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**Reg** · 04-19-2006, 08:58 PM

Hi Red,

I have used both the lead foil and the polyester tape you mentioned and had good results with both. Don't worry about being able to detect the lead foil when it is waved at a distance.

Now, the trick to the foil causing little problems is it has to be wrapped around the windings and not just placed in the housing. So, think of the lead foil shielding as a personal winding coat. If this is done correctly, then the lead foil doesn't become a major player in what you detect even though you can detect a strip of it some distance away. When it is close to the winding, other factors come into place and the foil detection is minimized. It does become a bigger problem when working with DD coils.

Now, I have also tried the coated polyester tape and personally, prefer it. The coils are lighter and the shielding is more than adequate, and there is no problem with detection of the foil in the quantity used for shielding the winding. The problem is, the stuff is getting harder to get in small quantities at a reasonable price.

Most places want you to buy about $300 of the stuff at a time.

Mouser is now selling the newer 3M version of the shielding tape, the CN3190 or something like that. It is about $100 per roll, but is 54 yards long, so there is plenty for multiple coils.

Now, LESSEMF sells a 25 foot roll of similar fabric tape for about $20 a roll which is more in the range of what people might need for just a few coils. I have used this, and it works fine.

Here is an intersting tidbit. I have used metallized ribbon I bought for a few bucks from Walmart also. One side of some of the silvery ribbon is conductive enough that it can be used as a shield. This works fine for electrostatic shielding but doesn't do quite as good of a job of noise shielding. However, if the detector is used away from major noise generators, then this isn't a problem.

In any case, one should space the shielding from the winding just a little, maybe using spiral wrap or even something as simple as plastic wire loom over the windings. Then place the lead foil over the spiral wrap tightly. Just make sure that everything is tight, so there is no movement between the windings and the shielding.

So, before giving up on the lead foil, just remember to use it as mentioned and I think you will find it will work just fine.

Reg

**Reg** · 04-19-2006, 09:27 PM

Here is a pic that, hopefully will display what I mean about the shielding and the windings. Once again, there has to be a space or a gap in the windings along the circumference. The best place to do this is where the wires come out of the coil. Normally, this gap can be quite small, maybe 1/8" to 1/4". I have displayed it larger just to show what I mean. I did not show the spiral wrap in this simplified pic. The spiral wrap would go on the windings and then the shield would go on the spiral wrap.

Reg

Attached Files

MONO WITH SHIELDING.jpg (15.5 KB, 2043 views)

**J_Player** · 04-21-2006, 08:35 AM

Ivconic has reported that he discovered several manufacturers use the anti-static plastic envelope material (like computer boards are shipped in) for a shield. He says this material works very well on the coils he built. This material is a plastic film that has a small amount of metal deposited on the surface by vapor deposition to make it partially conductive. Any grade of anti-static film can be used from the cheap pink poly bags to the more expensive dark silvery looking envelopes. I think the idea is to cut up a non-conductive envelope and wrap it around the coil, with a drain wire also enclosed in the wrap. These can be bought in quantity cheap on ebay and other sources. You can find this material cheap here: http://search.ebay.com/search/search...prclo=&saprchi=

Or go to the local computer store and ask for some discarded anti-static bags that circuit boards come in. They throw them away after installing the circuit boards.

**Reg** · 04-21-2006, 08:57 PM

Hi Player,

I have never tried the bags you mentioned. I have tried items with a higher resistivity and found they didn't quite provide enough shielding. One way to tell is to wave your hand near the coil or even touch the housing. If there is any distinct change in signal, then the shielding is iffy at best.

Also, the bags will not provide much if any shielding for noise. When away from any problem signal sources, this isn't a concern, but in other areas, a good shield does make a big difference.

Let me know how they work out for you if you decide to use the bags.

Thanks,

Reg

**J_Player** · 04-22-2006, 11:59 PM

Hi Reg,
I haven't used the bag material yet, and I think you are probably right. I am just going by what Ivconic posted. He says several manufacturers use it and it works for him. The beauty of the non-conductive film method is you can test it easy and it costs nothing if you have some old film laying around from circuit boards. If it doesn't work, then throw it away and on to plan-B at no cost except a few minutes to try it out.

