Quoting Ermi describing Dr. Kimber's statement "Retardation is a time delay, due to the finite speed of an electromagnetic wave along a conductor, that causes phase shift along the length of an antenna structure, and said structure must be large enough for sufficient phase delay to produce a significant amount of radiation resistance at the antenna port.".
I believe I understand the relationship between conductor length and phase delay but it is known and demonstrated that a single charged particle undergoing acceleration produces radiation, e.g. the so called "synchrotron radiation". Also, the use of the term "retardation" is a bit confusing since theoretical physicists Wheeler and Feynman postulated the concept of advanced and retarded waves to explain radiation. This theory predicts that without a "receiver" charged particle there is no radiation.
The words "sufficient" and "significant" used above would allow for radiation, though inefficient perhaps, from a short conductor so what is actually being said?. This doesn't imply that below a certain amount of "retardation" radiation ceases does it?
Neil
Here are a few selected quotes about radiation from antennas taken from ANTENNAS FOR ALL APPLICATIONS, 3rd Edition (2002) by John Kraus.
"Regardless of antenna type, all involve the same basic principle that radiation is produced by accelerated (or decelerated) charge."
"Thus, time-changing current radiates, and accelerated charge radiates."
"The radiation is perpendicular to the acceleration, and the radiated power is proportional to the square of I * L, or Q * v." [ I is the time-changing current, L is the length of the current element, Q is the charge, v is acceleration of the charge.]
"From the circuit point of view, the antennas appear to the transmission lines as resistance Rr, called the radiation resistance. It is not related to any resistance in the antenna itself but is a resistance coupled from space to the antenna terminals."
I paraphased Kimber's statement in my own words, expecting that there would be more clarity than in the original, but that turned out not to be the case. So I will quote Kimber more directly, but still abbreviating the statement. He was very probably referring to such short antenna designs as the CFA and the EH when writing his comments.
"...amazing claims are made for the radiation efficiency of certain unusual small antennas...such claims are false...
"...a small conductor (or set of small conductors) generates a local induction field when excited at a feed point. This field induces a back emf in the conductor, which is in quadrature phase to the original excitation, so no power flows...So how does anything radiate?
"In order for radiation to occur, some phase shifting needs to take place so that there is a component of the back emf which is in phase with the excitation. Then the feed point gains some resistive impedance, so real power can be fed. Where does this phase shift come from? The answer is...retardation.
"In any antenna of finite size, the fields take time to propogate from one part of the structure to another. This time delay causes a phase shift increasing as the antenna becomes larger...with respect to wavelength. Thus if the antenna is large enough, we get our resistive feed...there is no radiation without retardation."
"
Maybe there should be a control added to part 15 transmitters marked
RETARDATION
Thanks, Ermi, for the quotation. Here's how I understand what he has said and my comments:
"...amazing claims are made for the radiation efficiency of certain unusual small antennas...such claims are false...
OK, I follow this.
"...a small conductor (or set of small conductors) generates a local induction field when excited at a feed point. This field induces a back emf in the conductor, which is in quadrature phase to the original excitation, so no power flows...So how does anything radiate?
This one is a bit harder to parse. Local induction field and back EMF describe an inductive reactance, probably due to the inductance per unit length of the conductor but there is also a capacitive reactance due to the capacitance per unit length. Quadrature phase means at 90 degrees to the original excitation but it seems the capacitive reactance will swamp the inductive reactance in a short conductor. The effect is the same but the angle is capacitive and this doesn't detract from his statement's fundamental conclusion of no real power radiated. Note that the relative mix of inductive and capacitive reactances does not add a resistive component with the phase angle always being 90 degrees except at resonance.
"In order for radiation to occur, some phase shifting needs to take place so that there is a component of the back emf which is in phase with the excitation.
Yes, this will pull the phase relationship away from quadrature toward zero which represents resistance.
Then he explains: The answer is...retardation.
"In any antenna of finite size, the fields take time to propogate [sic] from one part of the structure to another. This time delay causes a phase shift increasing as the antenna becomes larger...with respect to wavelength. Thus if the antenna is large enough, we get our resistive feed...there is no radiation without retardation."
This is the part which I cannot accept since it doesn't explain why the phase shift occurs at the source. The phase shift to consider is that between the V and I at the feedpoint, not along the antenna. There is a "time delay" along a perfect transmission line of infinite length but the phase shift due to time delay seen is between the signal at a point along the line and the signal at the feedpoint which does not change the phase of V and I at the feedpoint. The feedpoint characteristics of a terminated transmission line are a function of the termination and the line length but in an antenna some other effect of length is causing the phase shift at the feedpoint and this effect is what we call the radiation resistance which exists because energy is radiated and which is defined in terms of radiated energy. The radiation resistance is the termination of the antenna but it appears to be distributed per unit length.
The "resistance" to acceleration of the charged particles representing radiated power is similar to the resistance to acceleration of a mass caused by inertia where in both cases real power (work per time) causes the acceleration (it goes into kinetic energy for mass and into radiated power for the particle). This seems like a "chicken and egg" problem where the radiated energy causes the resistive component and the resistive component causes the radiated energy yet the connection between the two is not explained.
The analogous mechanical inertia has been addressed with Mach's principle which says that inertia exists because of the entanglement of the accelerating mass with other masses in the Universe (and something has to be traveling backward in time). The Wheeler/Feynman advanced and retarded theory similarly connects the radiating particle to other charged "receiving" particles in the Universe. Both of these postulations do not easily answer the question of why the entanglement since they require fields or information traveling backward in time. To me, the most satisfying explanation of this is John Cramer's "transactional interpretation" which is built on Wheeler/Feynman's absorber theory of radiation. I encourage those who read this theory to approach it with an open mind. These ideas expressed as locality and non-locality (instantaneous actions at a distance) bothered Einstein yet they have been experimentally confirmed.
