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Insulation Dia for Impedance requirements

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6 years 8 months ago #620 by Archived Forum Admin
Hello again Sheks,

That is a question not easily answered from so far away because I cannot see what is going on. Your uncoiled measurement should have been, in my opinion, the reference so something really strange is going on. There must be more happening here than the information you have given us.

Make sure there are no shields and as far as the coiled sample goes, make sure it is not on a metal reel.

Hopefully another reader will have more to contribute but please note the following for twisted pairs:

For higher frequency transmissions in open line copper conductors, the characteristic impedance is determined by the following simplified equation:
Z = 276log D/r
where D is the distance between the centers of the two conductors and r is the radius of each conductor.
The characteristic impedance is therefore determined by the diameter of the conductors, the spacing between them and the type of insulation used. Any change in these will change the characteristic impedance.

Sincerely,

Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

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6 years 8 months ago #614 by Archived Forum Admin
In my experience, you need good, properly sized and well maintained extruders tandemized with with a good and well maintained intermediate drawing machine and annealer. The temperature controls for the extruders and crossheads must be PID, properly calibrated and in very good shape. All drives (DC, Digital DC or variable speed AC) must be quite accurate (1% drives for the extruders and drawing machine, 1/10% drive for the extrusion section capstan), well maintained, tuned, and properly geared. Use a Barr screw or modified Barr screw for pumping the foam.

You also need a low drag, stainless or aluminum cooling trough (preferably with covers) and a conductor preheater (re-heater) with good adjustment. Likewise, make sure you are using good, well maintained fixed center crossheads and tooling. The tips should should employ diamond inserts.

Finally you need a very good air wipe, laser diameter monitor, wire sparker and a reliable and well maintained parallel axis (or equivalent turret type or robot type) dual reel take up.

This will give you a properly sized conductor, without stretch and a proper outside diameter but the conductor could still be off-center and the specific gravity of the foam could change. This is checked on-line with a short, water filled trough containing a coaxial capacitance-to-ground monitoring head and a capacitance monitor and recorder mounted at the main operator control panel. It is definitely required for skin-foam. Now, the capacitance information verifying the coaxial capacitance to ground can be attached to each take up reel. Often the capacitance monitor also direct a movable leading section of the cooling trough (variable quench point) to control the thermally sensitive expansion of the foam.

For foam-skin, a referee trough is often used to verify the capacitive measurements made on-line. The operators also leave nothing to chance and manually check the wire dimensions on every reel.

Sincerely,

Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

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6 years 8 months ago #615 by Archived Forum Admin
I am sorry for not having put my question properly. Actually, I want to know the Insulation Diameter to meet the electrical parameters. Is there a formula to calculate the same with the following known variables:

Conductor: 0.5mm Bare Annealed Copper
Dielectric: Solid PE / Foamed PE
Resistance: 86 ± 3 Ohms/Km
Mutual Capacitance (only indicative, not to be complied): 45 ± 5 nf/Km
Characteristics Impedance: 120 Ohm ± 10
Cable Type: Individually Shielded (With Mylar tape) Pairs and overall Mylar Shield 16 Pair PCM Cable.

We have made samples with 1.3, 1.4 & 1.5 mm Solid PE Insulation diameter but are getting higher values of Characterisitic Impedance. Does Mylar shield effect Impedance as well apart from cross talk?

Thanks

Sheks

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6 years 8 months ago #616 by Archived Forum Admin
Hello again Sheks,

Your electrical engineers can easily calculate the characteristic impedance of each wire when the wires are designed (Conductor diameter, foam dimensions, foam specific gravity and skin dimensions.)

When 16 pairs are stranded to become a cable then you have to deal with the capacitance of the core as a whole and that is a bit more of a black art. (The thing affecting the core capacitance is the degree of core compaction and thus the tape over the individual pairs,the tension of the helically applied polyester core tape or a longitudinal polyester core tape followed by a binder all contribute to the core compaction and thus to the overall core capacitance.) If the core is manufactured incorrectly then you will have problems. The idea is therefore not to have the core stranded loosely so the dimensions of the core are properly stabilized. Then the engineers can tweak the DOD wire dimensions slightly to get the values they are looking for. It is all done experimentally but with good electrical engineering understanding.

I am not clear as to whether the polyester tape over the pairs and core is aluminized for shielding purposes or just straight polyester for additional dielectric protection and thermal (sheathing) protection. Thus I cannot answer your query re crosstalk (Near end? Far end?).

Sincerely,

Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

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6 years 8 months ago #617 by Archived Forum Admin
Dear Sir,

Actually my concern is Characteristics Impedance and not Capacitance or Cross-talk. I need a guidance for achieving the Characteristics Impedance Values.
The pairs are individually and then overall shielded with Polyester tape + Polyester Backed Aluminium Tape.

Regards

Sheks

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6 years 8 months ago #618 by Archived Forum Admin
Hello again Sheks,

Please read the following web pages:

en.wikipedia.org/wiki/Characteristic_impedance

www.allaboutcircuits.com/vol_2/chpt_14/3.html

Sincerely,

Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

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