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Is antenna a transducer to 377 ohms?
The impedance of free space / air is said to be 377 ohms. Impedance is ratio
of E/H. The feedpoint impedance of an antenna is usually 50 or 75 ohms. Can an antenna ever be regarded as a transducer that transforms a radio wave from 50 ohms to 377 ohms i.e. provides an impedance transformation? With a long tapered antenna, the feedpoint is at 50 ohms. Is the end of the antenna at 377 ohms to launch the wave easily into free space? In this case, antenna is a travelling wave antenna e.g. broad bandwidth biconical. Does the impedance gradually change from 50 ohms to 377 ohms over the length of the antenna? The impedance of the end of an antenna (open circuit), where it is a high voltage point, is usually 5K or 10K ohms. |
Is antenna a transducer to 377 ohms?
David wrote:
The feedpoint impedance of an antenna is usually 50 or 75 ohms. The feedpoint impedance of a resonant standing wave antenna is a result of interference/superposition and is a virtual impedance equal to (Vfor-Vref)/(Ifor+Iref) The characteristic impedance of a standing wave antenna is actually several hundred ohms. -- 73, Cecil http://www.w5dxp.com |
Is antenna a transducer to 377 ohms?
On Mon, 4 Sep 2006 20:36:00 +0100, "David" nospam@nospam wrote:
The impedance of free space / air is said to be 377 ohms. Impedance is ratio of E/H. Hi David, True only in free space. The feedpoint impedance of an antenna is usually 50 or 75 ohms. Rarely true, but to the point of your posting, this is not substantially wrong. Can an antenna ever be regarded as a transducer that transforms a radio wave from 50 ohms to 377 ohms i.e. provides an impedance transformation? A transducer changes energy between domains. That is from electrical to acoustic, or back again. Or between electrical to photo-electric, or back again. And so on. What you are describing is transforming and the appropriate device name would be transformer, not transducer. To the intent of your statement, yes, an antenna is a transformer. With a long tapered antenna, the feedpoint is at 50 ohms. Is the end of the antenna at 377 ohms to launch the wave easily into free space? The ends of an antenna are no more important than the middle. Radiation occurs on the basis of the entire radiator, not simply "some" parts. In this case, antenna is a travelling wave antenna e.g. broad bandwidth biconical. Does the impedance gradually change from 50 ohms to 377 ohms over the length of the antenna? In fact it is quite the opposite. The biconical is a classic constant impedance structure. Thin antennas such as the typical doublet or dipole are non-linear. The impedance of the end of an antenna (open circuit), where it is a high voltage point, is usually 5K or 10K ohms. This is a characteristic, not an explanation. 73's Richard Clark, KB7QHC |
Is antenna a transducer to 377 ohms?
David:
[snip] "David" nospam@nospam wrote in message ... The impedance of free space / air is said to be 377 ohms. Impedance is ratio of E/H. The feedpoint impedance of an antenna is usually 50 or 75 ohms. Can an antenna ever be regarded as a transducer that transforms a radio wave from 50 ohms to 377 ohms i.e. provides an impedance transformation? With a [snip] The answer is a "considered" yes! Although the [so-called] term "characteristic impedance", often labelled as Zo, and units of Ohms are often used to describe a certain characteristic of a propagation media in field theory, that governs the ratio of the E to H fields propagating in the media. This "characteristic impedance Zo" is not the same thing as "driving point or feed point impedance Z" in circuit theory. Although closely related, circuit theoretic concepts and field theoretic concepts are different views of electromagnetic phenomena. Characteristic impedance Zo and the units of Ohms are often used as the "name" for the square root of the ratio of mu the magnetic permeability is [u = 1.257E-7 for free space] to epsilon the electric permittivity of a propagating media [e = 8.85E-12 for free space] Maxwell's celebrated equations then result in the fact that... Zo = E/H = sqrt[u/e] = sqrt[(1.257E-7)/(8.85E-12)] = 376.7 Ohms ~ 120pi Ohms This Zo is not the same as a "feedpoint impedance" Z which is the ratio of voltage V to current I. Z = V/I Ohms. That said it should be recognized that any radiating antenna is "immersed" in a propagating media, usually free space, and the u and e of that media do have an important affect on the "characteristic impedance, or surge impedance" of the antenna which will in turn affect the driving point or feedpoint impedance of the antenna. For instance it is well known that the resonant feedpoint impedance (Ratio of V to I) at the center of a half wave dipole in free space is 73 Ohms. If that dipole were placed in another medium other than free space with correspondingly different u and e, the driving point impedance of the dipole would definitely be affected. So would it's resonant frequency, etc... And so in that sense, an antenna may be considered to be a transducer and not a transformer. Antennas may then be viewed as transducers that transduce the circuit theoretic variables of electric currents and voltages flowing in and between conductors into field theoretic variables of electric and magnetic fields flowing through a propagation media. And... indeed there is a "reaction" between the u and e of the media in which the antenna is immersed and the currents and voltages flowing in and on the antenna. The 73 Ohm driving point impedance of a free space half wave resonant dipole [in the ideal case this is the radiation resistance] is a direct result of the u and e of the free space in which the antenna is immersed. If all other things were held constant and the values of u and e of the medial were changed [i.e. move the antenna from free space where u/e=377 to be under water where u/e=x??? the driving point impedance of the antenna would most certainly change! [snip] long tapered antenna, the feedpoint is at 50 ohms. Is the end of the antenna at 377 ohms to launch the wave easily into free space? In this case, antenna is a travelling wave antenna e.g. broad bandwidth biconical. Does the impedance gradually change from 50 ohms to 377 ohms over the length of the antenna? [snip] No! Not really. Surprisingly, the actual surge or characteristic impedance Zo of a single wire antenna in free space, considered as a one wire transmission line placed high over a ground plane [the earth] is actually in the neighbourhood of several hundred Ohms... say 600Ohms or so. The exact value of Zo is easily calculated by well known transmission line formulas, that assume TEM mode propagation on the line, and this Zo basically depends upon the height over ground and the diameter of the wire. This is not a driving point impedance but is a "surge impedance". The driving point impedance of the single wire transmission line depends upon where and at what frequency it is "driven" by a source. Of course because this single wire is quite distant from it's return path [ground] this single wire transmission line is "leaky". That is it radiates and loses, or dissipates, power to some extent, as opposed to what it might do if it were placed very close to the ground where there was a nearby field "cancelling" current flow. [a microstrip transmission line for instance]. We know that if this single wire transmission line high above the earth is driven by a source it will exhibit a driving point impedance that depends upon its length relative to the wavelength of the driving voltage or current. [73 Ohms resistive if it is a 1/2 wave, some other in general complex Z if it is not 1/2 wave. [snip] The impedance of the end of an antenna (open circuit), where it is a high voltage point, is usually 5K or 10K ohms. [snip] I believe that you are referring to the driving point impedance of an end fed half wave dipole which is certainly high and in that neighbourhood. This is not a characteristic or a surge impedance. And so in summary... An antenna may be thought of as a transducer between a circuit theoretic electro-magnetic venue and wave propagation in a propagating media, but the relationship between the circuit driving point impedance and the characteristic impedance of the media is quite complex and is certainly not a simple linear relationship such as found in a transformer or other device. As far as I understand there is no practical application that has ever required anyone to quantitatively determine the exact relationship between the Zo of a propagating media and the driving point impedance Z of an antenna that is immersed in that media. In my opinion such a determination would be a very difficult experimental/engineering exercise. The experimenal problem is one of how does one vary the Zo of a media while measuring the effect on the Z at the driving point? Here's a thought experiment... Immerse an antenna in a liquid media with a given u and e in an anechoic tank then drive the antenna with a generator while measuring the driving point impedance (V and I) and then pour or mix in some other liquid with different u and e and observe the change in Z. Would that work? It could also be accomplished numerically on a computer by using a program [like the NEC programs] based on solving Maxwell's partial differential equations iteratively. As far as I know no one has ever attempted to do this... and notwithstanding the possibility for "invention" or "discovery" I might ask, why would one want to do this? Hey it might make a good Ph.D. or M.Sc. thesis... but what is the practical application? For all practical purposes, the characteristic impedance of the media in which antennas are immersed never changes! Who cares how Z varies when Zo varies? Thoughts, comments? -- Pete k1po |
Is antenna a transducer to 377 ohms?
