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Gene Fuller wrote:
I was going to drop this discussion, but I will respond to your request to share physics knowledge. Thanks, Gene, Please be patient with me. 1) I will repeat. E-fields, H-fields, voltages, and currents are all related through some very profound equations. However, shout THEY ARE NOT INTERCHANGEABLE. /shout Is the H-field around a wire proportional to the RF current in the wire? Is the E-field around a wire proportional to the RF voltage between the wires? Is the ratio of E-field to H-field fixed by Z0? Do I^2*R losses affect the E-field and H-field by equal amounts? This is not just a matter of semantics. These entities have different physical meanings, different units, and different dimensionalities. Of course that's true. However, they are not unrelated. 2) I offered a physics-based explanation for your proposed "current drop" in the 440 MHz RG-58 example a few days ago. Did you not read that message before responding to it? Yes, and I have been thinking about an example that would better illustrate what I was asking. A 1000 wavelength dipole located in outer space would have less current at the ends than at the source. Since there is no other path for current, what is the explanation for the decrease in the current at the ends? 3) What is not correct is the assertion that the coil exhibits a phase shift consistent with, for example, 20 feet of wire used to make the coil. Because nobody has made that assertion since the original eHam article, it appears to be a straw man. The coil occupies whatever number of degrees that it occupies and it does NOT occupy zero degrees. For instance, using a particular EZNEC segment model of a coil, the current at the bottom is 1.0 amp and the current at the top is 0.5 amp. Assuming the cosine distribution of standing-wave current is accurate, the coil occupies about 60 degrees. The whip would occupy about 30 degrees, the rest of the 1/4WL. Nobody has attempted to explain how one can obtain 90 degrees of a 1/4WL antenna on 4 MHz using a ten foot (15 degree) whip. That is one hell of a velocity factor. If the bottom-loading coil really occupies zero degrees, then the ten foot whip would be forced to occupy 90 degrees. That is so impossible as to be laughable. The notion that a coil replaces some sizable portion of the total phase shift in an antenna has been shown to be incorrect. Experiments reported by Roy and Tom R. convincingly demonstrate the phase shift behavior of coils. The total current undergoes virtually no phase shift since it is a standing wave. That's in the textbooks and nobody is arguing that point so it's just another straw man. It's the forward current and reflected current that is undergoing a phase shift through the coil just like they do on a wire standing- wave antenna. Nobody has measured those two current components so the jury is still out on that subject. There is no argument about the phase of the total current that Roy and Tom measured. Please, there are enough arguments already without having to introduce straw men. If you will look at my phasor diagrams of forward and reflected currents at: http://www.qsl.net/w5dxp/current.htm you will see that the phase of the total current is exactly the same in both cases. That's the phase that Roy measured. Since it is a standing wave current, the phase of the standing-wave current is almost constant. It is the magnitude of the standing-wave current that changes and it changes as a cosine function of electrical length in degrees. The coil has an electrical length in degrees. That's what causes the current to be different at the bottom and at the top in a 1/4WL antenna. Assuming the phase shift from the feedpoint current to the tip of the antenna is 90 degrees, if an accurate measurement of the current at the top and bottom of a bottom-loaded antenna coil is made, the number of degrees occupied by the coil can be calculated from arccos(Itop/Ibottom) just as it can be calculated between two points on a wire. This assumes that Ibottom is an Imax point on the standing wave. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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