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Current through coils
Roy Lewallen wrote:
Now explain how you'd do it with a box having only two terminals -- and assuming the box is very small compared to a wavelength. Assume a one-wavelength dipole off-center fed 1/4WL from one end. Using EZNEC with 60 segments, feeding at segment number 15 is 24.2% from one end and that's close enough for this example. This is actually done in EZNEC with a 130 ft. dipole on 7.2 MHz and I'll email out that file upon request. I'm going to describe the current distribution in the following diagram with 60 segments running from left to right in *fixed font*. Eash dash corresponds to a segment in EZNEC and F is the feedpoint. seg L L seg 1 v v 60 --------------F--------------------------------------------- ^ ^ ^ N N N The current distribution is sinusoidal. N stands for 'node' which is a current minimum point. L stands for 'loop' which is a current maximum point. Since I'm limited to ASCII, the reader will need to imagine a current envelope drawn from seg 1 up to 'L', down to seg 30, back up to 'L', and back down to seg 60. I'll follow this posting up with actual EZNEC graphics posted to my web page. Now we are going to replace part of that wire with a 6" long coil. A 6" long coil on 7.2 MHz is about 1/3 of one percent of a wavelength so that should qualify as 'very small'. And, to illustrate another fact, I'm going to make the coil from 1/4 wavelength of wire, 33' on 40m, and try to model that using the helical coil feature of EZNEC. That may or may not violate an EZNEC design rule - I just don't know yet. But it doesn't change the concepts being presented here. Let me say this is a very rough approximation to what happens in the real world. The concepts are accurate. The values may be off by a relatively large percentage. The coil certainly distorts the current away from that near-perfect sinusoid and certainly doesn't radiate like the wire it replaces. But roughly, here will be the results of placing the bottom of the coil at seg 30: seg L L seg 1 v v 46 --------------F---------------////---------------- ^ ^ ^ N N N The current at the left end of the coil will be low because that is roughly the location of a current node (minimum). The current at the right end of the coil will be high because that is roughly the location of a current loop (maximum). If one considers the current flowing from left to right, more current will be flowing into the coil than is flowing out of it, like the current at: http://www.qsl.net/w5dxp/qrzgif35.gif This is a standing-wave antenna so the standing-wave current displayed by EZNEC is flowing hardly at all. That standing- wave current consists of two component phasors, rotating in opposite directions. That's why the phase of the standing-wave current is relatively constant. The standing-wave phasor, the superposition of the forward and reflected current phasors, rotates hardly at all, usually by just a few degrees from end to end in a 1/2WL dipole. If the dipole is made of 'thin wire', the phase of the standing-wave current is fixed at zero degrees. (Can a phasor that doesn't rotate be called a phasor?) Taking 1/4WL of the antenna wire and winding it into a high-Q coil above replaces *roughly* 90 degrees of the antenna. The radiation pattern certainly changes because the coil doesn't radiate much. But we are not concerned about radiation patterns in this discussion. We are concerned about the current at each end of the coil, the same current that we measure and the same current reported by EZNEC. That current is certainly not constant through the coil and THE DIFFERENCE IN THE MAGNITUDE OF THE CURRENT AT EACH END OF THE COIL DEPENDS UPON WHERE IT IS PLACED IN THE STANDING-WAVE SYSTEM. The traveling-wave current through a coil is close to equal at each end. The standing-wave current at each end of a coil is NOT equal unless we locate the center of the coil at a current node or at a current loop. In a bottom-loaded mobile antenna, the coil is located very near a current loop where the slope of the current is near zero. In fact, the net current peaks inside the bottom-loading coil. So the concept that net current at each end of a coil installed in a standing-wave environment is equal is just a myth, an old wives' tale that needs to be banned from ham radio. The coil does indeed cause considerable distortion away from the perfect cosine current wave exhibited by a thin wire. But the macro effects of that cosine wave still exist when a coil is installed. The current at each end of a coil installed in a standing-wave antenna depends upon its location in the system. -- 73, Cecil http://www.qsl.net/w5dxp |
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