Peter O. Brackett wrote:
. . . In my opinion such a determination would be a very difficult experimental/engineering exercise. The experimenal problem is one of how does one vary the Zo of a media while measuring the effect on the Z at the driving point? Here's a thought experiment... Immerse an antenna in a liquid media with a given u and e in an anechoic tank then drive the antenna with a generator while measuring the driving point impedance (V and I) and then pour or mix in some other liquid with different u and e and observe the change in Z. Would that work? It could also be accomplished numerically on a computer by using a program [like the NEC programs] based on solving Maxwell's partial differential equations iteratively. As far as I know no one has ever attempted to do this... and notwithstanding the possibility for "invention" or "discovery" I might ask, why would one want to do this? Hey it might make a good Ph.D. or M.Sc. thesis... but what is the practical application? . . It would be one of the easiest degrees ever attained. NEC-4 allows setting the primary medium to any (reasonable) value of conductivity and permittivity, so you can have the answer in seconds with a free space analysis. Alternatively, you can bury the antenna deep in NEC-4's ground medium and define the ground characteristics for your test. I did a short consulting job a while back for some people interested in transmitting RF for short distances under water. Immersing the antenna eliminates the substantial signal loss incurred by reflection at the air-water interface when the antenna is out of the water. And antenna system design requires knowledge of the antenna feedpoint Z. I've seen numerous papers in the IEEE publications about antennas immersed in other media such as a plasma, and know that antennas buried in the ground are used. So it's of considerable practical interest. Roy Lewallen, W7EL |
Is antenna a transducer to 377 ohms?
Roy, David:
[snip] It could also be accomplished numerically on a computer by using a program [like the NEC programs] based on solving Maxwell's partial differential equations iteratively. As far as I know no one has ever attempted to do this... and notwithstanding the possibility for "invention" or "discovery" I might ask, why would one want to do this? Hey it might make a good Ph.D. or M.Sc. thesis... but what is the practical application? It would be one of the easiest degrees ever attained. NEC-4 allows setting the primary medium to any (reasonable) value of conductivity and permittivity, so you can have the answer in seconds with a free space analysis. Alternatively, you can bury the antenna deep in NEC-4's ground medium and define the ground characteristics for your test. I did a short consulting job a while back for some people interested in transmitting RF for short distances under water. Immersing the antenna eliminates the substantial signal loss incurred by reflection at the air-water interface when the antenna is out of the water. And antenna system design requires knowledge of the antenna feedpoint Z. I've seen numerous papers in the IEEE publications about antennas immersed in other media such as a plasma, and know that antennas buried in the ground are used. So it's of considerable practical interest. Roy Lewallen, W7EL [snip] Well thanks for that input Roy, that's very interesting and of course a useful application. But for the "thesis" idea I was thinking of something a little more "challenging", e.g. Clearly the antenna "driving point impedance" Z = R +jX is a complex function f (Zo) of the "characteristic impedance" Zo, where in general Zo = Ro +j Xo = sqrt[u/e] of the medium in which the antenna is immersed. Clearly this function f(Zo) is the "transducer" function that David [The OP] was seeking. It's clear of course that f(zo) will also be a function of complex frequency p = s + jw. Now expressing this functionality as: Z(p, Zo) = f (p, Zo) One might ask [the thesis candidate, (grin)] to derive/discover and tell us... What. precisely, is the functional form of this complex "transducer" function f that takes Zo to Z [377 Ohms to 73 Ohms! (grin)] Is f(Zo) a simple linear function? [e.g. like a transformer turns ratio as the OP David had asked] or perhaps... A non-linear function? or maybe... A differential or integral relationship? What? Except for a few isolated niche applications, such as those you mentioned having consulted on, I can't really think of any practical applications that demand knowledge of the functional form of "f". Which is likely why this subject is not mentioned in antenna textbooks or widely discussed. i.e. No one ever needed to know it and so no one worked out this relationship or even investigated it... Simply a matter of supply and demand (grin)! We have Ohm's Law and other such well known relationships such as V = IR because there was a demand by "scienticulists" (grin) to know these relationships to do real Engineering, i.e. build stuff they needed out of stuff they could get. The discovery of the functional form "f" of this relationship might perhaps be at least suitably hard for a Master's Thesis, a good challenge, and I believe that it is suitably "academic", since there is very little use for it (grin). What exactly is "f (Zo)"? Thoughts, comments. -- Pete k1po Indialantic By-the-Sea, FL |
Is antenna a transducer to 377 ohms?
Peter O. Brackett wrote:
What? ... What exactly is "f (Zo)"? Thoughts, comments. Peter, I for one, have missed your style. Consider the following: I(s) +--------------------------------------------open | V(s) 1/4 wavelength, Z0=600 ohms | +--------------------------------------------open Given: The ratio of V(s)/I(s) is 50+j0 ohms. Can you solve for f(Z0)? -- 73, Cecil http://www.w5dxp.com |
Is antenna a transducer to 377 ohms?
