I am working on an EAS (Electronic Article Surveillance) design. The objective is to design an active tag that a) beeps (screams) in the presence of the 8.2 MHz field generated by the surveillance antennas _and_ b) triggers the conventional alarm mechanism of the system.
To detect the RF field, we have built an LC tank where the inductor is made of some turns on a FR4 PCB (half on each layer) spiraled around a ferrite bar inserted through the PCB. This also triggers the basic alarm of the system.
The question: Which is the best way to increase sensitivity of this setup? [Sensitivity is ok by now, but we may expect that in the future someone expects "more"]
- We can increase the number of turns and decrease line thickness ("y" dimension is limited). This will increase inductance by a factor, but will increase resistance due to the increased length and the decreased cross section. Will there be net gain? Skin depth is roughly 20um at this frequency and so does not affect on 35um copper.
- Widening the loop in the "x" direction keeping the same ferrite? This does not make sense, since the effect of the ferrite gets more "diluted".
Additional constraints: power consumption has to be kept below 10 uA (3V) when not beeping.
Any suggestions/pointers are welcome.