Cable for Proximity Readers

[Darkness]

The strands are bare wire.

http://www.youtube.com/watch?v=yBF4uVqIp7U&list=PL5EE08A0B75E75744&index=2&feature=plpp_video

 

[JohnW2]

The strands are bare wire.

That's what I suspected. In that case, it will behave, electrically (even at 'high frequencies'), exactly the same as a single equivalent un-stranded solid conductor. ... leaving the reason for the size of the cable less certain! (the multi-stranded nature might just be a matter of flexibility)

You describe it as "6mm". Do you mean that the diameter of the collection of strands is 6mm (which would represent an overall cross-sectional area of about 28mm²), that the cross-sectional area of 6mm², or what?

Kind Regards, John

 

[Darkness]

By 6mm I mean it's like a core of 6mm T&E

 

[JohnW2]

By 6mm I mean it's like a core of 6mm T&E

Fair enough - so about 6mm² cross-sectional area. It might be that size, and stranded, simply for 'mechanical reasons' (a single 1mm² or 2mm² conductor would be fairly 'fragile'). It might conceivably be for reasons of low resistance, as suggested by bernard but, frankly, given its short length, I very much doubt that would be the reason.

6mm² single-insulated (stranded) 'single' cable, if that's roughly what you are talking about, is readily available.

Kind Regards, John

 

[bernardgreen]

It might conceivably be for reasons of low resistance, as suggested by bernard but, frankly, given its short length, I very much doubt that would be the reason.

The ratio of resistance (at DC) to the impedance ( at operating frequency ) of the wire ( not the complete coil ) affects the Q ( quality ) of the completed coil ( even when only a single turn coil )

 

[JohnW2]

It might conceivably be for reasons of low resistance, as suggested by bernard but, frankly, given its short length, I very much doubt that would be the reason.

The ratio of resistance (at DC) to the impedance ( at operating frequency ) of the wire ( not the complete coil ) affects the Q ( quality ) of the completed coil ( even when only a single turn coil )

I realise that, but did not think that one would be needing that high a Q. Maybe I'm wrong.

In the broadest of ballpark terms, to resonate at 125 kHz (and guessing a 3-4m loop circumference), we are probably talking in the ballpark of 1 μH and 1.5 μF. The reactance (of L or C) would therefore be about 0.8 Ω. Say 3.5m of 6mm ² cable would have a resistance of about 13 mΩ, hence a Q of around 60. However, what Q is required for the purpose, I don't know.

Whatever, as I've said, the OP should have no problem in finding a 6mm² single to replace his cable.

Kind Regards, John

 

[ericmark]

It is bringing back memories of doing my RAE and high Q values. I know my son has my books on transmission lines but thought I had seen my old RAE manual around but alas can't find it.

We use a meter to measure the voltage standing wave ratio it compares the power out to reflected power and one has many ways to tune a circuit. Be it a capacitor or a tuning stub there will likely be some method to tune it.

I remember with simple radios having a problem where the user damaged the shrink sleeve over the spring which formed the aerial. Even plastic will affect an aerial and when tuning I found if I slipped up and cut off a mm too much filling the centre of the spring with silicon sealant would bring it back.

So with your loop it could be some adjustment you don't even think as an adjustment which is used to fine tune. As radio hams we also use a wave meter and other instruments to assist both in getting maximum output on frequency required and ensuring it is not transmitting outside that frequency.

I took me around 3 months to learn enough to pass my RAE in spite before then being licensed as VP8BKM and after passing promptly forgot most of what was taught mainly as I only worked the 2 meter band.

To try and teach now would be the blind leading the blind there are web sites like this one which do give some guidance but to learn it all for one job does seem a little OTT.

In the main we realise it happens but don't bother doing anything about it at 50Hz not an issue.

So what you have to decide is learning all about aerial tuning worth it for one job?

 

[JohnW2]

So what you have to decide is learning all about aerial tuning worth it for one job?

Indeed - but what we don't have much of a clue about is how critical either tuning or Q is to the satisfactory functioning of this particular piece of equipment. It might be pretty tolerant in relation to both those things.

Kind Regards, John

 

[stillp]

The coil doesn't have to be anywhere near resonance, or to be an efficient aerial. All that matters is that a suitable tag within its field causes an identifiable change in the coil characteristics.

 

[JohnW2]

The coil doesn't have to be anywhere near resonance, or to be an efficient aerial. All that matters is that a suitable tag within its field causes an identifiable change in the coil characteristics.

That was my initial thinking but, on reflection, given what I presume will be a very small return signal from a passive tag (at appreciable distance), 'efficiency' may possibly be more of a consideration. However, I do very much doubt that it's going to be anything like as critical as some have suggested.

Kind REgards, John