Fast coils: worth it?

[Noemio]

Hi,

after having so much read about PI metal detectors and the need to build fast coils to obtain high values of performance, I realize that I do not have objective data to decide if it really worth to spend so much time and money to build this fateful “fast coil”.
Is there anybody who has comparative data that could make everyone understand the extent to which it is worth spending time and money to make a fast coil?
I also wonder why, for example, to prefer expensive Teflon insulated conductors when the polyethylene has a dielectric constant almost identical (2.3 vs. 2.1) and then spend a fortune instead of using the simple twisted pair.
Can anyone tell me WHEN it is worth achieving a fast coil? For what types of research, and possibly on what types of MD?
Thank you in advance.

[Ism]

I believe the fast coils are for use with fast PI detectors. I would guess 10us delay and under. As delay time increases the need for a fast coil is reduced.

[Kevco]

Noemio,

First, if you haven't read it already, read Joe Rogowski's (aka bbsailor) article on Fast Coils.

For me, I want a fast coil, to hunt for a low conductive metal -> gold. While slow coils work OK for normal coins, and even gold rings, they fail on small gold nuggets. I base that on my 30 uSec "slow" 11" dia coil. But, fast coils can detect small nuggets.

You questioned about teflon vs polyethylene. The common wire wrap wire is usually insulated with Kynar, which also has a dielectric constant (DC) matching teflon, but far cheaper in cost (and a thinner layer of insulation). I used a wire twister, to wind kynar wire wrap wire with a string of polypropylene (PP's DC is usually low, especially as fibers) - to increase spacing between coil turns, and reduce stray capacitance. I then wound an 8" mono coil with that wire & PP string combo. It ended up fast enough to use down to 8 uSec delay after being shielded. But the 8" coil can only detect small gold nuggets near the coil itself, and not in the center of the coil.

Kevco

[Tinkerer]

Noemio,

First, if you haven't read it already, read Joe Rogowski's (aka bbsailor) article on Fast Coils.

For me, I want a fast coil, to hunt for a low conductive metal -> gold. While slow coils work OK for normal coins, and even gold rings, they fail on small gold nuggets. I base that on my 30 uSec "slow" 11" dia coil. But, fast coils can detect small nuggets.

You questioned about teflon vs polyethylene. The common wire wrap wire is usually insulated with Kynar, which also has a dielectric constant (DC) matching teflon, but far cheaper in cost (and a thinner layer of insulation). I used a wire twister, to wind kynar wire wrap wire with a string of polypropylene (PP's DC is usually low, especially as fibers) - to increase spacing between coil turns, and reduce stray capacitance. I then wound an 8" mono coil with that wire & PP string combo. It ended up fast enough to use down to 8 uSec delay after being shielded. But the 8" coil can only detect small gold nuggets near the coil itself, and not in the center of the coil.

Kevco

Hi Kevco,

congratulations on your results.

I use the following calculatorhttp://www.mogami-wire.co.jp/e/cad/electrical.html for inter-wire capacitance and different dielectric constants.
Very important is the distance of the shielding also. The same string wrapped around the coil bundle makes a good spacer. The PP string as spacer probably has a similar effect as foam, because the space between the wires is then mostly air.

A PP/air space of 8mm between shield and coil seems to work well.

Tinkerer

[Kevco]

Using polypropylene string in coils

To add to my above post, to include more details for those who can use them...

I live in the US, and I will name the stores I bought these at - for the benefit of those who also live here.

First, I thought I had a great idea to use a combination of polypropylene string & wire to act as a spacer, and reduce capacitance. I used a wire twister to uniformly twist the wire and string together into one chord. Now, after reviewing other posts, I see this is nothing new. Using string as a coil spacer, especially polypropylene string, has been used by others, with good results. Mikebg (?) even posted part of a patent that used string as coil spacers.

I purchased polypropylene string at Home Depot. It was sold as "Masonry Line". But be careful when selecting it - because they also carry other strings that are sold with the same label - but are made with nylon, or other plastics other than polypropylene. The spools look the same, except for string color, and some differences on the label - but by the same company. Each spool is marked, if you read it carefully. Polypropylene has a low dielectric constant, and when woven as a fibrous string, the DC is far lower. It is a strong fiber, and is found in many strings, twines, and ropes.

I purchased the Kynar insulated wire wrap wire at Radioshack. It is sold in small spools, with 50 feet on each spool, and sells for US$ 3.99 each. After that coil worked so well, I ordered a 500 foot spool from: http://www.squires.com They sell 500 feet of Kynar 30 awg wire for US$ 23 (their site says US$ 21, but it's gone up).