While we are on the topic of Hammerhead coil shielding, maybe you can help with some questions I have.
I would like to build a Hammerhead PI, but I hesitate because I think I am not capable of building a suitable search coil. My feeling is that the HH PI is a first class circuit design, but the success of the machine depends on building a good search coil, and adjusting the electronics to take best advantage of the coil. I don't want to build a mediocre metal detector. I plan to use the best electronic components I can get, but I am lacking in the coil end of the machine.

Here are my coil questions:

1. I have seen coils wound flat, round bundle, and cylindrical. Does it make a difference? Which works best, if any?

2. What do you recommend for shielding a 10-inch beach-hunting coil for best performance? Details how to apply the shield would be helpful. At our local beaches, the power lines can interfere with the Excalibur VLF, but they don't bother the White's PI Pro.

3. I have a feeling that aluminum foil shown in Carl's article may not be the optimum material. But if I were to use an aluminum shield, would it work to pass the windings through a half-inch aluminum tube bent into a 10-inch circle shape? The tube could be filled with resin when the windings have been threaded. This could provide rigidity and shielding at the same time if it doesn't cause some other problem (just wondering).

4. What method of adjusting the inductance and resistance to the coil do you recommend? That is, how should you fine-tune the coil? What end results should you tune to? I assume the properties of the shield play a part in the fine tuning of the coil circuitry. According to Carl's text, we adjust R11 until the coil stops ringing. I guess what I am asking is nebulous, because there are so many variables, like coil size/windings etc. I just wondered if you have some recommendation at some approximate design parameters for the coil tuning and shielding. Maybe a step-by-step procedure for fine tuning after the main board is done and the coil is being finished and tuned to the board.

Thanks for any help you can give

**Reg** · 04-23-2006, 06:12 AM

Hi J,

I am curious as to who uses this bag shielding type of material in their coils. I have take a few apart and never seen it. However, all of mine I took apart have been coils built here in the US, except for one ML.

Now, to try to answer your questions about coils, I do have to say that I haven't built all of them. As such, I really can't comment on any specifics about some of them. However, from what has been written here on the forum, coils such as the flat one and the basket weave appear to reduce the capacitance of the coil. Hopefully, those who have built the flat coil or the basket weave will jump in and give some comparisons.

Now, this capacitance becomes important but I have found I can wind a coil that is conventionally wound and still produce a decent coil that operates at short delays. So, it can be done. There are no real tricks involved except for the insulation and shielding.

As for building the coils specifically, there has been quite a bit written on this forum already. BBsailor posted a lot of info regarding coils and wire used. As an example, just changing to Teflon insulation will significantly alter the capacitance. Even the use of stranded hookup wire can offer benefits over solid magnet wire.

One little trick when deciding to try different wire is to look for cheap prices. Ebay is a good source. I have picked up different sizes of teflon coated wire at very reasonable prices.

As for size of wire, I have use 26 awg and smaller wire, but have limited the current when using very small sizes. I personally do not think you need any wire size greater than 24 awg.

PI's will pick up noise via the coil, but will also reject certain noise also because of the circuit design. On the HH, the differential integrator circuit, where a later sample is subtracted from the main sample, does an excellent job of rejecting most low frequency noise from power lines. The coil shielding works better on the higher frequencies.

As for shielding, I still prefer to not use any solid form of material either aluminum or copper, so I wouldn't use the aluminum foil, copper foil, or any tubing, if other things are available. One could try the foils but should know what to look for. The copper plated tape is still my favorite because it works well. The plating is so thin that is isn't detectable for practical purposes.

Personally, I might try to find some aluminized or metallized ribbon before I would try any of the metal foils. I purchased some of this ribbon from Walmart for little or nothing quite some time back. I think I paid about $2 for it. It looks much like aluminum foil but is cleary a ribbon. It is shiny on one side and dull on the other. The resistance of this ribbon is a few ohms per inch so it is quite low. As such, it should work fine. I got it in the materials section. Just measure it with an ohmmeter to be sure. If it measures as a low resistance over a few inches, then it should work.

As mentioned before, some of the conductive materials or the fabric tape found on the Lessemf website work also. The phantom or silver mesh fabric should work fine. Some of the others might be to heavily coated such as the zelt or the flectron. So, I do not recommend those types.

I have even use conductive rubber tape used on high voltage wiring.

So, there are a lot of things that can be used besides the foils mentioned.