It may well be that we just do not know why an accelerating particle radiates energy but the fact that it does so explains the resistive component of the signal at an antenna feedpoint.
Class dismissed and let me know your thoughts.
Neil
One time I got transfixed by a book about time, and there was a discussion that under certain conditions it is expected that light, theoretically, radiates backward in time as well as forward, but this has not been observed.
I just spent 20-minutes in the dust trying to find the book, but all searched objects cannot be found at the time of the search.
So I found a different book: "The Philosophy of Space and Time", by Hans Reichenbach. The three chapters are I. Space, II. Time, and III. Space and Time.
I believe our problem is one of space and time together.
There is also a section on the retardation of clocks, something Einstein discussed, in which one clock was standing still and the other one was in motion.
Our theoretic transmitter/antenna combination is standing still.
That's my ramble.
OK Neil; maybe we are all in over our heads here. I should point out that retardation, quite apart from the funny(?) allusion to diminished mental capacity, is a major topic in antenna theory, especially for thin linear antennas.
Just out of curiosity, I wonder if it is a common belief among part15.us members that such antennas as the CFA, EH, and Isotron, in addition to several similar designs, actually work as advertised. A particular comment by RFB in this thread causes me to ask. I would like to know because I am interested about the state of the hobby, but I don't judge. I am very ecumenical when it comes to antenna belief systems.
Yes Ermi, I may well be in over my head about this. I have not heard the term retardation as applied to antennas but as I mentioned I have heard of it from theoretical physics. I wonder if the two uses of the term are the same thing. Maybe if I saw the math behind Dr. Kimber's explanation it would help.
Theoretical conjectures are fun and can be brain exercise but the practical models of the Rs, Ls, Cs and radiation resistance suffice quite well for understanding the basics.
On the subject of antenna "efficiency" I have no comment since I rely on measurements made by others. I would like to see claims backed by comparative data but other than the ubiquitous dBi gain claims I don't recall seeing any.
Neil
Neil observed: On the subject of antenna "efficiency" I have no comment since I rely on measurements made by others. I would like to see claims backed by comparative data but other than the ubiquitous dBi gain claims I don't recall seeing any.
One candidate might be the work of Dr George Brown, et al of RCA Labs in their 1937 I.R.E. paper on buried radial ground systems used with monopoles of various electrical heights.
They made careful measurements of the groundwave fields produced 3/10 of a mile from these antenna systems, when the applied power and frequency were held constant.
Their work shows that the field at that distance produced by a 0.25-wave monopole using 113 x 0.412-wave buried radials is within several percent of the maximum theoretical field of a perfect monopole system of that height when driven with that power against a perfect (zero loss) ground plane.
The theoretical field at 1 mile for 1,000 watts of applied power is 194.5 mV/m for a perfect system. Brown et al measured the equivalent of about 191 mV/m (a reduction of about 0.16 dB).*
The slightly lower field they measured in their experiments probably includes the reality that ground losses in their hardware and propagation path were not quite zero.
* The peak directivity of a perfect 0.25-wave monopole system over a perfect, infinite ground plane including the ground plane reflection is 5.16 dBi. The real-world system measured by Brown had that directivity, but its peak gain would be 0.16 dB less, or 5 dBi (antenna system gain includes losses).
The discussions being presented are wonderfully interesting, but not being conversant through the language of advanced radio engineering, I am having a good time here on the outskirts following a few related properties that play a key part in the equations. Particularly, matters of space and time, i.e., it takes a given TIME for RF energy to travel a given distance in the SPACE of a wire.
I found the book "On Time" by Michael Shallis, and this provocative statement from p. 189: "It seems that every moment contains the essence of all other moments within it, just as one particle contains within it the essence of all other particles, or that the moment of here and now in space and time also contains the past and future connected to it by light."
Now then, earlier I commented on Einstein's comments of "the retardation of clocks," in which one clock is standing still and the other in motion." At that time I remarked that our transmitter/antenna is standing still, but now I say further: But our radiant signal is in motion. Therefore our radio equipment, in it's space/time action, can be viewed as clocks.
If one is interested in the practical problems created and solved regarding simultaneity of events in time I recommend Peter Galison's book "Einstein's Clocks and Poincare's Maps: Empires of Time".
This is not a dry discussion of relativity but rather it presents the problems of world mapping, navigation, train scheduling, etc. which depend on time measurements in an understandable manner. Though it gives some of Einstein's special theory of relativity it, in an interesting way, presents these ideas as applied to practical problems. For example, the Eiffel Tower was used as a transmit antenna to send time signals around the world for mapping purposes but even the slight delay of the time signals transiting to the receivers caused problems.
The book was published in 2003 so it should be available for purchase or from a library.
Neil
Thank you Neil for the book suggestion, it is now on the list for the trip to the used book store.
You mentioned "simultaneity", and that reminds me of another Einstein remark. He said, if I recall correctly, "Nothing in the universe is simultaneous."
From your description of the ideas in Peter Galison's book, that might be a point that gets talked about.
How many times have we talked about ground? Many times, for the reason that good ground contact improves the AM radio signal.
But in the everyday search for ideas, how often do we look up? Never, is the answer.
Let's look up. You got your sky. Then up much higher is your ionosphere, which plays a key role in full power AM, but seemingly no role in low power broadcasting. But that's because it's so many miles up and our pea power doesn't go that high.
So, how about finding a way to put a lower ionosphere just above our 3-meter vertical stick? If we could generate a small ionosphere, we could get local skywave and bounce signals across the city limits.
My inventor's book says, "First things first," but it doesn't say what's first. That's where we need to start.