IB Double-D coil question

It seems the inductances are all over the map on these DD designs.  I understand the manufacturers will make a series of coils that will work on many of their platform designs, so they are just properly impedance matched. 

My question is more specific since I'm designing my own detector and can impedance match to whatever.  Is there any "rule of thumb" anyone knows about as far as a ratio goes from TX to RX and sensitivity?  Some manufactures have identical RX/TX, some have huge ratios.  My best understanding from magnetics and EMF is the higher the inductance the stronger the field given the same current.  Is this why a lot of the DD coils have inductances in the mH range on the RX side, that is to better receive the currents being coupled?

For instance:
2 identical DD coils perfectly nulled and impedance matched(let's say 1mH each) vs 1 coil (tx) using lower inductance(let's say 500uH) in relation to the RX coil having a higher inductance(let's say 2mH), but yet still perfectly nulled and impedance matched.  Would not the latter be more sensitive to currents?
Which one has the better sensitivity?   

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Dear there many other requirements for the sensitivity related issues  in the metal detectors,
Like the coil size, wire gauge, turns of wire of both coil of TX & RX,
1). At Transmitter end may be your TX coil turns did not mach the transmitter frequency which you are Appling,
I have circuits in which I use 555 IC as audio frequency oscillator then the output was given at one of the transistor base to derive the TX Coil, where variable resistor is used between 555 & Transistor for the adjust of power to the coil, because at maximum power to the coil it is hard to protect the receiver coil from the induction between them, the is also another variable resistor for the frequency adjustment (For proper setting), 555 ic oscillator can be easily found through net,

2). at receiver end proper gain control required, because high sensitivity of preamplifiers can create a lot of adjustments issue, it can be control by using variable resistors in your project,

But in my experience these two coils should be the same size, same wire gauge,
But it would be better to use RX Coil more turns than TX Coil of less gauge wire.

Best Regards,

Engr Obaid   


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