Discussion: Figure 1 shows an overall view of the test setup comprised of a pair of square wire (unshielded) loops connected to an Agilent N1996A spectrum analyzer set up to perform a two port insertion loss measurement. The two loops are positioned end-to-end. A close-up of the two loops is shown in Figure 2. The loops are about one inch, a few cm, on a side.

In Figure 2, the loops are reversed in position, that is the + side of one loop (center conductor of BNC) is opposite the - of the other loop (ground side of BNC). This will be referred to as the "reversed" direction. By inverting one of the loops in Figure 2 the "normal" direction is obtained  (+ side to + side of the loops). Data is presented for both directions. Capacitive coupling between the loops would cause the normal and reversed positions of the loops to yield different responses or cause resonant responses such as with the shielded loops discussed in the June 2008 Technical Tidbit.

If only magnetic field coupling existed between the loops, the change resulting when one of the loops is reversed would be a 180 degree phase shift in the output, which would not change the spectrum analyzer plot. If significant capacitive coupling existed between the loops, the output of the receiving loop would be the combination of the inductive and capacitive components. Since the phase of the inductive component is reversed when one loop is reversed but the phase of the capacitive component is not, the spectrum analyzer plot will be different.


Figures 3 and 4 show the loop-to-loop transmission from 10 MHz to 1 GHz for the reversed case and normal cases of the two loops as explained above. In last month's June 2008 Technical Tidbit, it was shown that the loop-to-loop transmission for a pair of shielded loops in the end-to-end configuration contained a deep resonance of about 30 dB due to inter-loop capacitance. Neither Figure 3 or Figure 4 shows such a resonance, with the frequency response being relatively flat by comparison. The differences between Figure 3 and Figure 4 are about 6 dB maximum at a few frequencies and the traces are not as smooth as the shielded loops. However the "lumps" in the frequency response in Figures 3 and 4 are only a few dB in peak amplitude, much smaller than the 30 dB resonant dip seen in the shielded loop case discussed in the June 2008 Technical Tidbit.




The relatively flat frequency response in Figures 3 and 4 are indicative that simple wire loops may be useful for injecting signals into circuit board paths and cables by inductive coupling. Using loops for this purpose will be covered in Part 4 of this series of Technical Tidbits.

Figure 5 shows the overview of a related case where the loops are overlapped to insure maximum coupling. Figure 6 shows a close-up of the loops. Notice in Figure 6 that the loops are arranged in the normal position as opposed to reversed in Figure 2, that is the side of the loops connected to the BNC center pin (or shield connection) are both on the same side.






Figures 7 and 8 show the coupling from 10 MHz to 1 GHz for the setups of Figures 5 and 6 in both the normal and reversed orientations. The difference between the two traces is only a few dB, and without shielding! The response curves look very close to the response of shielded loops that are overlapped, within a few dB.