PI Mono Coil Cable question

[6666]

ML "Dust bin lid" 18"coil. Ha I got one to

[kt315]

that may help. I got the info from an ML "Dust bin" 18"coil.
the coil was used with the SD2000 and it was one of the very early coils that they made.
it is labelled as fallows
AUSTEC cables 75 ohm Air spaced Co Axial UV Stabilised

mine cables are multistrand.

[Nexus]

Cable from Minelab mono coil. I would be curious to know if anyone knows where to buy something like this.


Once uploading image with a spelling mistake in the heading it was impossible to upload the correct file again. phu...

[kt315]

https://www.distrelec.de/en/cable-wi...AV_PL_09110201

[Polymer]

https://www.distrelec.de/en/cable-wi...AV_PL_09110201

I personally stay away from coax cables to the coil where the inside conductor only has 7 strands -like above- .
For testing on the bench they are OK though. Breakage can occur much faster than with say 19 strands inside.

Beware of the many coax types with copper plated steel inside like many have already mentioned. These break extra fast.
Often the composition is not directly mentioned, so read the data sheet. If this is not even in the datasheet then it is most likely cheap crap.
Test with a strong magnet if you come across some in 3d. I always have a magnet with me ... from an old hard drive.

Otherwise I am testing many different cable types myself, like:

microphone
s-vhs
twisted pair
RG58 CU
etc.

Once I'm finished -will that ever happen- I will post.

[kt315]

I personally stay away from coax cables to the coil where the inside conductor only has 7 strands -like above- .
---
nickel gave pics with ONE strand cable. its ML. so mine is much better
but i do not want disput with you, i have HARD cable i AGAIN write that. that does not the breakage.
i wrote its SOVIET MADE, NOT CHINESE MADE, not FAKE.

microphone - chinese
s-vhs - chinese
twisted pair - chinese
RG58 CU - chinese
etc.

you are living in era ALL cables making in China.

[dbanner]

Sputnik cable is excellent Soviet quality, Chinese know this very well.

[samsearcher]

Why coax? Would standard twin mains cable be OK, and low capacitance?

[Polymer]

I personally stay away from coax cables to the coil where the inside conductor only has 7 strands -like above- .
---
nickel gave pics with ONE strand cable. its ML. so mine is much better
but i do not want disput with you, i have HARD cable i AGAIN write that. that does not the breakage.
i wrote its SOVIET MADE, NOT CHINESE MADE, not FAKE.

microphone - chinese
s-vhs - chinese
twisted pair - chinese
RG58 CU - chinese
etc.

you are living in era ALL cables making in China.

Oh my KT ... Look at the data of the cable link you provided from distrelec, only 7x0.10mm ... that is skinny I'd say:
The silver plated copper shield doesn't look good for fast coils either. Why did you actually post this link?

I must add that I prefer durable AND very flexible cables, because I have to take detectors apart to pack them up in small cases for travel.
Stiff cables would simply be a hindrance here and the breakage risk higher than with good flexible ones.

The ML cable you mention looks solid to me and has an air gap too. Looks like fast coil material to me.

Most of my test cables are from the 70's 80's and 90's manufactured either in W.Germany or U.S.A.
The only exception here being the RG58.

Yes, I fully understand what you mean with the chinese stuff. They entered the WTO in 2001 I think.

I am also aware that the former Eastern Block countries produced some pretty solid stuff ... and still does.

Just as an example: NARVA, RFT, FROLYT - not to forget all the rocket propulsion stuff etc ...

Give us some data about your cables ... if you don't have any ... at least measure the capacitance
or start burning the plastic to find out what the insulation is.

[Polymer]

Cable from Minelab mono coil. I would be curious to know if anyone knows where to buy something like this.
[ATTACH]45899[/ATTACH]

Once uploading image with a spelling mistake in the heading it was impossible to upload the correct file again. phu...

Wow! That certainly looks like a great cable. Each filament is enameled - and so many of them too.

The white insulation looks stiff, but is it so? What is your perceived stiffness of this cable?

I will be on the lookout for stuff like this and post if I find something in this range. Just hope it isn't bespoke manufactured just for ML.

[Altair]

I do recommend Mogami 2964 audio cable.
Very low capacitance: 65pF / meter, 75 ohm impedance, very good double spiral shield.
This is the best I have found for my fast PI coils

[dbanner]

Can someone say how capacitance is measured on these cables? I mean is the shield part of the measurement? How is it measured?
Or are these numbers being quoted just specifications given by the manufacturer?
I found some nice looking cable. Problem is that there is big spool of it. Will I have to cut a piece in order to measure. I don't want to buy expensive cable only to find out it's no good. There is no maker name on it.

[waltr]

Probably cable capacitance numbers are from manufacturer's specs.

Typical for coax is the capacitance from shield to center. If unknown cable and you have a lot of it then cut off a 1 meter length and measure with a C-meter.

Same with twisted pair, C-meter across wires.
For a shielded twisted pair measure across pair and from shield to each wire.

[bbsailor]

Cable capacitance is just one case of total capacitance that determines the optimum value of the damping resistor.

Cable capacitance is determined by the capacitance between the center conductor and the surrounding shield as determined by:
1. The distance between the center conductor and the shield
2. The dielectric constant of the insulation materiel around the center conductor. Lower dielectric constant numbers are better. Anything near 2 is good.

Typically cables listed at a higher impedance have less capacitance such as 75 ohms or 93 ohms or higher.

The other things that add capacitance are:
1. The coil turn-to-turn capacitance as determined by the wire insulation thickness and the insulation dielectric constant
2. The coil-to-shield capacitance is determined by the space between the coil and the shield, the dielectric constant of the spacer materiel and the area of the shield materiel over the coil.
3. TX circuit capacitance a caused by the coil driver MOSFET COSS (output capacitance) and circuit wiring techniques. Putting a diode is series between the MOSFET driver and the coil will help reduce this capacitance.

If you observe the undamped coil response (with damping resistor removed) with a scope, you will see the high flyback spike, a voltage drop and a series of oscillations until the response reaches 0 volts when the RX signal can be turned on. However, this undamped coil response will take so long that all the eddy currents that the TX coil induced in the target will have died down and no target will be heard. The size and amount of these oscillations are the result of the total circuit capacitance caused by all the things I have identified above.

The optimum damping resistance value will suppress these oscillations and allow the earliest time when the RX circuit can be turned on. This is a "fast coil" and allows the earliest sample time and also allows small targets with low time constants, like gold nuggets, to be detected. Typically, higher values of damping resistors represent a coil and TX circuit with less capacitance.

By trying to create a mental model of the above will help you look at all the places in your circuit and coil design where you can reduce capacitance and allow you to sample sooner.

I hope this helps?

Joseph J. Rogowski

[Polymer]

Thought this might be interesting, esp. table III.