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From: Bill KT4YE
Date: 14 Nov 2002
Time: 19:38:45
Remote Name: 192.168.0.3
Comments on Professor D’Amico’s comments about my antenneX article this month:
Displacement Current Does Not Exist
Part 1: The Capacitor and Displacement Currents
By William C. (Bill) Miller, KT4YE
In the set of
equations presented, Professor D’Amico has established a fascinating proposition. By
postulating that Displacement Current does not exist, and using Maxwell’s Equations
to check the consequences, he has proven that a circular, parallel plate transmission line
(and by extension, all classes of parallel transmission lines) does not exist. After all,
transmission line theory neither requires nor supports the concept of Displacement
Current.
There are several issues here. Professor D’Amico has suddenly introduced an entirely new phenomenon – “relaxation time” – into the discussion. In the model we are discussing, this phenomenon does not exist. We would welcome seeing a capacitor model that incorporates the theoretical basis for this phenomenon, thus proving that – with the implementation of “relaxation time,” the E fields suddenly become uniform.
Professor D’Amico is correct in stating that drift velocity is not the phenomenon that “spreads” the electromagnetic perturbation along the conductor. However, there are still some people that belive that this is how current flows. (The author is not one of them.)
By pointing out that perturbations do not move instantaneously, but at near light-speed, Professor D’Amico is acknowledging that the E Fields between the plates of a capacitor ARE NOT UNIFORM. If the fields are not uniform, then any derivation using that assumption is fatally flawed. That is the basis of the author’s proposition, and the author thanks Professor D’Amico for acknowledging the validity of this basic concept.
Professor D’Amico is correct in assessing this effect as being of secondary value. But, with differential voltages on the same conductor, E fields from one point on the conductor to another point on the same conductor MUST exist.
But, by attacking the author’s knowledge of the phenomenon (or the measurement tools). Professor D’Amico neatly sidesteps the problem. (Ad hominem arguments are often used when there is no valid argument to be made against the proposition, aren’t they?) Classical theory REQUIRES infinite current flow when a step voltage is applied. This flaw is neither addressed nor explained by Professor D’Amico.
Professor D’Amico’s proposition can be summarized as, “If it works, it must be OK.” As a graduate engineer himself, the author has no problem with this concept – as long as the proposition continues to work. In virtually all Electronic Engineering, the presence or absence of Displacement Current is TOTALLY UNIMPORTANT to the design’s integrity.
This is NOT TRUE in antenna design. A variety of failed or flawed antenna designs are based in whole or in part on the existence of Displacement Current. Thousands of man-hours and much larger sums of money have been invested in (so far) futile attempts to “use” Displacement Current. The author asserts that one of the following two propositions must be true:
a) Quite a number of skilled and experienced theoreticians AND artificers are utter fools at their trades, or
b) The basic theory is flawed
Stateside, when a child loses a tooth, he or she places it under the pillow and, late at night, the “tooth fairy” retrieves it and leaves some money. This is a delightful myth. The tooth disappears and the child becomes richer. And the belief in the tooth fairy does no harm to the child – unless the child decides to get even richer by pulling all his (or his sibling’s) teeth en masse!
Displacement Current is also a delightful myth. Unlike the tooth fairy myth, it works in almost all applications. But let us not pretend it exists for ALL applications, lest we be like the child who, upon pulling his siblings teeth, learns that disappointment and pain – not riches – is the reward.
As a closing comment, the author would like to welcome Professor D’Amico, and all other interested parties, to review the second half of the article. In this, we believe that we demonstrate that the correct model for the capacitor is a transmission line launching a TEM wave.
reGARDS
William C. (Bill) Miller, KT4YE