David wrote: The impedance of free space / air is said to be 377 ohms. Impedance is ratio of E/H. The feedpoint impedance of an antenna is usually 50 or 75 ohms. Can an antenna ever be regarded as a transducer that transforms a radio wave from 50 ohms to 377 ohms i.e. provides an impedance transformation? I'd like to take a somewhat different tack on handling this question. It is a wave versus photon perspective. It seems to be very easy to slip into the thought mode where we view radio transmissions as a wave phenomena in some sort of space media. After all, most of the math we use for electromagnetic signal propagation work quite well assuming that. Maxwells math still works wonderfully even today, a century after Einstien described photons. Maxwell died more than a decade before Einstien published his seminal papers though. The real model of the radio operation is for there to be alternating electrostatic and magnetic fields surounding an antenna when it is driven by an RF power source. Through some yet poorly understood physical mechanism, either envolving the acceleration of electrons in the antenna's conductor or from the alternating E and H fields, photons are flung off the antenna. I interpret this to mean that what we call the "near field" around an antenna is the volume around an antenna where the electrostatic and magnetic fields are at an energy level significantly above natural background levels. The "far field" I interpret to mean where RF energy exists as a photon flux. To describe an antenna as a transducer is probably correct symantically. It does convert between electrical energy in the form of RF current and voltage and photons. This is both for transmitting and receiving. The 377 ohm thing though is a function of the releative intensities of the electrostatic and magnitic fields surrounding an antenna. It is a constant like pi or e. It is an emperically measured characteristic of our universe. It does not, however mean that an antenna transforms a feed impedance to this impedance. It simply tells us what to expect for when we feed an RF current into an antenna. It is much like knowing that a 1 foot diameter wheel will travel 3.14159265... feet for every revolution. Now, a point worth noting is that while RF current in a conductor produces photons, photons produce RF current in conductors. That, of course, is why antennas are able to operate for both receiving and transmitting. That is also why radio signals bounce. Photons are absorbed by objects such as wires or dirt and RF currents are produced. Those currents, in turn, generate new photons. The conductivity and dielectric constant of the absorbing material determine the amplitude and phasing of the current and thus the primary direction of emission of the new photons. So... Yes the antenna is a transducer. No, it does not transform 50 ohms into 377 ohms. 377 ohms refers to the eletrostatic and magnentic fields as they exist in the near field of an antenna or conductor. It does not refer to what is going on electrically in the antenna conductors. Anyway, that's my take on the subject. Gary N0GW |
Is antenna a transducer to 377 ohms?
On 10 Sep 2006 14:53:25 -0700, "N0GW" wrote:
Hi Gary, A number of comments: I'd like to take a somewhat different tack on handling this question. It is a wave versus photon perspective. Photons are no less wave oriented than RF - nor more. Also, RF is no less "corpuscular" (Einstein's term) than photons. The real model of the radio operation is for there to be alternating electrostatic and magnetic fields surounding an antenna when it is driven by an RF power source. Through some yet poorly understood physical mechanism, either envolving the acceleration of electrons in the antenna's conductor or from the alternating E and H fields, photons are flung off the antenna. Hardly "flung off" and even less, "poorly understood." I interpret this to mean that what we call the "near field" around an antenna is the volume around an antenna where the electrostatic and magnetic fields are at an energy level significantly above natural background levels. The background levels of EM fields are predominantly in the milli-micro-nano Kelvins of temperature. Nearly everything produces an energy level significantly above that. The "far field" I interpret to mean where RF energy exists as a photon flux. Photons don't exist in the near field? A flux is simply the bulk of them; like one electron flowing, or a trillion, is current. The 377 ohm thing though is a function of the releative intensities of the electrostatic and magnitic fields surrounding an antenna. It is a function of permittivity and permeability which exists even if the fields are not there. It is a constant like pi or e. Unless you happen upon something other than a vacuum. It is an emperically measured characteristic of our universe. It is empirically measured, but that does not create its value. Now, a point worth noting is that while RF current in a conductor produces photons, photons produce RF current in conductors. How much current is created by your antenna in sunlight? That, of course, is why antennas are able to operate for both receiving and transmitting. That is also why radio signals bounce. Photons are absorbed by objects such as wires or dirt and RF currents are produced. An 10 meter doublet of 1mm wire is exposed to 5 - 10 W of power. How much can you capture and bottle due to this production of current in conductors? Those currents, in turn, generate new photons. Photons are generated on the basis of an electron in an excited state falling to a lower ground state, not by current flow. The conductivity and dielectric constant of the absorbing material determine the amplitude and phasing of the current and thus the primary direction of emission of the new photons. Photons do not follow any particular channel of radiation, not unless you have a very large lens (few antennas do). So... Yes the antenna is a transducer. No, it does not transform 50 ohms into 377 ohms. 377 ohms refers to the eletrostatic and magnentic fields as they exist in the near field of an antenna or conductor. In fact, in the near field of an antenna, there is nothing that resembles 377 Ohms of Z. 73's Richard Clark, KB7QHC |
Is antenna a transducer to 377 ohms?
On Sun, 10 Sep 2006 16:01:34 -0700, Richard Clark
wrote: In fact, in the near field of an antenna, there is nothing that resembles 377 Ohms of Z. The page at: http://home.comcast.net/~kb7qhc/ante...pole/index.htm dramatically reveals that the near fields fluctuate wildly from 377 Ohms, and I have restricted my analysis to those values falling at roughly 100 Ohms or 1000 Ohms (the hot spots marking the feed point region and the tips of the dipole). Other antenna design's modification of the 377 near field around them can be observed at: http://home.comcast.net/~kb7qhc/ante...elds/index.htm 73's Richard Clark, KB7QHC |
Is antenna a transducer to 377 ohms?
N0GW wrote:
. . . So... Yes the antenna is a transducer. No, it does not transform 50 ohms into 377 ohms. 377 ohms refers to the eletrostatic and magnentic fields as they exist in the near field of an antenna or conductor. It does not refer to what is going on electrically in the antenna conductors. . . . 377 ohms does not describe the E and H fields in the near field. 377 ohms is the ratio of E to H in the *far field* when the medium is free space or, for practical purposes, air. In the near field, the ratio of E to H can be not only far from 377 ohms, but it's commonly also complex (that is, E and H not in time phase). For an illustration, model a short dipole or small loop with EZNEC or NEC-2, and use the near field analysis to find E and H at some point close to the antenna (within a fraction of a wavelength). When you divide E by H, you'll get a wide variety of results(*) depending on the type of antenna and the observation point. But as you get farther and farther from *any* antenna, you'll find that the ratio always converges to 377 ohms, purely real (that is, the E and H fields in time phase). (*) The ratio of E to H is called the "wave impedance". In the far field, and only in the far field, it equals the intrinsic impedance of the medium. And, as Gary and others have said, this shouldn't be confused with the ratio of voltage to current at an antenna feedpoint. They are not at all the same thing, in spite of having the same units of ohms. Roy Lewallen, W7EL |
Is antenna a transducer to 377 ohms?
Roy Lewallen wrote: 377 ohms does not describe the E and H fields in the near field. 377 ohms is the ratio of E to H in the *far field* when the medium is free space or, for practical purposes, air. In the near field, the ratio of E to H can be not only far from 377 ohms, but it's commonly also complex (that is, E and H not in time phase). For an illustration, model a short dipole or small loop with EZNEC or NEC-2, and use the near field analysis to find E and H at some point close to the antenna (within a fraction of a wavelength). When you divide E by H, you'll get a wide variety of results(*) depending on the type of antenna and the observation point. But as you get farther and farther from *any* antenna, you'll find that the ratio always converges to 377 ohms, purely real (that is, the E and H fields in time phase). Yes, I agree with that completely Roy. I apologize for simplifying my response so much as to not mention this. I was trying to answer the question at the same level as was asked. I did not mean to offend the more mathematically astute members of this group. I will stand by my comment that radiation from antennas, no matter how well predicted mathematically, is not well understood at a subatomic level. I personally prefer a model that assumes photons result from electron acceleration (or deceleration or energy level decrease). There are obviously competing models. Gary N0GW |
Is antenna a transducer to 377 ohms?
