electromagnetic theory That isn't a complete model of this particular antenna. The missing part is the velocity factor of the twin-lead when acting as a stub, which means that the electrical length of the stub is different from the physical length. Which of those two lengths would you use in the NEC model? The answer is easy for a single-band model; but it's not so easy to create one NEC model that will be valid for all the bands this antenna is designed to cover. -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek |
Antenna Theory
Ian White GM3SEK wrote:
The modeling challenge is that the ribbon operates in two different modes at the same time: a radiating common mode with a velocity factor of say 0.95; and a non-radiating "stub" mode with a VF of about 0.8. The problem is to model both modes simultaneously, for the whole string of stubs, without changing the physical dimensions of the real antenna. I'm not sure if NEC can do this, but maybe Roy can comment? Thanks Ian, for the spelling and for jogging my memory on the subject. I believe you have hit the nail on the head. EZNEC apparently cannot "model both modes simultaneously". -- 73, Cecil http://www.w5dxp.com |
Antenna Theory
J. B. Wood wrote:
Roy is absolutely right in a previous post that an antenna vendor is most likely blowing smoke by proclaiming that his/her antenna can't be modelled by a method-of-moments program like NEC. (My favorite antenna "myth busters" using NEC are Drs. John Belrose and Gerald Burke). Sincerely, and 73s from N4GGO, The problem that EZNEC has with this antenna, as I understand it, is that the same pair of parallel wires has common-mode current in them on some frequencies resulting in a high VF and differential mode current in them on other frequencies resulting in a low VF. I don't know how to model changing VF's with EZNEC without changing the physical length of the wires as frequency is changed. -- 73, Cecil http://www.w5dxp.com |
Antenna Theory
Ian White GM3SEK wrote:
The answer is easy for a single-band model; but it's not so easy to create one NEC model that will be valid for all the bands this antenna is designed to cover. Could a model be created for each band? What would be the VF of the wire when 50% of the current was common-mode and 50% of the current was differential mode? -- 73, Cecil http://www.w5dxp.com |
Antenna Theory
In article , Ian White GM3SEK
wrote: That isn't a complete model of this particular antenna. The missing part is the velocity factor of the twin-lead when acting as a stub, which means that the electrical length of the stub is different from the physical length. Which of those two lengths would you use in the NEC model? The answer is easy for a single-band model; but it's not so easy to create one NEC model that will be valid for all the bands this antenna is designed to cover. Hello, Ian. You would use the physical length for all wires that are interconnected and/or separated by free space. After all, that's what we're trying to model. You still must decide how many electrically-small segments would constitute, say, a 1 foot length of conductor. The higher the frequency, the more segments you will need. If transmission line is to be connected between segments, NEC has tools for doing that. BTW, my experience is with LLNL's NEC-4 (FORTRAN-77 source code) rather than the commercially-available packages. Sincerely, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
Antenna Theory
In article , Cecil Moore
wrote: The problem that EZNEC has with this antenna, as I understand it, is that the same pair of parallel wires has common-mode current in them on some frequencies resulting in a high VF and differential mode current in them on other frequencies resulting in a low VF. I don't know how to model changing VF's with EZNEC without changing the physical length of the wires as frequency is changed. Hello, Ian, and I guess I don't see how this is a problem provided you have described the antenna geometry correctly and have chosen the appropriate number of segments at the evaluation frequencies of interest. Velocity factor and other electromagnetic phenomena are implicit in the solution for the current distribution on the structure. Now, if dielectric material is distributed in the structure that does complicate things a bit. Can you point me to some further info on the antenna in question? I trust this is not another one of those situations where there is an attempt by vendors to "reinterpret" Maxwell's equations (or explain things that Maxwell "left out"). I would encourage frequent users of NEC to subscribe to the mailing list at . Lots of practical discussion there IMHO. Sincerely, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
Antenna Theory
"Cecil Moore" wrote in message om... Ian White GM3SEK wrote: The modeling challenge is that the ribbon operates in two different modes at the same time: a radiating common mode with a velocity factor of say 0.95; and a non-radiating "stub" mode with a VF of about 0.8. The problem is to model both modes simultaneously, for the whole string of stubs, without changing the physical dimensions of the real antenna. I'm not sure if NEC can do this, but maybe Roy can comment? Thanks Ian, for the spelling and for jogging my memory on the subject. I believe you have hit the nail on the head. EZNEC apparently cannot "model both modes simultaneously". -- 73, Cecil http://www.w5dxp.com |
Antenna Theory
On Tue, 3 Oct 2006 13:33:21 +0100, Ian White GM3SEK
