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Is there an ideal ratio of Rx coil diameter to Tx coil diameter for a balanced concentric co-planer? I would think that a relatively large Rx coil (i.e. closer to the diameter of the Tx coil) would decrease the effective magnetic flux density because of the larger bucking coil that would be required. So if my Tx coil is to be 11" diameter, what would be the ideal diameter for the Rx coil? Or does the diameter of the Rx coil make any difference, just the aggregate length of wire in the Rx coil?
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IB coils
[quote=KingJL;71719]Is there an ideal ratio of Rx coil diameter to Tx coil diameter for a balanced concentric co-planer? I would think that a relatively large Rx coil (i.e. closer to the diameter of the Tx coil) would decrease the effective magnetic flux density because of the larger bucking coil that would be required. So if my Tx coil is to be 11" diameter, what would be the ideal diameter for the Rx coil? Or does the diameter of the Rx coil make any difference, just the aggregate length of wire in the Rx coil?[/quote
Increasing the distance between the TX and RX coils, diminishes the coupling k factor between the two coils.
A lower coupling factor means that the flyback spike will be reduced in the RX coil, making its decay faster.
Tinkerer -
Tinkerer, KingJL
Everything said about balanced co-planar coils is correct. But what about unbalanced co-planar coils such as is used in the Garrett PI. See the coil datasheet at the link below and carefully look at the mono coil.
http://www.garrett.com/hobby/techsup...searchcoil.pdf
It is actually two separate coils, the outer coil for TX and the inner coil for RX. This coil arrangement is not designed to balance out the TX pulse spike.
What could be the benefit of doing this?
The analysis of this question and answer will lead to an intimate understanding of the coil discharge Time Constant.
bbsailorComment
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TX & RX coil arrangement
bbsailor, thanks a lot for the Garrett coil pdf.Originally posted by bbsailor View Post
The way I understand it, the smaller RX coil has less capacitance and is therefore faster.
It has less capacitance because it is disconnected from the Mosfet and its output capacitance and it has less inter-wire and coil to shield capacitance.
The distance between the coils reduced the TX to RX inductive coupling which reduces the TX flyback spike in the TX coil, making the decay curve shorter.
The question now would be, where is the ideal proportion between the relative RX and TX sizes?
TinkererComment
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Close-Concentric PI Coils
Originally posted by Tinkerer View Post
Tinkerer
In a previous post you said: " Increasing the distance between the TX and RX coils, diminishes the coupling k factor between the two coils.
A lower coupling factor means that the flyback spike will be reduced in the RX coil, making its decay faster."
Actually, the coupling is based on the relative area of the TX coil and the RX coil. That is why an RX coil that is exactly .5 times the TX coil diameter has exactly 1/4th the area and requires 1/4th the number of feedback turns of the TX coil to balance/neutralize the coupling.
So, given the above logic, putting the RX right next to the TX coil increases the coupling but does provide some other design alternatives to consider.
1. The TX mono coil value of Rd is only based on the TX circuit capacitance and is not effectively in parallel with Rin (input resistor) while the clamping diodes are conducting.
2. Separate TX and RX coils can be critically damped independently, although the RX coil will still see an underdamped TX coil. So, adjust the damping of TX coil first, then fine tune the RX coil damping.
3. Separate TX and RX coils allow the RX coil to have a few more turns for more sensitivity without adding more opamp gain. These extra turns equate to the equivalent capacitance not seen in the MOSFET COSS. Let's assume that you have the Hammerhead circuit with a 500uH coil and 680 ohm Rd. If you use separate TX and RX coils you might need a little lower Rd since Rin is no longer in parallel down to 0.6V. But you could use a slightly higher Rd on the RX coil since it sees less total capacitance. You could probably increase the inductance by about half to about 750uH and still use about the same Rd as before. These extra turns would provide a little increased sensitivity. Conversly, you could keep the same amount of turns and optimize the RX damping resistor for the fastest sampling.
4. Optimize the RX coil discharge TC for a of a specific target TC without affecting the TX coil TC too much.
This is an area that experimenters can run some pretty quick tests to see the benefits or opportunities of close concentric coil arrangements like in the "Garrett Mono" coil link. Since the diameter of both coil is rather large, it has the depth of a mono coil without the slight depth limitations of the 20% smaller DD coils in the coil housing compared to a mono coil in the same housing.
The question now would be, where is the ideal proportion between the relative RX and TX sizes?
This is an impossible question to answer in the abstract. It depends on the TC of the target, sixe of the target, workable and practical coil size, the ground mineralization and the power in the TX coil.
The fill benefit of a balance DD coil is 2 us to 3us over a mono coil. The close concentric benefit would only be about half, 1us to about 1.5us but is something to think about as a design alternative.
I hope this stimulates some experiments.
bbsailorComment
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