Low capacitance coils

[Tinkerer]

I have not been able to find information (numbers) on spider wound coils, basket wound coils. My initial tests showed that the capacitance is greatly reduced, compared to tight wound coils. My question is: how do these types of windings influence the tx field and the rx sensitivity?
Tinkerer

[bbsailor]

Tinkerer,

Winding low capacitance coils, by any method including spider winding, basket weave, thicker insulation, pancake spiral winding, etc will allow the coil to sample faster for low conductivity targets like thin gold bands and small nuggets. Like any coil that is spread out, it will have a lower inductance than a tightly wound coil and thus need a few more turns to achieve your designed inductance.

The more important aspect of these low capacitance winding techniques is how are you going to shield the coil? At a minimum your coil to shield capacitance will be about 30 to 40 pf per foot of coil circumference. However, if you wind a good low capacitance coil and have a high coil to shield capacitance, would will negate any benefits of the low capacitance winding technique.

The more important measurement is the total capacitance of the coil with it's shield and coax wire. This is what the PI circuit sees and what the value of the damping resistor will be based on. The total capacitance can be obtained by observing (on a scope) the self resonant frequency of the shield coil by connecting the scope to the coil connector at the end of the coax. Pulse the coil with another coil connected to the pulse generator and look for the ringing pulses on the scope and measure the self resonant frequency. This method eliminates the capacitance imposed by the scope probes which can lower the resonant frequency by several 10s of Khz. Once you know the self resonant frequency, you know the coil inductance and you can calculate the effective capacitance that creates that self resonant frequency. Anything you do in your coil design that raises the self resonant frequency is good!

If you are looking for coins and large objects, a fast coil is not necessary as you will be looking for target responses that last for a longer time. If you reduce the pulse delay and the coil suddenly stops responding to your target, you have reached the lower limit of your coil's fastest response and must reduce total capacitance somehow.

I find that using wire with thicker insulation and plastic spiralwrap to keep the shield farther away from the wire coil is a good coil making technique. Now you need to find a shield materiel that will be minimally detected at the fastest speed you set you PI detctor. Lead foil, thin alumimunized mylar, 3M copper fabric, and even unwound electrolytic capacitor plates have sucessfully been used to make coils shields.

Your overall performance will be based on how well you integrate all the factors that make up the coil that the PI circuit sees. It is not just the coil winding techniques, but a low capacitance coil is a good start.

bbsailor

[ElectroNovice]

Coil self resonance question

Quote from bbsailor:
"Anything you do in your coil design that raises the self resonant frequency is good! "

Could you be kind enough bbsailor to explain why this is so?
Regards
ElectroNovice

[bbsailor]

Raising the resonant frequency means that the total capacitance of the coil is lower. This higher resonant frequency can come from three primary sources: (1) coil layer to layer capacitance with thicker insulation having lower capacitance, (2) shield to coil capacitance with more space reducing the capacitance and finally (3) the capacitance of the coaxial cable connecting the coil to the PI circuit where 16 pf per foot is about as low as you can find.

So when comparing coil performance, the self resonant frequency is very telling about the potential performance, meaning speed of the coils. Your goal should be to adjust various coil parameters to obtain the highest resonant frequency possible or practical for your coil and the targets you seek. Remember that gold is a poor conductor and the induced response in gold dies away very quickly so a fast coil is needed to obtain a strong response from small gold targets.

Nickel coins are also poor conductors and respond like gold. Testing the coil response to a nickel will indicate more sensitivity to nickels at fast settings than larger quarters.

I hope this answers your question?

bbsailor

[ElectroNovice]

Thanks very much for your reply bbsailor but i have some more questions which your post raises. Which coil winding method will give the lowest interwinding capacitance,spider,basket,mobius or other etc? Rather than using thicker insulation to reduce the capacitance is it better to use the best insulator of the lot- air? ie to wind the coil so that the windings are separated by an air gap and also separated by an air gap from any electrostatic shielding or coil cover? Is there any advantage in using tank resonant circuits to lower the coil resonant frequency to the transmit Pi frequency so that the coil becomes a pure resistive load as the inductive capacitance and reactive capacitance at Q cancel each other out,which would allow much more power to be applied to the coil and hence higher coil magnetic moments( and depth?) could be obtained? Such a method is being explored by Zonge engineering where they use a short high amplitude pulse(20-50usecs) with the shape of a half sine wave which resembles an impulse function in a tuned circuit operating at its resonant frequency. Can this be done with square wave pulses?
Best regards,
ElectroNovice

[bbsailor]

There are whole websites that are dedicated to seeking the higest Q coil for homemade crystal radios. Some of these techniques also apply to PI coils.

http://www.csgnetwork.com/spiralcoilinduccalc.html

http://www.geocities.com/rook_coils/

While seeking to make the absolute lowest capacitance coil, hence the highest Q coil, could take a lifetime of experimentation. Many people have alrady done most of this work. Prime considerations for obtaining this are:

-core dialectric with air being one of the best.
-coil form and wire insulation dialectric.
-coil capacitance with minimal turn-to-turn capacitance.
-wire diameter, high frequencies ride on the outside of the coil, thus litz wire is typically used to lower the high frequency rf resistance to help raise the Q of the coil.