N0GW wrote:
Roy Lewallen wrote: 377 ohms does not describe the E and H fields in the near field. 377 ohms is the ratio of E to H in the *far field* when the medium is free space or, for practical purposes, air. In the near field, the ratio of E to H can be not only far from 377 ohms, but it's commonly also complex (that is, E and H not in time phase). For an illustration, model a short dipole or small loop with EZNEC or NEC-2, and use the near field analysis to find E and H at some point close to the antenna (within a fraction of a wavelength). When you divide E by H, you'll get a wide variety of results(*) depending on the type of antenna and the observation point. But as you get farther and farther from *any* antenna, you'll find that the ratio always converges to 377 ohms, purely real (that is, the E and H fields in time phase). Yes, I agree with that completely Roy. I apologize for simplifying my response so much as to not mention this. I was trying to answer the question at the same level as was asked. I did not mean to offend the more mathematically astute members of this group. I will stand by my comment that radiation from antennas, no matter how well predicted mathematically, is not well understood at a subatomic level. I personally prefer a model that assumes photons result from electron acceleration (or deceleration or energy level decrease). There are obviously competing models. I'm not the least bit offended; I just corrected a statement which wasn't true. Intelligent discussion of the subatomic and quantum physical aspects of electromagnetic radiation are for people mathematically much more astute than I, so I'll leave that for you. Roy Lewallen, W7EL |
Is antenna a transducer to 377 ohms?
Cecil:
[snip] "Cecil Moore" wrote in message om... Peter O. Brackett wrote: What? ... What exactly is "f (Zo)"? Thoughts, comments. Peter, I for one, have missed your style. Consider the following: I(s) +--------------------------------------------open | V(s) 1/4 wavelength, Z0=600 ohms | +--------------------------------------------open Given: The ratio of V(s)/I(s) is 50+j0 ohms. Can you solve for f(Z0)? -- 73, Cecil http://www.w5dxp.com [snip] Heh, heh... No of course not, you would need more than just this one experiment/measurement to determine Zo. The case you depict is at a "singularity" so to speak and is a "pathalogical case", because with the 1/4 wave line open at the far end, one sees only a short circuit with Z = V/I = 0.0 [zero] as the driving point impedance and one needs more equations than just this one singular situation to solve for Zo of the line. In fact though if you "vary" the Zo of the line, you would then see a change in the driving point impendance Z of the line from zero to another value. From such a thought experiment one should be able to formulate an expression for Z(Zo). Thoughts comments... -- Pete k1po Indialantic By-the-Sea, FL. |
Is antenna a transducer to 377 ohms?
Peter O. Brackett wrote:
"Cecil Moore" wrote: I(s) +--------------------------------------------open | V(s) 1/4 wavelength, Z0=600 ohms | +--------------------------------------------open Given: The ratio of V(s)/I(s) is 50+j0 ohms. Can you solve for f(Z0)? The case you depict is at a "singularity" so to speak and is a "pathalogical case", because with the 1/4 wave line open at the far end, one sees only a short circuit with Z = V/I = 0.0 [zero] as the driving point impedance and one needs more equations than just this one singular situation to solve for Zo of the line. Sorry I wasn't clear. One sees 50 ohms, not a short circuit, and the Z0 of the line is given at 600 ohms. The line is not lossless and not even low loss. There is enough resistance in the stub wire to cause a 50+j0 ohm impedance looking into the stub. I was just wondering what is the nature of your f(Z0) function. -- 73, Cecil http://www.w5dxp.com |
Is antenna a transducer to 377 ohms?
Cecil et al:
[snip] the stub. I was just wondering what is the nature of your f(Z0) function. -- 73, Cecil http://www.w5dxp.com [snip] I like working with Cecil! Like a Zen Master's rehtorical approach to facilitating understanding, Cecil's approach to this seemingly paradoxical "circuit-to-wave transducer" question is quite illuminating. Cecil has neatly sidestepped the fact that, even for the simplest practical antennas, elementary analytic formulae for antenna driving point impedances have never been discovered. Let alone formulae that explicitly show Zo as an independent variable. This [obscure?] fact [of delinquint formulae] often comes as a surprise to most electromagnetic novitiates, I know it did to me. [Aside: As far as I know, the fact that no one has ever worked out an exact analytic formula for the driving point impedance of a simple practical half wave dipole, is not a problem in practices since other approximate and/or "sledge hammer" style numerical methods provide appropriately accurate answers to all practical Engineering questions about such matters.] However, as a "seeker of truth", Cecil has noted an easier path as an approach to the apparently paradoxical question of the relationship of driving point impedance to the wave or characteristic impedance of free space or any other propagating media. Cecil has zeroed in on an alternative that might give us some insight! Namely the relatively simple "exact" formula, first revealed by Heaviside and Kelvin approximately two hundred years ago, the celebrated formula for the driving point impedance Z = V/I of a lossless transmission line of characteristic [surge] impedance Zo terminated in a load impedance ZL. This driving point impedance is given by the surprising simple relation... Z(Zo) = Zo[(ZL*cos(theta) + jZo*sin(theta))/(Zo*cos(theta) + jZL*sin(theta))] (1) Where theta = 2*pi*(d/lambda) is the relative fractional length of the transmission line, where d is the line length and lambda is the wavelength of a sinusoidal signal supported on the line at the particualr frequency of interest. Zo of course is the characteristic [surge or wave] impedance of the line. It is also well known [Again Kelvin and Heaviside] that Zo can be simply expressed in terms of the fundamental transmission line parametric constants [R, L, C, G] by the [equally] celebrated formula for the characteristic [surge or wave] impedance of the transmission line as Zo = sqrt[(R + jwL)/(G + jwC)]; Where, in the lossless case R=G=0.0, Zo - sqrt(L/C). [Aside: The terms Impedance, and Reactance were first defined by (Reg Edward's hero) Oliver Heaviside. I wonder if an equally simple formula for the driving point impedance in terms of the Zo of free space for some simple antenna is lying out there somewhere waiting to be discovered (grin). ] As can readily be seen, the driving point impedance Z is a function of the dependent variable Zo and... although the effect of this relationship is often referred to as an trasmission line impedance "transformer", the analogy between the so-called "transmission line transformer" (or should we say "transducer") described by (1) falls short of the simple turns ratio relationship where Z = ZL*N^2. To gain insight here, Cecil has obliquely suggested that, instead of searching for an antenna formula, that we invert the celebrated formula (1) and use it to determine unknown characteristic impedance Zo by assuming ZL known, and measuring Z. Inverting formula (1) we obtain the following relationship. Zo(Z) = ZL[(cos(theta) - jZ*sin(theta))/(Zcos(theta) - jsin(theta))] (2) [Aside: Apart from the fact that line parameters L,C are also implicit in the wavelength, Cecil is this right?] Thus we see that the relationship between Zo and Z is not a simple linear relationship as for the common transformer, but instead is, what mathematicians often refer to as, a so-called "bilinear relationship. I wonder, is it possible that such a simple relationship exists for some antennas as well as transmission lines? An interesting invention... now it will be public domain (smile). One could clearly construct a sensor to measure unknown Zo's by constructing a small piece of rigid air dielectric terminated transmission line and then "immersing" the sensor in substances of unknown Zo and then determine those unknown Zo's by measuring the driving point impedance Z. The calibration curve for this Zo sensor would be the inverse relationship (2). Sigh, it's too bad there is not a simple analytical relationship like (1) for antennas, for perhaps this would address the OP's question of the relationship between 377 Ohms free space wave impedance and 73 Ohms driving point impedance more directly. On the other hand we now can see that, contrary to Roy's recent assertion up the thread (grin), that certainly an exact analytic solution to this problem is likely a challenging Ph.D. thesis topic. For... after two hundred or more years [As far as I know...] no one has yet worked out an exact simple analytical expression [similar to (1)] for the driving point impedance of any simple practical antenna. A Ph.D. thesis indeed! Thanks Cecil! Thoughts comments... -- Pete K1PO Indialantic By-the-Sea, FL |
Is antenna a transducer to 377 ohms?