Now, one could try the foils such as the copper or aluminum foil as long as a mono coil is being built. If placed close enough to the windings (but not too close), it might not cause that dramatic of a problem but I don't know that for sure. Just look for a serious droop in the decay curve as it climbs back to 0V. If the foil is being detected, it will cause the same effect as if it were a target. I haven't tried it. When time permits, I will try it to see just how bad it works.

So:

1. I think all coil winding techniques will work ok to produce a coil that will work for your needs.

2. I still recommend the copper coated fabric tape if possible. Lead foil would be my next choice.

3. You might try the aluminum or copper foil, but I wouldn't recommend the tubing. Just don't seal the coil so you can change the shielding if necessary. Make sure to follow good building techniques such as tight windings, wrapped with spiral wrap, and then the foil tightly wrapped on this spiral wrap.

4. The damping resistor is a function of the coil design. The better the design of the coil, the larger value of resistor that can be used. This will allow for a shorter delay for sampling. The shorter the sampling delay, the greater the sensitivity. Try for a value greater than 500 ohms and preferably in the 700 to 800 range and still get a decent decay curve. This will be quite difficult to do, but it can be done.

I would increase the resistor value as much as possible and forget the pretty decay curve. As long as the signal levels out at 0V and is stable at the time of sampling then what happend prior is of little concern. It is preferable to have a nice decay curve, but not absolutely necessary.

Now, you can reduce the gain of the preamp to allow for a faster sampling time also. Signficant improvements can be made using this technique. The trick will be to make up the sensitivity at a later stage, maybe at the differential amp. You might reduce the gain to something like 200 to 400 max.

Now, you didn't mention the desired objects you will be trying to find. If they are to include small gold items, then my recommendation would be a coil in the 300 uh range. This will allow a faster sampling.

Hopefully, this will get you started.

Reg

**J_Player** · 04-23-2006, 09:04 AM

Hi Reg,
Thanks for the coil tips.
The anti-static film is used in Garret search coils according to Ivconic and Qiaozhi:

Originally posted by Ivconic

About faraday shield....A small tip...I am using a electrostatic "plastic" bags for it. Those bags you can find when buy some hard disk or mother board for pc.They are usually packed in it. When opened first time a Scorcher coil for Garrett GTI 2500, i founded exact same material used as shield in it! Simply cut in the round shape and glue it on the housing, after that try somehow to connect it with gnd. There are many ways to do that..But i guess, you already know how.

Originally posted by Qiaozhi

I have also found this electrostatic sheet in a Garrett GTAX1000 coil. The connection to the shield is made via a steel spring which presses against the sheet when the two halves of the coil shell are combined.

Ivconic says he built several hundred search coils, and they all work very well:

Originally posted by Ivconic

About coils....well coils are pure maths, nothing else.Trust me on this,i made over 200-300 coils so far. Every of them are working perfect! Pure maths, nothing else. No mithology,no abracadabra at all!

From what you said about coils, It would seem that It might be good to use a larger gage of teflon insulated wire in order to keep the coil resistance down, but only to the point of not making it too heavy (maybe 18 gage as a maximum size). Also there might be minor gains in low capacitance by winding the coil flat.

1. For sheilding, I think you are saying that very thin metal tape is better than a thicker walled metal tubing for shielding. I assume using metal is ok as long as it is a thin foil. From what you said I guess copper foil tape is best, followed by lead foil, then aluminum foil. Also the foil should be wrapped over a layer of spiral wrap, and a drain wire added.
It seems to me that even a thin foil of any of these materials would have a very low resistance, as these are basically considered conductors. I assume it is not necessary to have a few ohms per inch, but maybe that is the maximum limit, Am I correct that anything from zero to a few ohms per inch is ok?

2. I am still not clear what property of the shield supresses stray electronic noise like from power lines. Is it the conductivity of the shield material, or thickness of the shield material? I don't want to have problems from power lines. I am hunting for coins and lost jewelry at the beach, including gold and silver rings and bracelets.

3. The only question still in my mind about the different metal foils: It would seem they all work as well considering they are very low resistance compared to the ribbons you talked about. Is there a reason why aluminum is not as good a shield as copper or lead? You can buy 2" wide aluminum sticky-back tape at Home Depot air conditioning duct section really cheap.