wrote: The missing part is the velocity factor of the twin-lead when acting as a stub, which means that the electrical length of the stub is different from the physical length. Which of those two lengths would you use in the NEC model? Hi Ian, This is simply a veiled expectation for EZNEC not being able to model the "special attributes" of the antenna. The answer is easy for a single-band model; but it's not so easy to create one NEC model that will be valid for all the bands this antenna is designed to cover. The answer is even easier than that. The Lattin antenna has a basic rationale behind it that does not demand two different lengths: stub tuning which is an electrical quality (not physical). What acts like a stub, acts like a stub for any wire mesh modeling a stub. The Lattin antenna does not exhibit this action to any correlation to frequencies attributed to it. It is THAT simple. Appeals to physical size relate only to the far field radiation characteristic. Even here the Lattin is noted for being un-notable. You don't need to worry about velocity factor, or dielectrics when the basic rationale calls it a stub and it doesn't work as a stub for bare wire. The Franklin antenna employs some of the same geometries and nowhere makes a desperate grab for theoretical underpinnings called stubs. Yet the Franklin delivers as promised if or when you add dielectrics. The Franklin's simple distribution of currents (which works for every antenna) works without having stray wires tacked on like Irish Pennants. There are more apologists for this design than working Lattins flying their tuning wires (in their notorious disregard for the rationale of the design). The fact of the matter is that modeling lays bear the myth. 73's Richard Clark, KB7QHC |
Antenna Theory
Richard Clark wrote:
You don't need to worry about velocity factor, or dielectrics when the basic rationale calls it a stub and it doesn't work as a stub for bare wire. Consider a folded dipole made from Wireman #562. The radiating part of the antenna has a VF of 0.95 and the feedline has a VF of 0.8. Identical wires with 19% different VFs. How does EZNEC handle that? -- 73, Cecil http://www.w5dxp.com |
Antenna Theory
Last things first - I just read John's later posting, and rescued this message from the out-tray. I hope this message will supply the extra detail you need, John. Just one final thing: I trust this is not another one of those situations where there is an attempt by vendors to "reinterpret" Maxwell's equations (or explain things that Maxwell "left out"). Oh no. On that topic, I am an ironclad hardliner! If you remember where we came in, Roy was mentioning a few types of antennas that it is acknowledged cannot be modeled with NEC-based programs. Cecil then inquired if the Lattin was one of those... and, subject to correction, I think it may be (if you require one model that covers all frequencies). But every one of this small number of exceptions is for a clear and understandable reason, so they don't change the big picture, which is that "almost" all types of wire/rod antennas CAN be modeled accurately by NEC. If anyone thinks NEC doesn't work for their own pet antenna, the burden of proving that is entirely on them. We now hand you back to the original reply... J. B. Wood wrote: In article , Ian White GM3SEK wrote: That isn't a complete model of this particular antenna. The missing part is the velocity factor of the twin-lead when acting as a stub, which means that the electrical length of the stub is different from the physical length. Which of those two lengths would you use in the NEC model? The answer is easy for a single-band model; but it's not so easy to create one NEC model that will be valid for all the bands this antenna is designed to cover. Hello, Ian. You would use the physical length for all wires that are interconnected and/or separated by free space. After all, that's what we're trying to model. Certainly... but most of this antenna consists of pairs of parallel wires that are physically interconnected, but are *not* separated by free space - the wires that are part of the twin-lead. You still must decide how many electrically-small segments would constitute, say, a 1 foot length of conductor. The higher the frequency, the more segments you will need. If transmission line is to be connected between segments, NEC has tools for doing that. BTW, my experience is with LLNL's NEC-4 (FORTRAN-77 source code) rather than the commercially-available packages. Sincerely, Sorry, that model still wouldn't work (unless I've misunderstood the principle of this antenna). The whole point of modeling a multiband antenna is to get one model that is good for all its operating frequencies. That allows us to check that the SWR dips at all the right places, and to find out what's really happening in the supposedly "non-operative" parts of the antenna. AIUI, the central part of the Lattin antenna is a half-wave dipole at the highest operating frequency - call it 30MHz, so the wavelength is a nice round number, 10.0m. Outside each end of this 5m long dipole is a quarter-wave stub made of twin-lead. These stubs are resonant at 30MHz, so they cut off the rest of the antenna (much like a trap) leaving just the central half-wave dipole as the only functional part at of the antenna. The normal differential-mode velocity factor of the twin-lead applies to this stub, so its correct physical length is not a quarter-wavelength (2.5m) but about 0.8*2.5m = 2.0m. Moving to the next lower operating frequency, there will be another pair of quarter-wave resonant stubs isolating the ends of a half-wave resonant dipole. But part of the physical length of this longer dipole is the 30MHz stub. If you model it at its true physical length of 2.0m, this will be correct for the lower frequency, but if you ignore the differential-mode velocity factor, the stub won't be resonant at 30MHz any more. So the question remains: how can we model this "simplest" case of a two-band Lattin antenna, in a way that will be accurate at both frequencies? If we can solve that one, then extending it to the full 5-band Lattin should be child's play :-) -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek |
Antenna Theory
Ian White GM3SEK wrote:
So the question remains: how can we model this "simplest" case of a two-band Lattin antenna, in a way that will be accurate at both frequencies? A different model for each band that takes the varying VFs into account? -- 73, Cecil http://www.w5dxp.com |
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