Peter O. Brackett wrote:
Zo(Z) = ZL[(cos(theta) - jZ*sin(theta))/(Zcos(theta) - jsin(theta))] (2) [Aside: Apart from the fact that line parameters L,C are also implicit in the wavelength, Cecil is this right?] What got me going on this subject is months ago, someone asked what would be the feedpoint impedance of an infinitely long dipole in free space. Reg said it would be about 1200 ohms. Since that figure is obviously related directly to Z0, it got me to thinking about the similarity of dipoles to transmission lines. In fact, Balanis, in his 2nd edition "Antenna Theory" illustrates how a dipole is created by gradually opening up 1/4WL of a transmission line. That's on page 18. The current distribution on the dipole after unfolding is the same as the current distribution on the transmission line stub before unfolding. For transmission line analysis, we begin with simple lossless line formulas and then add complexity such as losses per unit length. For what we call near lossless feedlines, we often ignore the losses or at least consider them to be secondary effects. Going where angels fear to tread, I thought, why can't these same principles be applied to dipole antennas with admittedly reduced accuracy? Or as one of the r.r.a.a gurus said: "A wrong answer is better than no answer at all." :-) My thoughts didn't go to solving for Z0 as you did above. Using the well known Z0 formula for a single wire transmission line above ground, we get Z0=600 ohms for #14 wire 30 feet above ground and it certainly bears a resemblance to an infinite dipole made of #14 wire 30 feet above ground. Putting a differential balanced source in the middle of the single-wire transmission line would result in a balanced feedpoint Z0 impedance of 1200 ohms. 1/2 of this dipole resembles a 1/4WL stub. An infinite dipole is, of course, a traveling wave antenna. This is getting long but I think you can see where it is going. Make each 1/2 of the dipole equal to 1/4WL and we have the standard standing wave antenna. Analyze the 1/2 dipole as a lossy 1/4WL stub with a Z0 of 600 ohms not differentiating between radiation loss and other losses. (For this purpose, we are not interested in analyzing the radiation.) Hence, the earlier lossy stub where the impedance looking into the stub was 50 ohms and the Z0 was 600 ohms. Now quoting Balanis again, page 488 and 489: "The current and voltage distributions on open-ended wire antennas are *similar* to the standing wave patterns on open-ended transmission lines." "Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents If and Ib in Figure 10.1(a)." Figure 10.1(a) is very similar to the graphic depicting a single-wire transmission line over ground whe Z0 = 138*log(4D/d) D=height, d=wire diameter -- 73, Cecil http://www.w5dxp.com |
Is antenna a transducer to 377 ohms?
Cecil:
[snip] What got me going on this subject is months ago, someone asked what would be the feedpoint impedance of an infinitely long dipole in free space. Reg said it would be about 1200 ohms. Since that figure is obviously related directly to Z0, it got me to thinking about the similarity of dipoles to transmission lines. In fact, Balanis, in his 2nd edition "Antenna Theory" illustrates how a dipole is created by gradually opening up 1/4WL of a transmission line. That's on page 18. The current distribution on the dipole after unfolding is the same as the current distribution on the transmission line stub before unfolding. [snip] Well, as we all "know" the current wave on a dipole antenna is exceedingly close to sinusoidal, but not [exactly] sinusoidal, because if it were exactly sinusoidal it wouldn't be radiating. That [small] difference between the actual current distribution on an antenna and the actual current distribution on a transmission line is the [tell-tale] residual that separates us from an exact analytic expression for the driving point impedance of a dipole. Interesting stuff... Our man Reg [Edwards, RIP] always said that... "an antenna is just a lossy transmission line." And of course that is what it looks like approximately. Heh, heh... everyone always wanted to know the "mathematical" formulae and theory behind Reg's compact programs. He tantalized us all with a peek or two at some selections of his Turbo Pascal source code, but essentially left us all wondering... "How does he do that?" Apparently, at least one could infer so from his comments, Reg often modeled antennas as "lossy" transmission lines in some of his "nutcracker/lightweight" programs. Did he use the Heaviside/Kelvin formulae? I wonder... It would be interesting to compare how closely the input impedance of a 1/2 wave lossless feed line of appropriate Zo (Say 600 Ohms?) terminated in a 73 Ohm resistor would approximate that of a "real" dipole. At resonance it would be 73 Ohms at least. Such a comparison should be simple to check using EZNEC numerical readouts for the dipole and comparing to the numerical results obtained from the formula (1) for the input impedance of the terminated line. [snip] For transmission line analysis, we begin with simple lossless line formulas and then add complexity such as losses per unit length. For what we call near lossless feedlines, we often ignore the losses or at least consider them to be secondary effects. Going where angels fear to tread, I thought, why can't these same principles be applied to dipole antennas with admittedly reduced accuracy? Or as one of the r.r.a.a gurus said: "A wrong answer is better than no answer at all." :-) [snip] In many transmission line modeling programs [such as the ones used by the designers of xDSL modems who, unlike radio amateurs, need models that range from DC and on up over many decades of frequency range.] the fundamental transmission line parameters R, L, C, G are often replaced by [empirically derived] functions of frequency that represent "perturbations" from the constants to mimic skin and proximity effects. Both R and L are simultaneously affected by skin and proximity effects. Of course both Heaviside and Kelvin knew of these effects but could not include them in their simple derivations. Speaking of the "L" parameter and proximity... I thought the article by Gerrit Barrere KJ7KV in the most recent QEX was interesting because he points out that a large fraction of the "L" parameter in transmission lines results from the mutual inductance between and because of the proximity of the two conductors not the individual self inductance of the conductors. This is not obvious when looking at the "standard textbook" presentation/derivation of the Heaviside/Kelvin formulation for a differential section of transmission line. Such standard textbook derivations almost universally begin with a lumped differential model consisting of series R, series L, shunt C, shunt G per unit length with no mention of mutual inductance. In fact of course the "standard" model is "equivalent" to a model that explicitly exhibits the mutual inductance, [Because