4. Is there a point where the foil thickness starts to make problems for the coil? if so, what problems, and what thickness causes the problems?
Thanks again

**bbsailor** · 04-23-2006, 02:10 PM

Reg: Increasing Gain in IC8 and b

Reg,

Changing the value of the input resistor, R12 has some impact on the damping resistor value as R12 appears in parallel with the coil for a while when the flyback pulse returns to zero and the ringing starts. If R11 is adjusted with R12 being 1K and then when R12 is changes to 2K or 2.2K it (R11) will need to be readjusted to account for the above interactive effect of these components. Did you find this to be true?

By reducing the gain in IC6 to about 500 you must make up the gain in the two later gain stages of IC 8a and b. Did you increase R26 to about 150K in IC8a and R29 to about 150K in IC8b or did you lower the value of the input resistors R24, R25 and R28 to about 680 ohms or do a little of both?

By my math the total gain of Carl's Hammerhead is 1000 X 100 X 100 or 10 million. So, using a value of 2K for R12 you get a gain series of 500 X 150 X 150 or 11.25 million total gain, a little higher. Using a 2.2K for R12 you get a gain series of 455 X 150 X 150 or 10.23 million, a little closer to the initial gain.

Did you experience any problems or integration time adjustments necessary when you altered the gain of IC8a and b?

About using heavy coil wire, I believe it is necessary to keep the peak Hammerhead coil current below 2 Amps (maybe even lower) for a fast response at or below 10uS. This means that the total resistance for the coil plus the MOSFET (in the on-state) must be around 6 ohms. With a 300 uH coil at 6 ohms the Time Constant (TC) of the coil must be 50 uS and the coil needs about 3 or 4 TCs to reach full pulse before turn off. If the total resistance were 10 ohms the coil's TC would be 30uS and the peak coil current would be 1.2 amps. This is why Eric Foster uses a series resistor between the coil and the MOSFET to allow for a faster rise using shorter pulses on his machines operating at higher frequencies. Using heavy coil wire may be good for lower frequency PI machines but when operating in the mid frequency range, like the Hammerhead, one must consider how all these factors interact to affect the total performance on the desired targets. Reg, what are your thoughts on this?

Thanks

bbsailor

**Reg** · 04-23-2006, 06:53 PM

Hi bb,

I change the feedback resistor to a lower value, rather than the imput resistor. This will have a little effect on the decay curve but not as much as changing the input resistor. At one time, I was changing the input resistor, but as many tests proved, this increased the noise level. So, the input resistor is now kept as low as possible.

When you change the gain of the diff amp stage you will change the time constants so the filtering is changed as well as the gain. A little more filtering doesn't hurt. I haven't tried to figure out the best cap to resistor ratio to determine the maximum gain at a designated pulse rate.

As for heavier wire, I have found I can get the same depth using a low powered unit at shorter delay as I can with a higher powered unit at a longer delay. So, power isn't as dramatic as one might think when trying to obtain depth. Also, lower powered units are not bothered with ground signals nearly as much and DD coils work much better.

Now, as for the wire size, you might want to check the actual pulse current and see what the peak current is. If you have a short pulse on time, such as 50 usec, the peak current will only get to about 1/2 amp even with larger wire. So, the wire size makes little difference. Also, the 50 usec time constant works well for most gold items I plan on finding. You can nothing by increasing the wire size until you increase your pulse on time. Now, once you have done this sufficiently such that you see any marked difference in depth, then you have to begin worrying about how to carry the battery.

One might gain a little depth capability on silver or clad coins by increasing the pulse on time, but I am not sure just how much. I really haven't done much testing in that area. A real quick air test didn't show a dramatic difference between a 50 usec pulse and a 250 usec pulse.

Reg

**Reg** · 04-23-2006, 07:04 PM

Hi JJ,

I prefer the copper plated fabric tape, not the copper foil tape. There is a big difference between the two.

When I looked inside a garrett coil, I found some strange material that was more like a floppy disc than the plastic bag material. I am not sure just what it was.

You never mentioned just what you plan on detecting with your PI. That will make a difference in just how the coils are made.

As for heavier wire, they will make a heavier coil, but that is all I have found to be true. I haven't found them to go deeper. Maybe you will have better luck.

As for power line noise, I answered this one before. Most of the power line noise will be eliminated by the circuit design and not by filtering. The differential integrator will eliminate that type of noise. Once again, the shielding will work best on higher frequency type noise.

Any metal used as a shield will or could be detected as a target if it is far enough away from the coil winding. Now, I don't know just what happens when it is wrapped right around the winding itself. I haven't done that, except with lead foil. With the lead, I found it to have some effect on the decay curve.

