Dual coil for mpp

[dbanner]

Non-final SRFs are still useful. I suggest measuring the SRF at every stage: raw coil, add the shielding, add the cable, add the connector, add the MOSFET, add the clamp, add the preamp. In this way you get to see the specific effects of each step, and it will suggest where to focus your efforts if you are looking for more speed.

Carl, you are always so good at explaining things.

[dbanner]

A circuit with a value of resistor that causes it to be just on the edge of ringing is called critically damped. Either side of critically damped are described as underdamped (ringing happens) and overdamped (ringing is suppressed).
The critically damped response represents the circuit response that decays in the fastest possible time without going into oscillation.

So the objective here is to have CRITICAL DAMPING of the coil circuit (for PI).
Now the "coil circuit" consists of the coil AND THE ENTIRE FRONT END.
This is how I see it.
If the final damping resistor value is way off what you calculated for Rd, then you must assume the rest of the coil circuit is impacting on the ringing oscillations adversely. This would mean that you have a poorly designed front end which would retard the fastest possible decay time.
What all this means is that just simply winding a "faster coil" is no guarantee of improved performance if it is being undermined by a "slow" front end circuit.

[dbanner]

Q factor and damping

What about Q factor and damping? Is this an important consideration?
What effect does Q factor have on damping and response time?
Maybe this question is for another thread.

[Qiaozhi]

What about Q factor and damping? Is this an important consideration?
What effect does Q factor have on damping and response time?
Maybe this question is for another thread.

Q factor relates to the system as a whole. The system is critically damped when Q = 0.5.
For a system with no damping resistor, Q > 0.5 (i.e. underdamped)

[dbanner]

Q factor relates to the system as a whole. The system is critically damped when Q = 0.5.
For a system with no damping resistor, Q > 0.5 (i.e. underdamped)

So I assume overdamping is Q<0.5

It seems that the math involved in all of this PI business is tedious at best. I just don't have the skills.

[green]

Connected a completed mono coil(shielded with leads) to my TRT Tx. Want to try damping the coil. Made spice simulation using the coil parameters. Spice circuit resonance a little lower than TRT Tx, maybe due to a different Mosfet or added V2 and D2 for avalanche volts? The coil decay looks a little better slightly underdamped(1150 ohms)? 1100 is close to critical, 1100*1.1=1200 and 1100*.9=990. Including zip.
Not sure my scope is going to let me see the lower part of the decay.

[Carl-NC]

So the objective here is to have CRITICAL DAMPING of the coil circuit (for PI).

Actually, you can usually tolerate a little bit of underdamping as long as the overshoot remains well within the linear region of the preamp. Ergo, slightly underdamped will be slightly faster.

Now the "coil circuit" consists of the coil AND THE ENTIRE FRONT END.
This is how I see it.

Yes.

If the final damping resistor value is way off what you calculated for Rd, then you must assume the rest of the coil circuit is impacting on the ringing oscillations adversely. This would mean that you have a poorly designed front end which would retard the fastest possible decay time.

Maybe, maybe not. If you measure the final SRF with everything attached, then the calculated Rd should be correct. But that might not be the physical Rd you put in parallel with the coil. Remember that the clamp resistor also ends up, at a minimum, as a part-time damping resistor. So the physical Rd will likely be higher than the calculated Rd.

What all this means is that just simply winding a "faster coil" is no guarantee of improved performance if it is being undermined by a "slow" front end circuit.

That's why I suggest measuring SRF every step of the way. Then you get to see what matters.

[dbanner]

Thanks for the feedback.
Yes, I agree that the in circuit final SRF calculated Rd should be very close to the actual Rd. I was referring to using the measured SRF of the raw coil to calculate Rd.

Your comment about the formula in the whites patent intrigue me. That patent is page after page of repitition of the same thing. Now I think maybe the inner damping resistor could be a sham.
Now I suppose a Tx outer coil and a Rx inner coil of the same dimensions as the dual field would not run afoul of this particular patent.

[Carl-NC]

Your comment about the formula in the whites patent intrigue me. That patent is page after page of repitition of the same thing. Now I think maybe the inner damping resistor could be a sham.

No, it needs to be independently damped. This is generally true of multi-coil PI systems. I've designed both figure 8 and stacked coaxial systems, and the coils have to be individually damped, otherwise they feed each other and ring.

[dbanner]

No, it needs to be independently damped. This is generally true of multi-coil PI systems. I've designed both figure 8 and stacked coaxial systems, and the coils have to be individually damped, otherwise they feed each other and ring.