Leq = L1 + L2 + 2M] but it is much more physically satisfying to see the transmission line inductance presented the way Barrere did. Thoughts, comments? -- Pete K1PO Indialantic By-the-Sea, FL "Cecil Moore" wrote in message . .. Peter O. Brackett wrote: Zo(Z) = ZL[(cos(theta) - jZ*sin(theta))/(Zcos(theta) - jsin(theta))] (2) [Aside: Apart from the fact that line parameters L,C are also implicit in the wavelength, Cecil is this right?] What got me going on this subject is months ago, someone asked what would be the feedpoint impedance of an infinitely long dipole in free space. Reg said it would be about 1200 ohms. Since that figure is obviously related directly to Z0, it got me to thinking about the similarity of dipoles to transmission lines. In fact, Balanis, in his 2nd edition "Antenna Theory" illustrates how a dipole is created by gradually opening up 1/4WL of a transmission line. That's on page 18. The current distribution on the dipole after unfolding is the same as the current distribution on the transmission line stub before unfolding. For transmission line analysis, we begin with simple lossless line formulas and then add complexity such as losses per unit length. For what we call near lossless feedlines, we often ignore the losses or at least consider them to be secondary effects. Going where angels fear to tread, I thought, why can't these same principles be applied to dipole antennas with admittedly reduced accuracy? Or as one of the r.r.a.a gurus said: "A wrong answer is better than no answer at all." :-) My thoughts didn't go to solving for Z0 as you did above. Using the well known Z0 formula for a single wire transmission line above ground, we get Z0=600 ohms for #14 wire 30 feet above ground and it certainly bears a resemblance to an infinite dipole made of #14 wire 30 feet above ground. Putting a differential balanced source in the middle of the single-wire transmission line would result in a balanced feedpoint Z0 impedance of 1200 ohms. 1/2 of this dipole resembles a 1/4WL stub. An infinite dipole is, of course, a traveling wave antenna. This is getting long but I think you can see where it is going. Make each 1/2 of the dipole equal to 1/4WL and we have the standard standing wave antenna. Analyze the 1/2 dipole as a lossy 1/4WL stub with a Z0 of 600 ohms not differentiating between radiation loss and other losses. (For this purpose, we are not interested in analyzing the radiation.) Hence, the earlier lossy stub where the impedance looking into the stub was 50 ohms and the Z0 was 600 ohms. Now quoting Balanis again, page 488 and 489: "The current and voltage distributions on open-ended wire antennas are *similar* to the standing wave patterns on open-ended transmission lines." "Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents If and Ib in Figure 10.1(a)." Figure 10.1(a) is very similar to the graphic depicting a single-wire transmission line over ground whe Z0 = 138*log(4D/d) D=height, d=wire diameter -- 73, Cecil http://www.w5dxp.com |
Is antenna a transducer to 377 ohms?
Richard:
[snip] "Richard Clark" wrote in message ... On Sun, 10 Sep 2006 16:01:34 -0700, Richard Clark wrote: In fact, in the near field of an antenna, there is nothing that resembles 377 Ohms of Z. [snip] Correct, but don't we all believe that the wave impedance of "free space" is approximately 377 Ohms... Everywhere... Even in the near field of an antenna. That is an antenna itself has no effect on the fundamental u and e of the media in which it is immersed. u and e are defined only in terms of and as affecting "plane wave" [TEM mode?] propagation, and... After all the antenna is very small, and free space is very large (grin), and so a tiny antenna cannot change u and e everywhere! The fields E and H in the "near region" of an antenna where the waves are not "plane" on the other hand may not be related by 377 Ohms, simply because the waves emanating from the "near" antenna are not plane, but... There might just also be plane waves passing through identically the same region of space, say emanating from a more distant antenna. The ratio for those plane E and H fields will indeed be 377 Ohms over the exact same region of space where Zo is different because of simultaneous but non-planar waves. So in fact... the wave impedance of free space can have many values simultaneously, one [universal?] constant value of ~377 Ohms for plane waves, while it may have many other [arbitrary] values for waves passing through the same region of space that are not plane. Thoughts, comments? -- Pete K1PO Indialantic By-the-Sea, FL The page at: http://home.comcast.net/~kb7qhc/ante...pole/index.htm dramatically reveals that the near fields fluctuate wildly from 377 Ohms, and I have restricted my analysis to those values falling at roughly 100 Ohms or 1000 Ohms (the hot spots marking the feed point region and the tips of the dipole). Other antenna design's modification of the 377 near field around them can be observed at: http://home.comcast.net/~kb7qhc/ante...elds/index.htm 73's Richard Clark, KB7QHC |
Is antenna a transducer to 377 ohms?
On Tue, 12 Sep 2006 16:00:24 GMT, "Peter O. Brackett"
wrote: In fact, in the near field of an antenna, there is nothing that resembles 377 Ohms of Z. [snip] Correct, but don't we all believe that the wave impedance of "free space" is approximately 377 Ohms... Hi Peter, Beliefs. -sigh- Is this one of those transcendental statements about navel gazing? Everywhere... Even in the near field of an antenna. No. Not even in the near field of an antenna. That is an antenna itself has no effect on the fundamental u and e of the media in which it is immersed. Wrong. After all the antenna is very small, and free space is very large (grin), and so a tiny antenna cannot change u and e everywhere! Abstracting from near space to everywhere is the source of your error. The fields E and H in the "near region" of an antenna where the waves are not "plane" on the other hand may not be related by 377 Ohms, simply because the waves emanating from the "near" antenna are not plane, but... The waves are not plane where the waves are not plane, but... Is this a Zen "but?" There might just also be plane waves passing through identically the same region of space, say emanating from a more distant antenna. Wrong. The ratio for those plane E and H fields will indeed be 377 Ohms over the exact same region of space where Zo is different because of simultaneous but non-planar waves. Wrong. So in fact... the wave impedance of free space can have many values simultaneously, one [universal?] constant value of ~377 Ohms for plane waves, while it may have many other [arbitrary] values for waves passing through the same region of space that are not plane. Thoughts, comments? Wrong. Peter, are you trying to bust loose a seized bearing? Most of this reads like the Molly Bloom citation from a technical translation of "Ulysses." 73's Richard Clark, KB7QHC |
Is antenna a transducer to 377 ohms?
