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Miller effect - was: What is the antenna current path or route
Dear Group:
A hot button has been pushed. The Miller effect (not to be overly confused with the Miller theorem) stems from an exhaustive analysis of the amplification vs. frequency of very low mu triode amplifiers (the tubes available at the time). The work was published just after WW-1! It was noted that the effect on frequency response of the capacitance between grid and plate was magnified by the amplification of the stage. Miller's paper showed that the effect of the aforementioned capacitance could be approximated by a capacitor in series with a resistance with the combination placed in shunt with the grid-cathode. Several conditions need to be true that were overlooked by later commentators! Many years latter, Miller's work became corrupted into Miller's theorem that erroneously contended that the effect of the feedback capacitor was the same as a gain dependent capacitor in shunt with the input and output of the amplifier. Various "proofs" have been published that "prove" Miller's theorem. They remind one of HS proofs involving dividing by zero. It is true that the frequency effect of capacitance between input and output (note, those are nouns) of an amplifier is detrimentally increased as the gain of the amplifier increases. [as gain goes up - the upper half-power frequency goes down] [note the notion of approximately constant gain-bandwidth product for well behaved amplifiers] However, the simple modeling indicated by "Miller's theorem" only occasionally estimates the relationship between feedback capacitance and upper half-power frequency. Fortunately, the allege convenience of using Miller's theorem in the slide rule days has now gone away. Today, one may use PSpice (or the like) and receive close estimates of the expected performance of an amplifier. It is difficult for me to see gain at the end of an antenna's wire. The apparent capacitance that exists at the end of an antenna's wire simply exists and is not magnified by anything. Lecture mode off. 73, Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Richard Harrison" wrote in message ... snip Energy from the magnetic field is converted to energy in the electric field for an instant (Cecil`s famous conservation of energy). This produces an insreased voltage at the open circuit end. This incresed voltage has more capacitive effect, akin to the "Miller effect" caused by the higher signal voltage on the plate of an amplifier vacuum tube than on its grid. snip Best regards, Richard Harrison, KB5WZI |
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What is the antenna current path or route
On Oct 19, 6:52 am, "Richard Fry" wrote:
"art" wrote And "end effect" is the confusion created at the top of the radiator ____________ Those wanting a more accurate description can find it here... http://books.google.com/books?id=U-3...&dq=antenna+en... RF Thanks, Richard. I notice that article references King, I assume Ronold W. P. King, who wrote some very nice qualitative explanations of various linear and loop antennas in King, Mimno and Wing's "Transmission Lines, Antennas and Waveguides." There's also a nice explanation that I feel is quite accurate in Joseph Boyer's "The Antenna--Transmission Line Analog," from Ham Radio magazine, April and May 1977. I believe I have it somewhere as a PDF, scanned from the article. Cheers, Tom |
#3
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What is the antenna current path or route
art wrote:
On 19 Oct, 03:55, "Dave" wrote: "art" wrote in message oups.com... Pseudo experts of fractional wavelength antennas. Where does the current flow when it reaches the END of a fractional length? verticle antenna and why? How does this relate to the term "end effect"? If you have already written a book then tell us what the auther said. Art KB9MZ.....XG it turns around and goes right back down the way it came. So a electrical generater doesn't keep turning in one direction but instead it occillates at the desired frequency. I have never seen one do that! And "end effect" is the confusion created at the top of the radiator I don't have to be an engineer to question this one. An electrical "generator" does indeed keep turning in a single direction. It has brushes and split commutator rings to keep the current flowing in only one direction. That is in the very nature of a "generator." The device I suspect that you are alluding to is an "alternator" in which a magnetic field is rotated through a coil of wire and passes through both halves of the coil at the same time. Since in a well designed, single phase alternator the center of the magnetic field and the center of the coil of wire are coincident the current does indeed alternate because unlike a generator the alternator does not include any means of flopping the connections on the coil in time with the magnetic field. So as the negative and positive fields of the magnet pass through each half of the coil of wire in turn the current reverses direction. -- Tom Horne, W3TDH |
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