Analysis of a tank circuit and notch circuit with the DG8SAQ VNA

image series lc dip IMG_0459

Experiment 1: I did an S11 analysis with my VNA on a 500pF transmitting capacitor in parallel with an airwound copper coil of good cross section. The question is how to interpret the first picture. The voltage over the parallel circuit is in phase with the drive voltage and almost as high in amplitude as the drive voltage when there is a real component present. (Blue trace, left image). The drive voltage cannot drive a current when the circuit is in steady state oscillation since the circuit has almost the same voltage over it as the drive voltage itself. Low voltage differential, results in low current. This is the definition of high impedance. Low in frequency the resultant voltage is not in phase however the voltage amplitude close to the resonance is almost as high as the drive voltage. The circuit presents a inductive load. Above the frequency there is a change from inductive to capacitive load. (Red trace). It looks like the phase changes abruptly, but I interpret it as a high impedance that occurs only due to a change from high inductive to high capacitive voltage. The phase goes “over the top” but only since someone defined capacitive reactance as minus. The impedance is still high and almost real close to resonance. |Z| represents the resistance to AC current in this context. It is the length of the R+jX vector.

Experiment 2: after that I did a S21 sweep of the same components in a series notch configuration as per VE2AZX’s paper http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb_2012/QEX_1_12_Audet.pdf  to try to determine the Q (of the coil primarily – I assumed the Q of the capacitor very high compared to the coils Q).

The second picture gives the notch depth so the Q can be calculated as per VE2AZX’s QEX paper http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb_2012/QEX_1_12_Audet.pdf

Note1: the picture to the right is of the S21 test, not of the parallel circuit and the frequency may have changed slightly as the coil were changed between the two experiments if I recall correctly.

Note2: don’t worry about the long leads. This is an exercise in understanding the design and operation of antenna trap circuits and loss in reap circuits due to resistive losses in the inductor primarily. The leads would be almost as long in a real trap.

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