After winding the pair of string and wire, the string made it "want" to come unwound (while wire will want to partially assume the shape it's been bent to). Long ago I had to re-work wiring harnesses (bundles of wires), and we commonly would lace the harnesses with a waxed nylon chord. It's expensive, so for binding the string & wire bundle together, I used waxed dental floss to lace it together. I wound a bunch on a small spindle, so I could weave it around the wound coil, and then laced it together. It worked perfectly! I then added two layers of plastic spiral wrap, and shielding to complete it.

I used the entire spool of 50 feet, in an 8" diameter coil. Equations said it should take 53 feet to obtain 300 uH, but I measured it after lacing it, and obtained 314 uH. The coil bundle was 7-8mm diameter, before adding plastic wrap and shielding. After adding those, it fit nicely inside of the 8" PI coil housing from Hays Electronics.

I have been using the following site, to compare wire to wire capacitance, using their "twisted pair" computations for capacitance. It make a great way to compare wires & dielectrics, to find wire with insulation which should give a low capacitance.
http://www.mogami-wire.co.jp/e/cad/electrical.html (just as Tinkerer has been using).

30 awg kynar as a twisted pair should have 46 pf/meter. Polypropylene string measured 33 mils (i.e. 0.031 inch) diameter, and can probably be compressed to 25 mils wide when wound in a coil. Using that distance between two kynar wires, should yield about 21 pf/m (*). But it would average half way between 21 & 46 pf, giving 33.5 pf/m. But in reality, the capacitance is probably lower, due to all of the air mixed in the fibers of the string.

(*) The mogami website only lists five dielectrics: air, pvc, ptfe, cpe and pe. Since polypropylene's DC is ~1.5, you can use "air" as the dielectric (air DC~= 1), and then multiply the result by 1.5 to obtain the expected capacitance (capacitance is directly proportional to dielectric constant).

Kevco

[bbsailor]

To add to my above post, to include more details for those who can use them...

I live in the US, and I will name the stores I bought these at - for the benefit of those who also live here.

First, I thought I had a great idea to use a combination of polypropylene string & wire to act as a spacer, and reduce capacitance. I used a wire twister to uniformly twist the wire and string together into one chord. Now, after reviewing other posts, I see this is nothing new. Using string as a coil spacer, especially polypropylene string, has been used by others, with good results. Mikebg (?) even posted part of a patent that used string as coil spacers.

I purchased polypropylene string at Home Depot. It was sold as "Masonry Line". But be careful when selecting it - because they also carry other strings that are sold with the same label - but are made with nylon, or other plastics other than polypropylene. The spools look the same, except for string color, and some differences on the label - but by the same company. Each spool is marked, if you read it carefully. Polypropylene has a low dielectric constant, and when woven as a fibrous string, the DC is far lower. It is a strong fiber, and is found in many strings, twines, and ropes.

I purchased the Kynar insulated wire wrap wire at Radioshack. It is sold in small spools, with 50 feet on each spool, and sells for US$ 3.99 each. After that coil worked so well, I ordered a 500 foot spool from: http://www.squires.com They sell 500 feet of Kynar 30 awg wire for US$ 23 (their site says US$ 21, but it's gone up).

After winding the pair of string and wire, the string made it "want" to come unwound (while wire will want to partially assume the shape it's been bent to). Long ago I had to re-work wiring harnesses (bundles of wires), and we commonly would lace the harnesses with a waxed nylon chord. It's expensive, so for binding the string & wire bundle together, I used waxed dental floss to lace it together. I wound a bunch on a small spindle, so I could weave it around the wound coil, and then laced it together. It worked perfectly! I then added two layers of plastic spiral wrap, and shielding to complete it.

I used the entire spool of 50 feet, in an 8" diameter coil. Equations said it should take 53 feet to obtain 300 uH, but I measured it after lacing it, and obtained 314 uH. The coil bundle was 7-8mm diameter, before adding plastic wrap and shielding. After adding those, it fit nicely inside of the 8" PI coil housing from Hays Electronics.

I have been using the following site, to compare wire to wire capacitance, using their "twisted pair" computations for capacitance. It make a great way to compare wires & dielectrics, to find wire with insulation which should give a low capacitance.
http://www.mogami-wire.co.jp/e/cad/electrical.html (just as Tinkerer has been using).

30 awg kynar as a twisted pair should have 46 pf/meter. Polypropylene string measured 33 mils (i.e. 0.031 inch) diameter, and can probably be compressed to 25 mils wide when wound in a coil. Using that distance between two kynar wires, should yield about 21 pf/m (*). But it would average half way between 21 & 46 pf, giving 33.5 pf/m. But in reality, the capacitance is probably lower, due to all of the air mixed in the fibers of the string.

(*) The mogami website only lists five dielectrics: air, pvc, ptfe, cpe and pe. Since polypropylene's DC is ~1.5, you can use "air" as the dielectric (air DC~= 1), and then multiply the result by 1.5 to obtain the expected capacitance (capacitance is directly proportional to dielectric constant).