Yup


"LET'S KICK THE TIRES AND LIGHT THE FIRES, BIG DADDY...." quote from the movie "Independence Day"
I scoured the forum for any information on the TDI DUAL FIELD coil, and look what I found, see anything familiar?
YES THERE IS A DAMPING RESISTOR IN THE COIL HOUSING FOR THE INNER COIL. And what's more, we can read it's value!!!
I make it out to be 4.7K@1% tolerance. Yellow violet black brown brown. A four band jobby.

[dbanner]

The wire looks to be a multi strand type, but not silver coated?? The insulation looks to be thickish. Pretty straightforward actually. No magic there, just as how the patent describes.
Now question is what can be inferred from such a high Rd value for the inner coil?
And what about the other damping resistor, I assume it's in the control box. I guess it's value would be pretty low? Say between 500-700 ohms.

[godigit1]

Keeping with the idea of front end impediance and or probe impediance effecting calculated SRF.
I would like to propose again that the method of live testing gives you all the information needed.

In greens post #105 ill use as a example and my post #136 we come up with very similar #s for the different methods of loop pick up.

SRF
6.7MHz____PI coil exciting small coil, loop pickup
3,87MHz___small coil connected to Tx with diode, loop pickup
2.82MHz___small coil connected to Tx with diode, x10 probe across coil
550kHz____small coil connected to Tx no diode, x10 probe across coil
1.3MHz____PI coil exciting small coil, x1 probe across small coil
3MHz_____ PI coil exciting small coil, x10 probe across small coil


I would like to propose that the higher 6.7 mhz is the free air or spacial resonance.
The 10 X probe across the coil is half loaded down and can be used for inductance and capacitance calcs as SRF.
The 1 x probe across coil is used for damping resistor calculations as the probe is heavily loading the coil.

The Srf drops by half or there about for each method.

In post #118 I use the lower 1x probe SRF# to calculate the damping resistor.

I damped first with a resistor set up as fast as it could go without ringing and also used the calculated resistor.

The calculated resistor actually was slightly overdamped.

When testing with targets on the resistor set up you could see a slight oscillation pop in for just a second, the inner coil would ring just a flash when hit with a target.

With the calculated resistor and being slightly overdamped the ringing was gone.

Thinking the slight overdamp makes up for or adjusts for the effects of coil coupling.

The whites patent formula seems to work for me.

What do you guys think is that logical at all?

[godigit1]

Quick note for fun tested the 12" dual field with the dip meter. 3.154Mhz. thinking that dip meter #s should be devided in half for a PI coil to be more accurate.

Ill hit it with a genny later.

[dbanner]

4.7K Rd for the whites inner coil makes me think it is overdamped because of the reflective interaction of the two coils.
I think I'm confusing myself.
Does increasing the Rd increase(overdamp) or decrease(underdamp) the coil??? My brain is starting to hurt. I had to refresh my scientific notation skills just to do the algebra using those pF and uH variables. Then I came up with a simplified shorthand for these calculations that I can do in the mind quickly. But it is still a chore.


Green's shorthand of pi* L(uH)*f(mHz), don't know how he arrived at it, and I haven't compared any results. The standard damping resistor formula is:
Rd = 0.5*sqrt(L/C)

The new formula:
Rd = pi * L * 10^-6 * f * 10^6 (in µH and MHz)
Rd = pi*L*f (in H and Hz units now)

1.
Rd = pi*L*f

Resonant frequency:
2.
f = 1/(2*pi*sqrt(L*C))
we put f in the first formula.

Rd = pi*L*1/(2*pi*sqrt(L*C))
Rd = 1/2 * L/sqrt(L*C)
Rd = 0.5 * sqrt(L*L)/sqrt(L*C)
Rd = 0.5 * sqrt(L*L/L*C)
Rd = 0.5 * sqrt(L/C)
END
So it ends up to the same thing. But using Green's shorthand formula is easier for our purpose here since we are dealing with mHz and uH.


Another interesting experiment would be to test a preferred target for maximum distance while adjusting the Rd with variable damping tool, then checking on the scope to see how much the coil is underdamped.

[green]

4.7K Rd for the whites inner coil makes me think it is overdamped because of the reflective interaction of the two coils.
I think I'm confusing myself.
Does increasing the Rd increase(overdamp) or decrease(underdamp) the coil??? My brain is starting to hurt. I had to refresh my scientific notation skills just to do the algebra using those pF and uH variables.

Low resistance overdamps, Open circuit oscillates. Rd=pi*L*frequency, L*frequency=4700/pi or L*frequency=1500. If L=300uH, frequency would=5MHz. I would guess underdamped.