twisted pair for RFID (RI-RFM-007B)

The cable voltage rating of 380Vrms shall be sufficient.
The voltage is depending on the resonance Q of your antenna resonance circuit.

If a TI antenna is used the Q factor will be high (about 100)

Twisted pair and shielding can help to reduce emissions.

The maximum length is determined by the tuning capability of RFM and must be around 27uH@134kHz measured at the antenna connectors including the cable.
Please see RFM reference manual you can find on the WWW.

 I hope the information is helpful.

SDLA

Hello Luc...allow me to comment on your issue...

We sometimes separate the RFM 007 up to 3metre from the panel coils...we use "dual" heavy duty(current) speaker cable for this connection...ie 4 separate wires

These are in parallel ....now we tightly twist the 2+2 or foursome(dual figure-eight)...and by this peculiar method achieve superior "magnetic cancelling"...ie emanating from a simple single pair... four wires make a better fit in circular cross-section and therefore better for flux cancellation (at distant points)

We then wrap a simple Faraday shield (a stringer wire) around this dual-dual twisted bundle to early terminate the electrostatic field associated with the energy passing along this cable....we then slip this into flexible conduit to tidy it up ...etc.

Be aware, that the "inductance of this strange configuration will be about 1.5- 2.5+ microHenries and this must be subtracted from the 27.5uH the RFM007 wants to see...etc..as we wind our own large aperture coils we can take this into account, however if you buy a commercial coil you will need take other compensation action...etc

By this mechanism we greatly mitigate the emission from and ingress into to the linking cable of "noise"....reciprocity...it is worth the effort

We know that nearby laptop computer screens are very noisy for instance and will diminish detection range...you speak of co-mingled wiring/co-located equipment...etc...it is my strong advise to keep heart pacemakers/pain-block implants etc well away from the high power RFM007 aerial...for obvious reasons

Cross coupling between adjacent wires can be mitigated....the casual resort to thick co-axial cable is not what you need here...confirm it yourself.

Be careful with really long simple figure-eight cables, if their loop inductance is compatible to the coils 27uH then half your "tag-charge-up power is being lost in the cable duct..etc...thin "weedy" cables add too much series resistance and therefore lower the net Q factor...etc...be careful.

Do you wind your own aerials?...do you have a spare circuit diagram of the RFM007 by any chance?

Kind Regards

Ray

Thanks SDLA and Ray for your quick respond, I find your answers very helpful,

I am sorry to say that my worries in order. We used a 1mmØ TP foiled cable to extend the antenna for 11.5 mtr’s.

It is right we are using the standard antenna of 27.5µH.

 We want to use a 4xTP cable (each tp foiled with drain wire and al four foiled together with drain wire)

 Shielded twisted pairs 1,00 mm2 bare copper conductor insulated with PE. Two conductors twisted

together with a tinned copper drain wire and aluminium/polyester foil (4 ea.)

Filling compound The cable is filled with cable filling compound

Shield Aluminium/polyester foil and tinned copper drain wire, coverage 100%

Operating voltage 600 V

Test voltage 4000 V DC for 1 min. (cond./cond.)

3000 V DC for 1 min. (cond./shield)

500 V DC for 1 min. (shield/shield)

Conductor resistance ≤ 20,0 ohm/km

Insulation resistance ≥ 10000 Mohm*km (cond./shield)

≥ 100 Mohm*100 m (shield/shield)

Capacitance 100 pF/m

Impedance 54 ± 5 Ohm at 1-10 MHz

Attenuation 5.5 dB/100m at 1 MHz

14.0 dB/100m at 10 MHz

To connect 2 antenna for position indication.

 I think this cable is oke, I only worry about the foil is not thick enough to protect the rest of our equipment from noise.

 The antenna will be tuned for resonance, and impedance, to meet the requirements of the RFM 007B

 Good we have no pacemakers….

 Ray, What do you mean by spare circuit diagram of the RFM007 ? we don’t have a schematic of the RFM007, at least not more than on the TI site available.

Best Regards,

Luc

Luc...we tried science on connecting cables and winding of detection coils, and came to the conclusion it was better thought of as an "art form".

If you cannot relocate the RFM 007 closer to the aerial...remember you can easily separate the RFM007 from the MB6 and join them with say 22.5 metre of Cat5 cable. This works.

If you must pipe 134.2Khz down to the distant aerials, and want to stop feeder leakage to other equipments and insulate against ingress of RF/Motor/Machine...etc noise  you will probably end up where did..."Very Thick" multistranded copper cables+tight twisting+rudimentary E/S shielding..

You must convert your thinking over to issues about "skin effect"..that is RF current travels on the surface not in the core of the conductor...therefore we use multi-stranded "speaker cable" ie >120 strands is good (about 3mm diameter....big and chunky is good...we even once used multi-stranded welding cable (about 1cm diameter).

This was of course to increase the "effective surface area of the transporting cable.  Remember at these frequencies over 70% of the current is within an annulus of only about 0.1mm...skin effect is a surface phenomena....Litz wire is really the best even for fabricating the connecting cable...but it is very expensive.

As an aside we found tin-plated copper-wire was worse than multi-stranded oxygen free copper....the tin being on the surface had lower RF conductivity and as the RF currents "swim" in the layer then the effective series resistance (RDC+RAC) lowered the Q factor and lowerer the radiated magnetic flux,,,etc...bit strange ...try it.

When you say you are concerned about the "thickness" of the shield, keep in mind that even a layer a few atoms thick is enough to transport surface current...the mechanism of E/M shielding as distinct from E/M shielding is like comparing a boat and a baby.....lot of difference.....carbon spray with a stringer is more than adequate to terminate the E/S leakage-ingress matter...water pipe thickness is not necessary....your real concern will be with magnetic coupling into adjacent wiring...this is wy we tightl twist ie one twist be 20-30mm is good....do not add other equipment cables along in ducting along side the RFM007 feeder cables.

If you are in a nuclear power station or submarine please contact me ahead of time before you switch the RFM007 on....just in case.

Dedicate a steel duct (not plastic) for the wiring for extra precaution....SDLA is correct about the +360 V RF AC....it ignited a coil we sprayed with carbon paint once as a spark jumped over the insulation....lots of smoke and a red face.

Regards

Ray

Thanks Ray,

This helps a lot, when working with power transmitters; I am always ready for a surprise!

I experimented already some time with Wireless transmitters, but that is less power, but easier with cable :-)

At least my concerns were right, but with your details!

Thanks a lot