Cable for Proximity Readers

[Darkness]

I am trying to fix a proximity reader for a business, one which you can walk through and the reader reads the code from the tag etc. The reader consists of a single core type cable aroud 6mm which forms a loop/ring from a control panel around the reader and back to the control panel.

The cable was partly broke and I joined it with choc block but works slightly better but nowhere near perfect.

Markings on the cable are : E34722 AWM STYLE 1015 VW-1 TEW 105C

I hope that's right as it wasn't the easiest to read but I would like to source the same cable online and replace the damaged cable.

I get the impression that this is a specialised cable because of its numerous small cores that creates an rf field or whatever that is that energises the tag that comes into proximity.

Does anyone know where I can find this cable online? Thanks

 

[stillp]

That looks like a standard UL spec solid core cable to me. Have you tried the manufacturers of the reader?

 

[ericmark]

An aerial has a critical length and often it's impossible to get it exactly right so we use something to tune in the aerial. Even the metal work around the aerial can affect it. I would guess there is a method of tuning and likely any repair will mean it needs re-tuning.

 

[Darkness]

Spoke to the agent of the kit today and they told me to try and source the cable myself and if I found a supplier, I had to let them know so they can get it.

They can only sell the complete unit that contains the cable and not to bother asking for a price cause it was just stupid.

I had a feeling the length of the cable had to be similar so I was going to pull the old one out and cut to the same length.

What would you need to retune such a thing? It's a passive system operating at 125kHz if my research into passive proximity systems are correct.

A device for testing the strength etc of the reader would be very useful.

 

[JohnW2]

What would you need to retune such a thing? It's a passive system operating at 125kHz if my research into passive proximity systems are correct.

If the frequency is anything like that low, then the length of the wire is going to be essentially irrelevant in relation to any 'tuning'. The wavelength of 125 kHz is 2,400 metres, about 1.5 miles.

Kind Regards, John

 

[bernardgreen]

What would you need to retune such a thing? It's a passive system operating at 125kHz if my research into passive proximity systems are correct.

The wire loop is an aerial and has to be resonant at the operating frequency of the system. This requires a carefully selected value of capacity connected to the loop to make a tuned circuit. The resonant frequency is set by the inductance of the loop and the capacity.

Any alteration to the loop or the construction of the frame holding it will alter the inductance of the coil and thus the capacity will have to be changed to suit. This requires special equipment and while it can be done on site it is better to be performed in the manufacture's workshop where the necessary equipment is available ( it has to be there to tune the units as they are made.

It is difficult to re-tune a loop on site as the usual procedure is to add or remove capacitors by trial and error using measuring equipment to detect resonance. Very often the cover has to be fitted as the cover affects the capacity.

 

[JohnW2]

The wire loop is an aerial and has to be resonant at the operating frequency of the system. This requires a carefully selected value of capacity connected to the loop to make a tuned circuit. The resonant frequency is set by the inductance of the loop and the capacity.

It would surely be all-but-impossible to get the (tiny) inductance of a loop consisting of a few feet of wire to resonate at 125 kHz just by adding capacitance alone. I would have thought that the loop would be part of an essentially 'external' tuned circuit consisting of both inductance and capacitance and, as I implied, the loop itself would be an almost insignificant part of the tuned circuit as a whole. For example, short vertical antennas tuned to 'moderate' frequencies usually have pretty sizeable inductors at their base.

Kind Regards, John

 

[bernardgreen]

as I implied, the loop itself would be an almost insignificant part of the tuned circuit as a whole.

It is small and thus several microFarads of capacity is needed to get resonance.

Tuning a tag detector is similar to tuning a metal detector ( the type where products go through the loop )

Tuning the loop used in a metal detector will involve soldering many capacitors across the loop with paper capacitors of a few microFarads and silver mica capacitors from 10 to a few hundred picoFarads.

The paper caps have the opposite temperature co-efficient to the mica and with the right mix of types the effect of temperature can be minimised.

 

[ban-all-sheds]

The reader consists of a single core type cable aroud 6mm

That's huge.

Why so big?

 

[Darkness]

I think it's to do with the large numbers of strands that make up the core. I read that the more strands/wires, the greater the range/distance so the tag can be energised further from the reader.

I think

 

[bernardgreen]

The current in the coil ( loop ) is very high when resonating so the wire has to have a near zero resistance at DC. Hence a large cross section.

At high frequency the current flow is concentrated in the skin of the conductor. A multi-stranded cable has far more skin than a solid conductor of the same cross sectional area.

A thin walled copper pipe has the practically the same impedance at high frequency as a solid copper rod of the same diameter.

 

[JohnW2]

I think it's to do with the large numbers of strands that make up the core.

Are the (many) strands individually insulated, or are they just bare wire?

Kind Regards, John

 

[JohnW2]

At high frequency the current flow is concentrated in the skin of the conductor. A multi-stranded cable has far more skin than a solid conductor of the same cross sectional area.

That is true if the strands are individually insulated (as in Litz wire, which exists for this very reason). However, if the strands are bare, and in contact with one another, I would imagine that, in terms of skin effect, it behaves essentially like a single solid conductor, doesn't it?

The 'skin depth' of a copper conductor is about 0.2mm at 125 kHz. Hence there is not a lot is gained (in terms of reduction in impedance) by increasing diameter of a conductor beyond about 0.4mm (around 0.125mm² csa) at that frequency.

Kind Regards, John

 

[bernardgreen]

The larger the diameter ( solid or hollow ) the more skin there is to carry the high frequency currents.

Designing coils using Litz wire was complicated and had to take into account whether the strands were simply twisted or woven. From memory ( from the 1980's ) woven Litz wire reduces the effect of the capacity between turns in the coil.

 

[JohnW2]

The larger the diameter ( solid or hollow ) the more skin there is to carry the high frequency currents.

Well, yes, that goes without saying!

Designing coils using Litz wire was complicated and had to take into account whether the strands were simply twisted or woven. From memory ( from the 1980's ) woven Litz wire reduces the effect of the capacity between turns in the coil.

I'm pretty sure that the main reason for using Litz wire was/is to reduce the effects of skin effect at appreciable frequencies. However, as I said, that only works because the individual strands are insulated from one another. If (and I'm waiting to hear) the strands in the OP's cable are bare, then skin effect would essentially be the same as with a single solid conductor of the same effective (total) csa.

Kind Regards, John