10.25" coil for HH

[Gimp]

Gretings, new to the forum and this is my first (probably of many) Questions.

I'm building an HH system on breadboard and have the power supplies, oscillator and coil driver working.

I made a stab at a coil last night using 27 truns (probably 8 too many) of #28 Kynar wire wrap wire. This was taped with yellow transformer (mylar?) tape, then wrapped with spyral wrap and finished with a shield of copper tape (1/8" gap at ends near the leads from the coil.

DC resistance is 4.8 Ohms.

I have access to a HP4193 Vector Impedance meter, but can't make much of the results.

Without shield connected:

26K ohms at 717Khz, phase angle =0 degrees
23K ohms at 700Khz, phase angle = 30 degrees
23K ohms at 738Khz, phase angle = -30 degrees

With Shield:

27K ohms at 673Khz, phase angle =0 degrees
24K ohms at 688Khz, phase angle = 30 degrees
24K ohms at 658Khz, phase angle = -30 degrees

This looks like my resonance is near 700KHz since this is wehre the phase angle passes through zero, so not a bad first attempt, but certainly lots of room to improve.

What I'm trying to understand is how to use the impedance change at 30 degrees to determine the Q of the coil. THe phase shift of 30 degrees with a frequency change of 17Hz out of 700 seems like a fairly quick drop off, but I normally don't work with rf so I'm not sure.

Thanks.

[Gimp]

Typo's - Last sentence should have read 17KHz out of 700KHz.

I got access to a Genrad 1658 RLC Bridge and it shows an inductance of 519.7mH at 1KHz with a Q of 0.69. The inductance seems a tad high based on calculations I've seen.

Comments?

Steven

[bbsailor]

Gretings, new to the forum and this is my first (probably of many) Questions.

I'm building an HH system on breadboard and have the power supplies, oscillator and coil driver working.

I made a stab at a coil last night using 27 truns (probably 8 too many) of #28 Kynar wire wrap wire. This was taped with yellow transformer (mylar?) tape, then wrapped with spyral wrap and finished with a shield of copper tape (1/8" gap at ends near the leads from the coil.

DC resistance is 4.8 Ohms.

I have access to a HP4193 Vector Impedance meter, but can't make much of the results.

Without shield connected:

26K ohms at 717Khz, phase angle =0 degrees
23K ohms at 700Khz, phase angle = 30 degrees
23K ohms at 738Khz, phase angle = -30 degrees

With Shield:

27K ohms at 673Khz, phase angle =0 degrees
24K ohms at 688Khz, phase angle = 30 degrees
24K ohms at 658Khz, phase angle = -30 degrees

This looks like my resonance is near 700KHz since this is wehre the phase angle passes through zero, so not a bad first attempt, but certainly lots of room to improve.

What I'm trying to understand is how to use the impedance change at 30 degrees to determine the Q of the coil. THe phase shift of 30 degrees with a frequency change of 17Hz out of 700 seems like a fairly quick drop off, but I normally don't work with rf so I'm not sure.

Thanks.

Typo's - Last sentence should have read 17KHz out of 700KHz.

I got access to a Genrad 1658 RLC Bridge and it shows an inductance of 519.7mH at 1KHz with a Q of 0.69. The inductance seems a tad high based on calculations I've seen.

Gimp,

Here are a few comments related to your coil.

Your coil inductance is not 519.7mH but is more likely 519.7uH (u=micro while m=milli).

The no shield reactance of a 519.7uH coil at 717KHz is 2341 ohms (not 26K ohms) and has a capacitance of 94.8pf.

The shielded reactance of a 519.7uH coil at 673KHz is 2197 ohms and has a capacitance of 107.6 ohms.

The capacitance difference is the effect of the distributed capacitance of the shield over the coil and is 107.6-94.8 or 12.8pf. This represents a fraction of the total coil to shield capacitance which I estimate to be about 50 pf total. 50pf is a good low number.

To measure coil to shiled capacitance clip one of lead the capacitance meter to the shield and the other to one of the coil wires. Depending on which coil wire you choose, the readings will be slightly different by a few pfs based on how the wire lays in the coil bundle.

Measure the coil resonance with the coax cable connected and the resonance point will be lower. Connect the coil to the circuit and adjust the damping resistor value for the least amount of overshoot at the output of IC6 when observing it with a scope.

I hope this helps.

bbsailor

[Gimp]

Thanks bbsailor,

I'll bring the coil back into work tomorrow and measure it again at lunch time when I can get to the equiptment in the lab.

[Gimp]

Yup, I typed mH when it should have been uH. I need proof read better.

I tried the HP4193 Vector Impedance meter again and am convinced it is wrong. It was red tagged for out of calibration since no one uses it and the company didn't want to pay the calibration fee. I figured it would still function properly although it would be out of cal.

I still get a resistance in the range of 23K ohms at resonance (688Khz,) with just the coil and shield connected. So the resistance reading is certainly suspect.

If I connect 50" of RG-59U from the coil to the board, and connect the meter at the RG-59U connection point, I can not find a resonance point all the way down to 400KHz (limit of the meter), even if I remove the dampening resistor (820 ohm).

I then measured the inductance and Q with the RLC Bridge with the coax connected (no dampening resistor), and got .545mH with a Q of 0.63 so is probably in the ballpark.

I was expecting something around 330-370uH based on a calculation I did, plus what I've read about the inductance of similar coils.

I may wind a new one with fewer turns (18?) using 28AWG magnet wire with a 0.042 OD teflon sheith. The teflon sheith should help hold down the capacitance and up my resonance point.

I'm trying for something around 330uH because it is going in a PI detector and I understand that this is a better value than the 500+uH I currently have.