Peter O. Brackett wrote:
Well, as we all "know" the current wave on a dipole antenna is exceedingly close to sinusoidal, but not [exactly] sinusoidal, because if it were exactly sinusoidal it wouldn't be radiating. That [small] difference between the actual current distribution on an antenna and the actual current distribution on a transmission line is the [tell-tale] residual that separates us from an exact analytic expression for the driving point impedance of a dipole. Of course, but for conceptual purposes, both Balanis and Kraus seem to give us permission to consider the current distribution to be sinusoidal for "thin-wire" dipoles. Kraus says: "... it is assumed that the current distribution is sinusoidal. Current-distribution measurements indicate that this is a good assumption provided that the antenna is thin, i.e. when the conductor diameter is less than, say, lambda/100." Balanis agrees: "If the diameter of each wire is very small (d lambda), the ideal standing wave pattern of the current along the arms of the dipole is sinusoidal with a null at the end." It would be interesting to compare how closely the input impedance of a 1/2 wave lossless feed line of appropriate Zo (Say 600 Ohms?) terminated in a 73 Ohm resistor would approximate that of a "real" dipole. At resonance it would be 73 Ohms at least. Of greater interest might be the construction of a stub using resistance wire to simulate the "loss" to radiation plus all the other losses. As one of gurus on r.r.a.a said, "One can replace the resonant antenna with a dummy load without changing anything." :-) -- 73, Cecil http://www.w5dxp.com |
Is antenna a transducer to 377 ohms?
Richard:
[snip] Beliefs. -sigh- Is this one of those transcendental statements about navel gazing? [snip] No... it's not transcendental it's purely algebraic! (grin) [snip] Everywhere... Even in the near field of an antenna. No. Not even in the near field of an antenna. [snip] Where then is Zo = 377 Ohms? [snip] That is an antenna itself has no effect on the fundamental u and e of the media in which it is immersed. Wrong. [snip] Surely uo, eo, Zo and c (velocity of light) are fundamental and invariant properties of "free space", no? [snip] After all the antenna is very small, and free space is very large (grin), and so a tiny antenna cannot change u and e everywhere! Abstracting from near space to everywhere is the source of your error. [snip] No, I'm "contracting" from outer space to near space... using the contravarient tensor! [snip] The waves are not plane where the waves are not plane, but... Is this a Zen "but?" [snip] If a tree falls in the forest and no one is there to hear it, does it make a sound? If an antenna radiates somewhere in the Universe and there are no receivers, does it really radiate? [snip] There might just also be plane waves passing through identically the same region of space, say emanating from a more distant antenna. Wrong. [snip] Oh, and here I thought that at least tiny remnants of all radiation eventually passes through every part of space, filling all of space as it expands throughout the Universe.. [snip] The ratio for those plane E and H fields will indeed be 377 Ohms over the exact same region of space where Zo is different because of simultaneous but non-planar waves. Wrong. [snip] I know that Special Relativity [Maxwell's equations] is not supported in full by General Relativity, but surely even though space is warped by mass, superposition must still be supported. The radiation in your neighbourhood is a superposition of suitably delayed and reduced (by path attenuation) of all radiation, no? [snip] So in fact... the wave impedance of free space can have many values simultaneously, one [universal?] constant value of ~377 Ohms for plane waves, while it may have many other [arbitrary] values for waves passing through the same region of space that are not plane. Thoughts, comments? Wrong. [snip] And here I thought I was going to be able to sell you a little corner of the Universe [very near the Brooklyn Bridge] that has any Zo you want. What? [snip] Peter, are you trying to bust loose a seized bearing? Most of this reads like the Molly Bloom citation from a technical translation of "Ulysses." 73's Richard Clark, KB7QHC [snip] Molly Bloom? How did she get into this... I thought she was still living in the house on Eccles Street". What? Now Ulysses, he's my man! I miss Reg Edwards already :-( Regards, -- Pete K1PO Indialantic By-the-Sea, FL |
Is antenna a transducer to 377 ohms?
On Tue, 12 Sep 2006 18:39:25 GMT, "Peter O. Brackett"
wrote: Where then is Zo = 377 Ohms? Hi Peter, To how many places? Your question is rather oblique when we are discussing near fields and antenna as "transducer" [not a choice of term I subscribe to]. I seriously doubt that you've unhinged from the origins of that value, however, it bears only tangentially on the matter. That is an antenna itself has no effect on the fundamental u and e of the media in which it is immersed. Wrong. [snip] Surely uo, eo, Zo and c (velocity of light) are fundamental and invariant properties of "free space", no? And some toothpaste makes our teeth whiter, no? Your reply does nothing to answer your error, however. [snip] After all the antenna is very small, and free space is very large (grin), and so a tiny antenna cannot change u and e everywhere! Abstracting from near space to everywhere is the source of your error. [snip] No, I'm "contracting" from outer space to near space... using the contravarient tensor! Then you have misapplied it, clearly. Arguing does not take the place of easily demonstrable facts. AH! forgive me, wrong forum, arguing is classic substitution. However, the entertainment value is rather poorer this round. There might just also be plane waves passing through identically the same region of space, say emanating from a more distant antenna. Wrong. [snip] Oh, and here I thought that at least tiny remnants of all radiation eventually passes through every part of space, filling all of space as it expands throughout the Universe.. Are "thoughts" related to "beliefs?" Bloated speculations of background radiation don't change the basic assertion that in the near field, there is nothing that remotely approaches the presumed 377 Ohm specification. You've both (earlier) acknowledged this and (have since) challenged it with a semantic fog such as: I know that Special Relativity [Maxwell's equations] is not supported in full by General Relativity, but surely even though space is warped by mass, superposition must still be supported. The radiation in your neighbourhood is a superposition of suitably delayed and reduced (by path attenuation) of all radiation, no? EZNEC demonstrates the violation of your "beliefs," yes? I miss Reg Edwards already :-( Certainly you're a poor substitute for Punchinello. (and Kelvin is winding up a pitch to wing a chunk of chalk off your noggin.) 73's Richard Clark, KB7QHC |
Is antenna a transducer to 377 ohms?
