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Magnetic Loops
On 10/19/2015 2:28 PM, wrote:
Brian Howie wrote: In message , bilou writes "Brian Howie" wrote in message ... I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. Brian GM4DIJ -- Brian Howie Hi My own experience is that ,at least for receive, multi turn loops are useless. Instead you can use a single turn one with a good coil in serial. The tuning range for a given variable capacitor is much greater especially if ,at low frequency, the coil is using ferrite . Switching the coil can increase the tuning range easily. The coil, with a secondary winding,is also very useful to adjust the coupling to the receiver. I'd have thought I'd get a better signal from more turns, but maybe better coupling and a higher Q from your suggestion would do the same. Brian To be a bit simplistic, the amount of signal captured is proportional to the loop area; the number of turns has little to no effect on that. I'm pretty sure that is not correct. The signal strength is proportional to the number of turns *and* the loop area. I will have to dig out my notes on this, but some factors (like Q) even out with various changes in antenna parameters such as number of turns, loop size, etc. But signal strength is proportional to the area of the loop and the number of turns. From http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm E = 2pi w S µR e / λ λ is the wavelength in meters w - the number of ML turns; S – is the area of the windings in m2; μR is the effective magnetic permeability of the ferrite rod SML. μR is always less than the permeability of the material used and depends from the size, geometry and the way the windings are constructed. μR = 1 for aerial loops. The product: А = w μR S (3) is called effective area of the SML. If you don't like this reference, I know I have seen this info in other places too. The number of turns greatly effects the inductance. I believe it is N squared. Twice as many loops *and* twice as much interaction between the loops. Picture a triangle formed by the sum of the progression 1, 2, 3. That area is the inductance as you add loops. 1 1,2 1,2,3 1,2,3,4 1,2,3,4,5 I spent a great deal of time once trying to understand the formulas for inductance. Seems the problem is the non-idealities of coils significantly affect the results and vary a lot for different form factors, etc. So it is *very* hard to produce an equation that is good for all. The result is a number of different equations for different shapes and many different equations over the years as better approaches are found. I think the Lundin formula was the best one I found, even if a bit complex. The Wheeler formula is not as general or accurate, but simpler. Every formula I found used an N^2 term for the number of turns. Wheeler formulae http://home.earthlink.net/~jimlux/hv/wheeler.htm I can't find a good reference for Lundin's formula, but if you want I will copy my spread sheet data here or email a copy. -- Rick |
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