You might be able to get the aluminum foil to work. I just do not know at this point since I don't know what you plan on trying to detect, so I don't know the minimum delay you should be shooting for.

Reg

**J_Player** · 04-24-2006, 12:57 PM

Hi Reg,
I am hunting for lost jewelry at the beaches in wet or dry sand. Sometimes submerged in 4 ft ocean water and buried in submerged sand. Also in lake water areas. These are Southern California beaches where there are some areas of mineralized sand pockets, I have also found hot rocks that sound like coins on the White's PI Pro. The targets I am looking for are gold, platinum and silver rings, bracelets, chains, and ear rings. My #1 target is a ring that could hold a diamond, like a ladies ring. #2 target is man's ring. Lower on my dream list is ear rings, bracelets and small chains. What I usually find are clad coins. I will take them too. I dig pull tabs because rings can sound the same as a pull tab on the White's PI Pro. I can usually discriminate a pull tab on the Excalibur, And I dig a lot less iron trash too with this VLF detector. But it lacks the depth of the White's PI Pro. Many of the west coast beaches have clean areas where you find little iron trash, but some are very dirty with rusty nails, large bolts and construction trash as well as bottle caps, pull tabs and other aluminumn trash.

So far, I think I understand:

A) The shield does not shield AC line noise, only high frequency noise. The Hammerhead circuit design cancels AC noise.

B) Increasing the wire size will not help noticably with depth. #24 or #26 teflon should work fine. Use coil currents as described above.

C) Tinker with the amplifier stages and R11 and R12 as shown in posts above to improve depth and reduce time constant.

What I still don't understand is what makes different shield materials work.

1. You say there is a big difference between how a foil performs and a ribbon. What difference? What disadvantage will I find when using a foil that won't be a problem using ribbon? What exactly am I looking for? a ribbon with a few ohms per inch is better than a ribbon with a few hundredths of an ohm per inch?
2. Is it preferable to wind the ribbon/foil or whatever shield directly on the coil or not? If it should be separated, How far should it be separated? Is there a limit to how much separation works well? What is the best separator material?

3. I am not interested in getting an inferior shield to work. I would rather find an excellent shield material and method of applying it, optimized for locating lost jewelry in wet ocean sand. I can do the waterproofing needed after the coil is done. I understand that the coil parameters work together with the tuning of the circuitry. But I suppose the first step is to build a decent coil, then start tuning.

Thanks

**Reg** · 04-24-2006, 01:40 PM

Hi J,

The shield performs two functions. First, it is there to reduce or eliminate the electrostatic field. Basically, this field interacts with the ground causing some really strange things to happen, especially at very short delays. At long delays, it isn't a problem.

The second function is to absorb higher frequency noise signals so they are minimized. This is a problem when hunting relatively close to civilization. If there is nothing for miles, then it is possible the noise level is low, so a really good shield isn't a necessity.

Now, for hunting for ideal objects, ie diamond rings, most are made of gold or other precious metal so a PI with a short delay is preferable. If you can get the delay down to 15 usec on the HH, it should suffice. If you can reduce it further, then that is good too.

There are different techniques used for shielding. The most common talked about here is wrapping the windings with some form of shielding material. Normally, some spacer is installed over the windings and the shield is placed over the spacer. Spiral wrap works well as a spacer.

Ok, what happens if conductive metal is used for the shielding. In the case of lead, the decay curve is short so there is little effect. However, if copper or aluminum is used, the decay curve of these two metals is much longer, so it can cause the signal to "droop" since it could be detected. In other words, if the shield isn't just right, it becomes a constant target and will cause the decay curve to droop just like what happens when an external object is detected. If this happens, then the detector may not work until the delay is turned way up.

Now, what I don't know is if the foil is placed around the winding and is close and snug, then it might be possible to minimize the negative effects. For some reason, if the shielding is wrapped tightly around the windings, there is little detection of it. Right now, I don't know for sure just how well it would work since I haven't tried it in a very long time. It is something I should try when time permits so I know exactly what happens.

What I do know is the copper plated fabric tape works very well and isn't detected enough to cause any major problems. I can get coils to work down in the less than 10 usec range on a high powered PI.

You could try the aluminum foil to see just what happens, but you should look at the signal with a scope if possible. You need to look at the coil signal without any shield and then with the shield to see just how much it changes. This is the only way to know for sure.

