Buck coil effects on TX Inductance?

[Jeremy]

I am an analog guy with few decades under my belt, but, coils and magnetism are not my strong point. Therefore, I would like a little coaching in a coil project of mine. When designing a (large) co-planar coil to a given inductance, I assume (oops) that the buck, being wound in a counter-TX coil fashion, is going to impact the effective inductance of the TX windings. Is it correct to assume that TX coil inductance will be altered by the buck windings' inductance in a linear fashion? For example, if I want 3.3mH and need some of that to be buck windings to cancel field at the RX - does the buck inductance just subtract from the inductance of the outer TX windings for a total inductance of something less than the outer windings? In doing the calculation of a proper buck winding for a given TX winding and just adding a "proper" amount of buck windings, the thought occurs to me that the TX winding inductance will now be reduced and so I must add some more windings - Then I need a touch more buck because of the added windings - then I affected my TX inductance a smidge and so on, ad nauseum.. If this is so, then there must be some successive approximation process whereby one can reach a final value of 3.3mH with proper cancellation for the RX coil. That having been said, is there a handy formula or calculator for this process? Or, and this may be a perfectly valid answer, do I have an invalid assumption and no freakin' idea of what to do?

[Jeremy]

Maybe a light is going off?

OK, I see perhaps a fallacy in my original assumptions. I had assumed that a buck coil was just in series with the TX coil and 180 degrees out of phase. Is this not so in a typical application? Or, is the buck driven seperately?

[Jeremy]

Mutual Inductance

OK, this variometer calculator shows the mutual coupling to which I refer. Ham radio nerds are cool!

http://www.qsl.net/in3otd/variodes.html

[Qiaozhi]

Have you looked here? -> http://geotech.thunting.com/cgi-bin/...anar/index.dat

[Jeremy]

Yes, I looked at that and found some great ideas as to how one may mechanically construct a coplanar coil. But, in an attempt to design to a given inductance I came up with my questions. What I plan to do is start with a number of turns for a given TX coil inductance that is some ratio (dependant upon area coverage) greater than the TX coil needs to be. Then I will calculate the appropriate buck windings and use the variometer calculations for 180 degree negative coupling. I will enter the TX windings as if they were the outside coil (as they will be in real life) and the buck windings as the inner coil. Because my buck windings will be at -180 degrees, I will then use the "minimum inductance" output from the calculator as my end value. If it's not within a certain tolerance of my desired coil inductance goal I will tweak a little until it is. So I think I may have answered my own question now that I found the variometer calculations. Thanks for the interest. I'll be sure to let folks here know how I do. My end goal at present is just to build a large coil (for my Bounty Hunter Land Star). If the Land Star doesn't "like" my coil, I can build my own detector to fit it. I'm better at electronics than magnetics, so , I want to start by designing the coil. Once I have it built and balanced I have confidence that the rest is just a matter of circuit design. And I'll also know how to build a coil to spec., so building one of another value should be easy. I have a friend who lives near an old stage coach line and knows where a coach stop was many years ago (yes, "exactly" where - it's a big desert out there). We want to two-man carry (or pull behind a horse - yes, it's bumpy and coil microphonics other issues will come to play) a large coil to look for artifacts that will then be directed towards a local museum.

[Aziz]

Yes, it is affecting the total inductance, when there are more than two tx-coils with interleaving/overlapping magnetic flux. This is called mutually inductance between the coils. As anyone mentioned above, it is not easy to build such coil arrangements.
There is an algorithym called "Neumann's formula" to solve such problems.

See following link for more info:
http://www.answers.com/topic/inducta...cat=technology

Aziz

[Carl-NC]

Jeremy,

The bucking coil is normally wired in series with the TX coil. You could drive the bucking coil independently, but there is the danger of having the drive current shift and messing up the balance.

One way of figuring out the effect of the bucking coil is with a little experiment. Wind up a coil as outlined in Dave Emery's article. It doesn't have to be to some exact inductance, just make sure the bucking coil is half the diameter and 1/4 the turns. Measure the inductance. Now snip the two coils apart, and measure their inductances independently. This will tell you how much the bucking coil affects the overall inductance.

I did a little exercise in math, and it came up with the bucking coil causing a relative shift of 1/32. That is, if your goal is a 1mH coil and you calculate the TX coil for this value, then the addition of the bucking coil will shift the total inductance to 31/32 mH, or 0.96875mH. It would be interesting to see if my math is correct.

- Carl