11.2.2. Transponder

The transponder consists of a dipole antenna with a Low Barrier Schottky Diode HSCH-5340 of centred inductivity. The transponder is 10.5 mg in weight and 12 mm in length. It consists of a silver or gold wire with a diameter of 0.3 mm and a loop inductance of 1.3 nH (Fig. 22B, 23A). The dimensions of the wires, the loop and the conductive glue (two component conductive epoxy, Chemtronics, Kennesaw, GA, 30152, USA) used to connect the diode across the loop to the wire are very important and define the strength of the harmonic signal. The loop is stabilized with nail polish. A simple device can be used (Fig. 23B) to stretch and stiffen the silver wire in order to produce a precise loop both with respect to loop diameter (0.35 mm) and distance of the loop at the location where the diode is glued across the loop (Fig. 23A). A laboratory-based calibration radar unit may be helpful for estimating the quality of the transponders, but such a unit is not essential. The calibration can also be performed in the field. The price for such a custom-made transponder is about 20 €.

Fig. 23A. A collection of transponders. Notice the loop bridged by the diode, the nail polish for stabilizing the loop, the PVC tubing into which the wire is inserted, the number tag and the double sticky tape. (Photos: Christoph Grüter). B. A simple device for producing a loop with an inner diameter of 0.35 mm and a defined distance between the distance of the wire in the loop. One end of the wire is pulled below the horizontal wire, bent around the vertical filament above the horizontal wire and then strongly stretched.

1293PN revised Fig 23