Kevco

Kevco, and all interested in PI coils

When using the coil calculator on the following link http://www.wetnet.net/rf_design/airind.main.cgi, use the 1% answer. This is closer to actual winding measurements. If you are using the "hook method" in my Fast Coil article, then wind an extra turn or two and measure the coil unductance while on the hooks. It will be about 10% less than the inductance once bundled in the spiralwrap which holds the wires in a tighter bundle and increases the inductance. Cut off any extra turns so you get an inductance reading of about 285 uH for a trageted 300 uh coil while on the winding hooks.

One last important tip is how to enter the coil length in the above coil calculator. The cross section diameter of the coil wire bundle should be the coil length in the above calculator. Use this formula: D = 1.155 X d X the Square Root of the coil turns number (N) where:
D = the coil bundle diameter
Constant = 1.155
d = Outside Diameter (OD) of the coil wire, including insulation
N = number of coil turns

19 turns of AWG 30 Teflon insulated wire is: .024" (wire OD) X 1.155 x SQ RT 19 turns (4.3589) = 0.1208" for the coil length in the above calculator.

This tip should help you get more consistant results when making coils. Note that thicker wire and insulation makes the coil bundle get pretty thick, in smaller diameter coils of 8" or less since they have more turns to reach the desired coil inductance. Add the spiral wrap and shielding and the .5" wide space of some coil housings gets pretty tight.

Kevco is right about fast coils. Small gold targets need to be sampled faster to obtain a response from PI metal detectors. Try detecting a thin gold chain which looks like a bunch of individual small gold links. Anyone hunting for coins or rings can get away with delays between 10uS and 15 uS on the beach. It is only on very small gold targets that the ability to use low delay settings can make a difference between being detected or not!

When making coils, measure the coil self resonant frequency and post the results. This will be most helpful for those making their own coils and it tells you alot about the Dielectric Constant of the wire insulation. Here, higher self resonance is better as it indicates less capacitance between turns. Adding the shield will lower the self resonance by adding about 25% of the measured coil-to-shield capacitance.

Less coil, coax and MOSFET capacitance (look for low COSS ratings) means that the value of the damping resistor can be higher because of the need to damp less stored energy in the form of all these TX circuit capacitances. Higher damping resistance means that the turn-off slope of the TX pulse is steeper and can stimulate smaller gold targets better.

I hope this helps.

bbsailor

[Qiaozhi]

When using the coil calculator on the following link http://www.wetnet.net/rf_design/airind.main.cgi, use the 1% answer.

This calculator is only valid for single-layer solenoid coils. Note that it asks for the coil length.

[Monolith]

Uncovering the secrets of PI coils

When we increase the space between the coil windings, we reduce the inter wire capacitance. We also reduce the inductance a little. Therefore we may need one turn more of wire to get the same inductance.
What is the ideal distance between the wires?
For the shielding we have again the capacitance problem. Increasing the distance between the coil bundle and the shielding reduces the capacitance.
The only limitation to the distance seems to be the size of the coil housings.

Now, capacitance in parallel increases. Capacitance in series gets less. If we could find a way to get some of these capacitances in series, we could really make some improvements.

Monolith

[N1PS]

Interesting thread...fast coils. Have builders noticed better performance using spiral type coils or other variations that reduce wire-wire capacitance? The concept is not new...a half century ago hams and antenna engineers created magnetic loop antennas (Google this phrase) in this manner using either face wound (spiral) or edge wound conductors. By carefully spacing the wires, the null (and node) could be clearly defined. The same goes for shielded loops. I'm not certain there is a relationship to the MD coil, but suspect there is.

I have built several loop antennas over the years...now about to make some PI coils. I look forward to playing with these as well as my first MD (a Surf PI)! The trick is not to let the smoke out.

Peter

[Monolith]

Interesting thread...fast coils. Have builders noticed better performance using spiral type coils or other variations that reduce wire-wire capacitance? The concept is not new...a half century ago hams and antenna engineers created magnetic loop antennas (Google this phrase) in this manner using either face wound (spiral) or edge wound conductors. By carefully spacing the wires, the null (and node) could be clearly defined. The same goes for shielded loops. I'm not certain there is a relationship to the MD coil, but suspect there is.

I have built several loop antennas over the years...now about to make some PI coils. I look forward to playing with these as well as my first MD (a Surf PI)! The trick is not to let the smoke out.

Peter

One of the first differences between the PI coil and a magnetic loop antenna that comes to mind, is the bandwidth.
the magnetic loop antenna is designed for an extreme narrow bandwidth.
The PI coil needs to be able to receive a bandwidth from about 3Hz to 200KHz.

Monolith

[bklein]

Please elaborate on your bandwidth statement.
I don't understand why it would be spec'd so wide.

