I see the BBC reporters don't understand RFID tags.

[NY]

Given that even *we* can't agree on how the power is "transmitted" from the reader to the RFID device so it can send back its unique ID, what hope is there for a journalist who may not have as much (or any!) technical knowledge?

Can I check what we know is fact and what we are unsure of:

- power is transmitted by RF (radio *frequency*) signals, as opposed to mains frequency

- power may be transmitted by induction (coil in reader and coil in RFID device behave like primary and secondary windings of an air-cored transformer) or by radio waves (coil in RFID behaves like an aerial): this is the bit that we are unsure about!

[Andy Burns]

Isn't this where it all comes down to being electromagnetic waves?

e.g. with an induction coil low frequency RFID tag, it's short range so the magnetic field that counts. For the high frequency RFID tags it's long range so the electric field that counts.

Even for the short range tags, surely the tag can't harvest enough energy to signal back to the reader inductively, that must be radio transmission?

[The Natural Philosopher]

Oh dear.

I am sorry but I simply do not have the time to teach you the WHOLE of the physics O level course

[The Natural Philosopher]

They are all RF transfer

There is no fundamental difference between a loosely coupled coil at 3" and a loosely coupled coil at 300 miles.

there are no 'magentic fields' there are electro magnetic fields as soon as the strength varies with time

[The Natural Philosopher]

This is like the arguments that rage permanently on avionic web sites. Is lift on a wing generated by Newton's laws of momentum transfer or Bernoullis equations of pressure differential?

Of course when you do the necesarry transforms, they are expressions of the exact same physics.

But you try getting Believers to accept that

Its just about possible for a sweep oscillator to detect a dip as it goes past the frequency of a loaded tuned circuit in very close proximity.

But I wouldnt rely on it.

Way too easy for the body to affect the tuning and Q

[Roger Hayter]

Not exactly, in the far field it is the EM radiation, magnetic and electric in a fixed reltationship, that counts. You could in theory have a coupling system that relied on near field electric field being predominant, but that would still be a near field effect.

It is symmetrical. If inductive coupling is much more important in one direction then it also must be in the other direction. A bit like light rays being bidirectional.

[Roger Hayter]

This is inaccurate. While there is always some EM radiation from a varying current it is quite unimportant in most elecronics, including inductively coupled RF (which doesn't mean much more than above audio frequency) tags.

[Andrew]

I thought they were the new 'loft living' anyway. I'm sure I have seen a project in east london somewhere where these are stacked 3 or

4 high in geometric 'shapes' and converted to residential.

[The Natural Philosopher]

And how many years did you spend designing analogue electronics?

Sheesh

[Roger Hayter]

You've forced me to look it up. The earlier deployment of RFID was at something like 13.56MHz, and I stick to my argument above about those. They're the ones I remember. They presumably account for the two metre high coils at supermarket exits. However, I understand we are now moving towards 800MHz band apparatus, where what you say makes more sense. And I don't think transmissiion line theory came into the design of 20 metre circuits, though you're undoubtedly right about

800MHz!

[dennis]

Many are 125 kHz.

My phone reads/writes 13.56MHz tags.

[Roger Hayter]

That's interesting. No-one is going to convince me *those* worked by radio waves. The fact you can call that RF simply means RF is a pretty broad term. Though interestingly radio waves are used down to a few kHz, which is certainly radio but arguably not RF; although maybe it is, because it is a frequency used for radio! Radio frequency is not necessarily radio waves, though.

[Andy Burns]

some of the anti-theft tags are described as acousto-magnetic and go as low as 56kHz, must admit that type seems to have fallen out of use though

>

[Andy Burns]