You could study all these characteristics in the abstract and find some really creative coil designs. However, when you apply these techniques to making a PI coil there are some practical coil design characteristics that now come into play.

-Fitting the coil into a coil housing.
-Maintaining coil rigidity to minimize false signals due to the movement of the coil.
-Maintaining a low shield to coil capacitance with no relative movement of the shield to the coil.
-Matching the coil resistance to desired power needed.

If you want to explore the typical coil shape designs that are used in PI coils look below.

http://www.mindspring.com/~eganger/coils.gif

When planning a coil for a PI detector you must depart from the abstract and answer some very fundamental questions first.

-What am I looking for?
-What frequency will the coil be operating?
-What amount power will be going through the coil?
-What are my physical size constraints?
-Am I only making a test coil or must my coil be a finished coil to be submerges in water?
-Do I need a mono coil or a DD coil?

I will not answer these questions as they have all be answered on the forum before by people much smarter than I. Use the search feature and search on key words and phrases. You will find a wealth of technical information on making coils, obtaining fast responses, optimizing circuit designs by using low capacitance MOSFETS. Look to obtain a coil housing from Charles (upstate NY) or from Hays Electronics. Once the coil housing is in hand, you can start to get down to making a specific coil to fit that housing. Hints: See what Reg says about using stranded teflon coated wire. See what I said about using AWG30 Kynar coated wirewrap for a coil.

You will need a few pieces of test equipment to gather some useful data from your experiments.

-Function generator with frequency meter.
-Oscilloscope, dual channel, 10 to 20Mhz (minimum).
-LCR meter.
-Digital Volt Meter

Keep good notes about the following.

-Coil inductance in uH, measured with LCR.
-Coil resistance, measured with LCR.
-Coil self resonant frequency, measured using oscilloscope and Function Gen.
-Capacitance of coil, calculated.
-Capacitance of shield to coil, measured with LCR and calculated by comparing self resonant frequencies before and after shield is added.
-Damping resistor value calaculated from ideal damping resistor formula (look it up on the Internet) compared to actual damping resistor value as see on the oscilloscope.
-Capacitance of coaxial cable, measured by LCR and calculated with before and after self resonant frequency measurements.

Be careful, scope probes add capacitance and lowers the true self resonant frequency. Pulse the coil with another coil using a square wave, and see ringing in the coil-under-test hooked directly to the scope.

The quest for a good PI coil continues!!!

bbsailor

[kt315]

basket coil I do.

[Largesarge]

KT:


Are you using 2 turns of wire and then 1 turn of twine (string) as a spacer between the wire? How many turns and what was the inductance? Lastly, I notice you have ears attached, how did the coil perform?

Many thanks
Dennis

[kt315]

> Are you using 2 turns of wire and then 1 turn of twine (string) as a spacer between the wire?

no. wire and twine were wound together. as the twine is ordinary paper so a paper is good penetrated by an epoxy.

> How many turns and what was the inductance? Lastly, I notice you have ears attached, how did the coil perform?

I still did not measure. nothing of. 24 turns. I hope it will be OK

TAKE ATTENTION, THERE ARE 11 SECTORS - ODD NUMBER

[nncunew]

About coils???

I am making a PI detector with the following coils

20 awg, 28 turns, 8.2 ID 10 OD planar (flat).

My detector is a Sandbank with a MOSFET IRF 740 and a LM 709 in preamplifer but i notice that when i use a damping resistor more than 170 Ohms aparece a ring. I see that my detector have minuscule sentitivity, it is only capture a coke can 333ml about 40 cm in air.

The pin 6 of LM 709 present a wave form that up extremedly rapidy.

I make another coil with 0.5 mm wire, 21 turns, 36 cm diameter but no planar and with no shield but the same target is located at about 45 cm i think that is small distance too.

What do you think about that???

Thank

[email protected]

[kt315]

take attention on a clip. it is amazing really at end of the video you will see THE basket coils.
http://dailymotion.alice.it/video/x3...pe-triode_tech

[Esteban]

Yes, some time ago...

http://thunting.com/geotech/forums/s...ad.php?t=13931

[Tinkerer]

KT315,

How does your coil (above) perform? Could you tell us about it's sensitivity and depth?

What shape does the detecting field sensitivity have compared with the drawing below?

Tinkerer

[Kevco]

Results of basket coil?

kt315,

Did your basket coil work better than simple coils?

Did the basket coil improve sampling less than 10 usec?

[mikebg]

kt315,

Did your basket coil work better than simple coils?

Did the basket coil improve sampling less than 10 usec?

Kevco, basket coil and flat spiral coil make better pinpointing if internal diameter is enough small.
If you wish to sample below 10us, you must avoid cable capacitance. As bbsailor described in posting #4, this is the largest capacitance in the circuit. For this purpose, theTX circuit should be placed near to search coil.
Below is shown a design where TX circuit is placed inside plastic clevis tubing.