[Monolith]

Please elaborate on your bandwidth statement.
I don't understand why it would be spec'd so wide.

I wish I knew exactly how this works.
It is a proven fact that "fast" coils are needed for detecting targets that have a TC of less than 10us.
I have also noticed that if I reduce the bandwidth of the preamp below 150KHz, the signal of targets that have a fast decay, is attenuated.

There are other things involved, like the PPS or pulse per second (maybe 3000) and the sweep speed.
A standard sweep speed of 1m/s, amounts to about 3Hz of signal response for a 300mm diameter coil.

We usually sample at the rate of the PPS and then integrate the samples.

Monolith

[Bill512]

As the primary field collapses, every target reacts as fast as it allowed by its properties. Small TC, means the fastest reaction at the given rate of collapsing field.
We can consider the voltage on every element (small, big targets, coil itself) , as a superposition of several frequencies, with respect on their magnitude and phase.
This is well known as Fourier transform.
So, the signal from the fast targets consists of a "packet" of the higher frequencies .Also the slow targets maybe have some fast components, but these are not critical to their detection.
these ,are just some thoughts...

[dfbowers]

To add to my above post, to include more details for those who can use them...

I live in the US, and I will name the stores I bought these at - for the benefit of those who also live here.

First, I thought I had a great idea to use a combination of polypropylene string & wire to act as a spacer, and reduce capacitance. I used a wire twister to uniformly twist the wire and string together into one chord. Now, after reviewing other posts, I see this is nothing new. Using string as a coil spacer, especially polypropylene string, has been used by others, with good results. Mikebg (?) even posted part of a patent that used string as coil spacers.

I purchased polypropylene string at Home Depot. It was sold as "Masonry Line". But be careful when selecting it - because they also carry other strings that are sold with the same label - but are made with nylon, or other plastics other than polypropylene. The spools look the same, except for string color, and some differences on the label - but by the same company. Each spool is marked, if you read it carefully. Polypropylene has a low dielectric constant, and when woven as a fibrous string, the DC is far lower. It is a strong fiber, and is found in many strings, twines, and ropes.

I purchased the Kynar insulated wire wrap wire at Radioshack. It is sold in small spools, with 50 feet on each spool, and sells for US$ 3.99 each. After that coil worked so well, I ordered a 500 foot spool from: http://www.squires.com They sell 500 feet of Kynar 30 awg wire for US$ 23 (their site says US$ 21, but it's gone up).

After winding the pair of string and wire, the string made it "want" to come unwound (while wire will want to partially assume the shape it's been bent to). Long ago I had to re-work wiring harnesses (bundles of wires), and we commonly would lace the harnesses with a waxed nylon chord. It's expensive, so for binding the string & wire bundle together, I used waxed dental floss to lace it together. I wound a bunch on a small spindle, so I could weave it around the wound coil, and then laced it together. It worked perfectly! I then added two layers of plastic spiral wrap, and shielding to complete it.

I used the entire spool of 50 feet, in an 8" diameter coil. Equations said it should take 53 feet to obtain 300 uH, but I measured it after lacing it, and obtained 314 uH. The coil bundle was 7-8mm diameter, before adding plastic wrap and shielding. After adding those, it fit nicely inside of the 8" PI coil housing from Hays Electronics.

I have been using the following site, to compare wire to wire capacitance, using their "twisted pair" computations for capacitance. It make a great way to compare wires & dielectrics, to find wire with insulation which should give a low capacitance.
http://www.mogami-wire.co.jp/e/cad/electrical.html (just as Tinkerer has been using).

30 awg kynar as a twisted pair should have 46 pf/meter. Polypropylene string measured 33 mils (i.e. 0.031 inch) diameter, and can probably be compressed to 25 mils wide when wound in a coil. Using that distance between two kynar wires, should yield about 21 pf/m (*). But it would average half way between 21 & 46 pf, giving 33.5 pf/m. But in reality, the capacitance is probably lower, due to all of the air mixed in the fibers of the string.

(*) The mogami website only lists five dielectrics: air, pvc, ptfe, cpe and pe. Since polypropylene's DC is ~1.5, you can use "air" as the dielectric (air DC~= 1), and then multiply the result by 1.5 to obtain the expected capacitance (capacitance is directly proportional to dielectric constant).

Kevco

Kevco,

Hey, do you mind if I ask how your coil ohms out for 50 ft? For my 10", 18t coil using Radioscrap wire wrap, I'm measuring a little over .5 ohms. My measurement seems kind of low for 47 ft of wire.

Thanks.
Don

[hobbes_lives]

That does seem low. It should be closer to 5 ohms. 30 AWG wire is approximately 103.2 ohms per 1000 feet. 47/1000*103.2 = 4.85 ohms