As I said before, I will try the aluminum and copper foil when time permits. Unfortunately, that may take a while.

Reg

**J_Player** · 04-24-2006, 09:34 PM

This is some dang excellent information that can only come from lots of experience. It is nice to have folks like you and bbsailor around.

From what you said, I should be using a shield because of the people who have homes in the sand near where I hunt and the stray electrical noise that I often hear on the Excalibur near these homes. There is municipal lighting near some of these beaches that emits some kind of high frequency I think, as well as fluorescents.

It sounds like one of the best shields you found is copper fabric tape wound over spiral wrap on the coil. I suppose If I wanted to improve that the best place to look is some very thin lead tape. Maybe there is a form of lead ribbon with a very thin layer of lead. In any case that is for experimenting. We already have copper fabric tape that works pretty good in spite of a slower decay. I suppose I could also experiment with some very thin aluminum foil like found the ribbons you talked about, or maybe aluminized mylar or something like that. I tested the resistance of a circuit board anti-static bag, and found it to be very high. No where near the few ohms per inch like you said I should have. I guess it works ok to dissipate high voltage static from circuit parts, but not for a faraday shield.

In the area of spacers, It sounds like some experimenting could show what works best. I suppose I could try with and without the spiral wrap, just to see which way works better. I also wonder if it would help to wrap the coil with teflon tape before shielding. I used to use teflon tape when soldering assemblies for military specs, and I remember how it had excellent insulating properties. You say it is good for reducing the capacitance in the coil wires. Maybe it is also good for a shield spacer. I guess I won't know till I try.

From what you say, I should connect the coil to the scope with no shield and look at the trace. Then watch what changes with each experiment until I get the best results.

Thanks for the tips on the coil shielding

**bbsailor** · 04-24-2006, 11:30 PM

J. Player

Try Polyetheleyne (PE) spiral wrap as it has a dielectric of about 2.4 compared to 2 or 2.1 for Teflon. PVC,on the other hand, has a much higher dielectric constant. Teflon spiral wrap is just too expensive for the small improvement.

One other thing you can try for making a shield spacer are Davis Instruments Cable Covers that are used on sailboats. These are stocked by West Marine in the sailing section. There cable covers are tubes that come in a variety of diameters from 3/23" to 3/8" and are slit length-wise to allow easy placing on sailboat rigging wire to protect the sails from wear. Thay all come in standard lengths of 6 ft. Go to the "Davis Instruments" web site (do a web search) and download the pdf file technical data about the cable covers.

Just multiply the OD or your coil wire by 5 and the use that number to obtain a cable cover with an ID that will fit about 20 turns of this wire. If you want more turns, then go to 6 times the individual wire OD. Cut the cable cover length to snugly fit inside your coil housing. Use duct-tape to tape the ends together and cut a slit in line with the cable cover slits. Arrange the slit to be aligned on the outside of the circumference of the cable cover circle. The use a wood ice cream stick to spread open the cable cover slit followed by Teflon insulated wire as you slide the stick and wire around the coil 20 times making the coil winding. Make sure the start winding is on the inside and continue to wind approximately 20 turns until the cable cover is full. Then you can place the shield around the outside circumference of the cable cover and make 1/4" long snips at 2" intervals on each side of 1" wide lead foil shielding tape on the cable cover and bend the shielding tape around the cable cover. Before you get to the end with attaching the shield tape, be sure to attach the shield wire with a small metal heat sink under it to keep from burning the shield tape or add the ground wire before you start.

This method allowed me to make a perfect size coil that fits snugly inside my commercial coil housings on the first try.

Here is another tip: only about 20% to 25% of the shield-to-coil capacitance effects the distributed capacitance of the coil by lowering the coil's self resonance. Take one coil self resonance measurement before adding the shield and another measurement after the shield is added. The amount of self resonance lowering will only be equal to about 20 to 25% of the coil-to-shield capacitance. I have made coils using AWG30 Teflon wire, 10.5" ID and a cable cover with about 135 pf coil-to-shield capacitance. The lowest shield capacitance comes with using the lowest dielectric spacer with the least shield total surface area. My coils with thicker coil wire bundles had a higher coil-to shield-capacitance due to the extra shield surface area.

Post you measurements.

bbsailor

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New Hammerhead/Lead Foil Shielding Experience

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