Elliptical Coil
 Fisher 6.5" Elliptical Search Coil for Gold Bug-2  |
 Audio Technica AT440MLA Dual-Moving Magnet Phono Cartridge List Price: Sale Price: $160.99 You save: $139.00 (46%)  Eligible for free shipping!Availability: Usually ships in 24 hours |
 Grado - Reference Platinum MM Cartridge 4.0mv Output Sale Price: $350.00 Eligible for free shipping!Availability: Usually ships in 24 hours |
 Grado Prestige Black P Mount Turntable Cartridge Sale Price: $60.00 Eligible for free shipping!Availability: Usually ships in 24 hours |
What is the effect of a elliptical coil compared to a circular coil on magnetic induction?
I am trying to implement a RFID system, in the near field range, the 125kHz system uses magnetic induction.
Every diagram and example and formula i have found gives details of induction coils being 'circular' (i.e. wrapped around a bar), and using the formula:
Field Strength = ((2 x (Number of turns) x current)/Pi) x ((radius of coil)^2 / ((Radius of coil)^2 distance^2)^3/2)
It shows that by increasing the radius or the number of turns in the coil, you can increase the magnetic field strength.
SO that brings be back to my question: If i had an elliptical 'coil' instead of a standard circular coil, what would the effect be on the magnetic field strength?
Assuming that the circumference is the same (say, as if you were to "squash" the circular coil a little), I would expect the magnetic induction at the center to increase, but I don't think that that is the number that is important for your application. First let me explain the first part of my statement. imagine that you squeeze the cool so much that the wires almost touch, let's say that the cross section is rectangular with one side approaching zero, the other that of half the circumference of the coil. The magnetic field at the center would be almost entirely due to the two long wires and come close to that of two infinitely long wires. Furthermore, if the wires almost touch, that field strength appoaches infinity, unless you correct for the thickness of the wire. Now, the ellipse would do the same thing, but I won't vouch that the field strength would necessarily increase monotonically with the excentricity (as you squeezed the ellipe more), so you may want to investigate that further. The exact solution would be rather involved, and probably is best done with a simple computer program, however, you can show that the field at the center of the ellipse is proportional to the ratio of its circumference to its area, which agrees with the physical argument I gave.
The other point I made is that what you probably want is not the magnetic induction (B) but the total magnetic flux, which is the integral of B over the cross sectional area, because that is what determines the self-inductance, L, of the coil. I suspect that that would decrease as you made the coil elliptical, by the following simple argument. Suppose I started with a circular coil and imposed a slight deformation, would the internal forces on the coil (due to the interaction of the magnetic field with its current) try to return the coil to its circular shape or would they try to increase the deformation? I leave it to you to show that the forces are of a repulsive nature, so that the coil will try to return to the circular shape. You could try to calculate the inductance of a rectangular wire. I think you will find that it is largest for a square, assuming the circumference of fixed length.
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I forgot to point out that I only addressed the problem of single turn, although I believe that the conclusion applies to any number. There is at least one more loose end, however, so I am sorry that I couldn't give you a very cohesive answer. One other suggestion from a simple practical viewpoint: If the inductance of a circular coil were not the largest for a given amount of material, inductors would be made in other shapes than circular.