Richard:
[snip] To how many places? Your question is rather oblique when we are discussing near fields and antenna as "transducer" [not a choice of term I subscribe to]. [snip] OK, ok... you've busted me... I admit that circuit theory is on really shaky ground. Although circuit theory was developed by Ohm, Kirchoff and others before Maxwell presented the world with his celebrated equations, we can all agree that circuit theory is a very poor (one dimensional) approximation to field theory. Circuits are a thoroughly useless affair dealing only with poorly understood approximations to the "real deal"...waves! I admit it, there is no such thing as "voltage", which after all is only the value V of a definite line integral of the vector field that depends upon the somewhat arbitrary path of integration, chosen by the integrator, through the appropriate E field, and so consequently there is no such thing as a "real" driving point impedance Z = V/I. The only reality is the characteristic or wave impedance! There I've said it! So... youv'e got me... I agree... we should not really be messing about trying to define phoney "transducer" functions between circuit theoretic variables (V, I) and wave theoretic variables (E, H) since the former have such an ephemeral existence. Still in all... one wonders... do circuits and waves, charge particles (electrons) and waves particles (photons) have any truck with each other or... do they lead entirely separate lives? What would you call the intermediaries between reality and approximation? I'm very sorry to have brought up the subject... I've probably confused the OP, and I am here and now prepared to recant my heresy, before you light the kindling beneath my feet, heh, heh... I solemly swear that an antenna is not a "transducer" between circuits and waves! (grin) But... ahem... the sun still has spots! Kathy, bring me another glass of that Rivaner! Molly Bloom indeed! -- Pete K1PO Indialantic By-the-Sea, FL I seriously doubt that you've unhinged from the origins of that value, however, it bears only tangentially on the matter. That is an antenna itself has no effect on the fundamental u and e of the media in which it is immersed. Wrong. [snip] Surely uo, eo, Zo and c (velocity of light) are fundamental and invariant properties of "free space", no? And some toothpaste makes our teeth whiter, no? Your reply does nothing to answer your error, however. [snip] After all the antenna is very small, and free space is very large (grin), and so a tiny antenna cannot change u and e everywhere! Abstracting from near space to everywhere is the source of your error. [snip] No, I'm "contracting" from outer space to near space... using the contravarient tensor! Then you have misapplied it, clearly. Arguing does not take the place of easily demonstrable facts. AH! forgive me, wrong forum, arguing is classic substitution. However, the entertainment value is rather poorer this round. There might just also be plane waves passing through identically the same region of space, say emanating from a more distant antenna. Wrong. [snip] Oh, and here I thought that at least tiny remnants of all radiation eventually passes through every part of space, filling all of space as it expands throughout the Universe.. Are "thoughts" related to "beliefs?" Bloated speculations of background radiation don't change the basic assertion that in the near field, there is nothing that remotely approaches the presumed 377 Ohm specification. You've both (earlier) acknowledged this and (have since) challenged it with a semantic fog such as: I know that Special Relativity [Maxwell's equations] is not supported in full by General Relativity, but surely even though space is warped by mass, superposition must still be supported. The radiation in your neighbourhood is a superposition of suitably delayed and reduced (by path attenuation) of all radiation, no? EZNEC demonstrates the violation of your "beliefs," yes? I miss Reg Edwards already :-( Certainly you're a poor substitute for Punchinello. (and Kelvin is winding up a pitch to wing a chunk of chalk off your noggin.) 73's Richard Clark, KB7QHC |
Is antenna a transducer to 377 ohms?
On Tue, 12 Sep 2006 19:55:52 GMT, "Peter O. Brackett"
wrote: OK, ok... you've busted me... Hi Peter, Sure, and without being... elliptical. Myself, I find Joyce rather a Mick awash in beer soaked imaginings in comparison to Henry Miller and The Cosmodemonic Telegraph Company of North America. 73's Richard Clark, KB7QHC |
Is antenna a transducer to 377 ohms?
Peter O. Brackett wrote:
Correct, but don't we all believe that the wave impedance of "free space" is approximately 377 Ohms... Everywhere... Even in the near field of an antenna. That is an antenna itself has no effect on the fundamental u and e of the media in which it is immersed. u and e are defined only in terms of and as affecting "plane wave" [TEM mode?] propagation, and... After all the antenna is very small, and free space is very large (grin), and so a tiny antenna cannot change u and e everywhere! The fields E and H in the "near region" of an antenna where the waves are not "plane" on the other hand may not be related by 377 Ohms, simply because the waves emanating from the "near" antenna are not plane, but... There might just also be plane waves passing through identically the same region of space, say emanating from a more distant antenna. The ratio for those plane E and H fields will indeed be 377 Ohms over the exact same region of space where Zo is different because of simultaneous but non-planar waves. So in fact... the wave impedance of free space can have many values simultaneously, one [universal?] constant value of ~377 Ohms for plane waves, while it may have many other [arbitrary] values for waves passing through the same region of space that are not plane. Thoughts, comments? I don't believe I've ever encountered the term "wave impedance of free space", and its use is certain to cause confusion, as I sense here. The *intrinsic* impedance of free space is 377 ohms. The *wave* impedance of an EM wave in that medium is 377 ohms if it's a plane wave in the far field of a radiator, and some other value if it's close to an antenna or other conductor or dielectric. The *intrinsic* impedance of free space is determined only by the conductivity, permittivity, and permeability of the medium; the impedance of a wave is governed not only by the intrinsic impedance of the medium but also other factors. If you have a reference that defines and uses the term "wave impedance of free space", I'd like to look it up to see how the author deals with this potentially confusing combination of terms. If it does indeed "have many values simultaneously", it's pretty useless in my opinion. Roy Lewallen, W7EL |
Is antenna a transducer to 377 ohms?
Peter O. Brackett wrote:
Circuits are a thoroughly useless affair dealing only with poorly understood approximations to the "real deal"...waves! What? No lumped inductors in reality? :-) What about the measured 3 nS delay through a 100 uH coil? -- 73, Cecil http://www.w5dxp.com |
Is antenna a transducer to 377 ohms?
Cecil:
[snip] "Cecil Moore" wrote in message . .. Peter O. Brackett wrote: Circuits are a thoroughly useless affair dealing only with poorly understood approximations to the "real deal"...waves! What? No lumped inductors in reality? :-) What about the measured 3 nS delay through a 100 uH coil? -- 73, Cecil http://www.w5dxp.com [snip] Delay through a coil... what is that? Heh, heh... Nickola Tesla would be rolling over in his grave if he could hear us discussing a possible dichotomy between lumped and distributed systems. Nickola "lived" in the intersticies between the wave theoretic and circuit theoretic fabrics of reality. It is interesting is it not, that the "only" element of circuit theory that allows for "action at a distance" [a.k.a. "field effects"] is that of mutual inductance "M". Circuit theoretic elements; R, L, C, and M... Other than "M" circuit/network theoretic concepts are devoid of the wave theoretic aspects of the celebrated Maxwell/Heaviside partial differential equations. Funny what you can do with "M". Variometers, Tesla coils, and such... -- Pete K1PO Indialantic By-the-Sea, FL |
Is antenna a transducer to 377 ohms?
On Wed, 13 Sep 2006 02:38:37 GMT, "Peter O. Brackett"
wrote: It is interesting is it not, that the "only" element of circuit theory that allows for "action at a distance" [a.k.a. "field effects"] is that of mutual inductance "M". Hi Peter, What happened to... capacitance? The Leyden jar proved "action at a distance," literally, long before folks started choking. 73's Richard Clark, KB7QHC |
Is antenna a transducer to 377 ohms?
Peter O. Brackett wrote:
Nickola Tesla would be rolling over in his grave if he could hear us discussing a possible dichotomy between lumped and distributed systems. Nickola "lived" in the intersticies between the wave theoretic and circuit theoretic fabrics of reality. Some very interesting information on that subject is contained in: "Class Notes: Tesla Coils and the Failure of Lumped-Element Circuit Theory" by K. L. Corum (KB1EUD) and J. F. Corum (K1AON) http://www.ttr.com/corum/index.htm More very good information on RF coils is found in an IEEE paper by Drs. Corum at: http://www.ttr.com/TELSIKS2001-MASTER-1.pdf It's hard to look at the Figure 2 diagram of "a capacitively tuned resonator" and not see a 75m mobile antenna with top hat. Equations (32) for velocity factor and (51) for characteristic impedance are of particular value to amateur radio operators. Having a good approximation for characteristic impedance and velocity factor for a bugcatcher mobile loading coil allows an understandable analysis of a mobile antenna system. I'm working on a web page based on those two papers. -- 73, Cecil http://www.w5dxp.com |
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