a reply to: BASSPLYR
E-field tags use propagating radio waves. You have several reasons why this is a bad idea in meat.
1) the tags use fairly long wavelengths. So the chip part may be small, but the total tag assembly is not. (it's the old radio antenna size is a
function of frequency thing rearing its ugly head again) So e-field tags aren't the size of grains of rice.
H-field tags use fairly low frequencies, but it's magnetic, so you can put in a tiny bit of ferrite and a coil of wire, which is the gold color thing
you see in a Digital Angel part. So even though an h-field tag might have an operating frequency orders of magnitude lower than a UHF tag, they're
also orders of magnitude smaller because you don't have a propagating wave to interface with.
2) Radio waves have that pesky set of Maxwell's laws attached. The dissipation in blood/lymph/meat is directly proportional to frequency. So the
higher you go in frequency to get the antenna size down, the worse the loss.
3) Radio waves have this annoying tendency to diffract and reflect. So at every impedance step you lose a metric crapton of the signal. That's air to
skin, skin to subcutaneous fat, fat to muscle, muscle to tag on the way in, and then again at each step on the way out. So you lose close to all of it
to reflection and diffraction. What you don't lose there you lose to dissipation. Radios buried in meat don't work well. This is directly analogous to
the goop you use on ultrasound probes, by the way, and for the same reason. The goo is to reduce the impedance step between the transducer to air, air
to skin, and skin to air, air to transducer on the way out. There's a reason they can't aim an ultrasonic scanner at you from across the room, and
it's the same for burying e-field tags in meat.
4) The e-field tag itself uses something called quarter-wave backscatter. It doesn't transmit. At all. It takes some of the power from the e-field
interrogator, powers up a very simple circuit that returns a serial number. It returns it by varying the reflectivity of the tag. In essence, the
interrogator is a very simplistic radar that only works with tags. The tag essentially varies its RCS to return some of the interrogator's signal. In
fact, the equation you use for e-field tags is the basic radar equation. However, when the thing's buried in meat, you have countless reflections on
the way in and out, and the end reflectivity coefficient differential between the tag's mark and space states closely approximates to zero.
H-field tags use load signalling instead of quarter-wave backscatter. MUCH easier to detect close in. However, h-field has its own set of problems.
The power density of the interrogator falls as the sixth power of the distance. And the load signalling is read as a differential load on the
interrogator's output. The bigger the drive, the less the percentage change. There is a truly awful signal-to-noise ratio with h-field load signalling
that's exponential with distance. Between the SNR, the signal return falloff and the sixth power distance problem, you can't practically read h-field
tags at a distance. However, they're dandy for burying in meat.
More, since h-field tags only communicate in the near field, you can't read them outside the near field at all, because they don't use propagating
waves. Like e-field tags, h-field tags don't radiate signal either.
On the other hand, in close proximity, you can couple a lot more power to the tag with h-field, which is why credit cards and passports use h-field
signaling and not e-field - they've got little microprocessors in. H-field can get you enough power to run one, whereas passive e-field tags can't.