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Fisher 6.5" Elliptical Search Coil for Gold Bug-2 |
DescriptionThis is an elliptical search coil for use with the Fisher Gold Bug-2 metal detector. |
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Audio Technica AT440MLA Dual-Moving Magnet Phono Cartridge List Price: Sale Price: $160.99 You save: $139.00 (46%) |
DescriptionAUDIO TECHNICA AT440MLA, DUAL MOVING MAGNET PHONO CARTRIDGE Enjoy remarkably accurate tracking, separation, and performance when listening to vinyl with this dual-moving magnet phono cartridge. The 1/2-inch mount cartridge features a MicroLine shaped diamond stylus that tracks portions of the groove that other styli simply cannot reach, ensuring that high-frequency passages sound clear and crisp. The multi-level shape also wears remarkably evenly, extending the lives of both the record and the stylus itself (up to 1,000 hours). In addition, the AT440ML boasts a small curving rate that improves playback over the entire 5 Hz to 32 kHz frequency range, even for the inner part of the record groove. Other features include a gold-plated beryllium cantilever for added durability and a tracking force of 0.8 to 1.6 grams. The stylus is covered by a one-year warranty. |
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Grado - Reference Platinum MM Cartridge 4.0mv Output Sale Price: $350.00 |
DescriptionGrado's Reference series of wooden-body cartridges are a fixed coil design, utilizing manufacturing and assembly techniques of fantastic accuracy and precision. These striking to see and hear cartridges are handcrafted at Grado from a specially selected species of mahogany. An intricate procedure of curing between production steps ensures optimal sound quality. Unlike Grado's popular Prestige series, the Reference series generator/stylus module is not replaceable, allowing a redesigned one-piece magnetic circuit and a reduction of chassis resonance for impressive neutrality throughout the product line. The Platinum is the entry-level model in Grado's new wooden-body Reference line, moving Ken Kessler to wonder, "If this is the bottom of the range, then what does the $1200 flagship model offer?" in the January 1997 issue of Hi-Fi News & Record Review. The newly redesigned Reference Platinum1 moving coil cartridge has had its coil design reconfigured, and the effective moving mass of the generating system has been reduced by 17%. This decrease in stylus mass allows for enhanced tracking with improved imaging, giving a quicker, more accurate reproduction of the music. All this is hand-assembled within a machined, new processed, Australian Jarrah wood housing. The Platinum1 uses a modified four piece OTL cantilever technology achieving a 10% tip mass reduction over the Prestige series and ultra-high purity long crystal (UHPLC) oxygen free copper wire in the coils. Reference cartridges use a 1/2" mount, and the recommended tracking force is 3/4 to 2 grams. All models are handmade in the USA. |
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Grado Prestige Black P Mount Turntable Cartridge Sale Price: $60.00 |
DescriptionThe Black and Green models from Grado use a three piece OTL cantilever technology, standard oxygen free wire in the coils and Grado's specially designed elliptical diamond mounted in a brass bushing. The Green model is selected from the Black production run and meet higher test specifications. Approximately 15% of the production run will meet these standards and become Green models. What People are saying about the Black... "The Grado brought out most of the tonal accuracy and excitement and, though I have heard better reproduced, that was only with cartridges costing 10 times the price." Gramaphone / John Borwick "If you thought a true "high-end" cartridge had to cost $100 or more, then you've never heard the Grado Prestige Black." Opera News / Anthony Chiarella Vol.64, No. 8 |
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Grado - Blue1 - Prestige Series - Phono Cartridge Sale Price: $110.00 |
DescriptionHigh Output and Excellent Stability Newly Upgraded for Even Better Performance The Grado Blue moving magnet phono cartridge delivers high output and excellent stability. It uses a four-piece OTL cantilever technology, standard oxygen free wire in the coils, and Grado's specially designed elliptical diamond mounted on a brass bushing. A reduction in the mass tip results in a frequency response from 10 Hz to 55 kHz. Output is 4.5mV with channel separation of 25 dB at 1 kHz. "I recommend that you consider this $ensible cartridge, as it is good value for the price," urges James T. Frane in $ensible Sound magazine. Newly Redesigned Blue1 The newly redesigned Red1 and Blue1 cartridges have had their coil design reconfigured, and the effective moving mass of their generating system has been reduced by 17%. This decrease in stylus mass allows for enhanced tracking with improved imaging, giving a quicker, more accurate reproduction of the music. The Red1 and Blue1 models use a four piece OTL cantilever technology, oxygen free copper wire in the coils and Grado's specially designed elliptical diamond mounted on a brass bushing. The Red1 model is selected from the Blue1 production run and meets higher test specifications. Approximately 10% of the production run meet these standards and become Red1 models. |
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Nuts and Bolts:
Price
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Warranty
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Engine Size/Type
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Transmission/Driven Wheels
Four-speed automatic, front-wheel-drive.
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