VNA measurement of an air-core transformer. - Page 1
Forum section: electronics / beginners

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: August 03, 2020, 09:21:49 am
content: I have made a measurement of S21 magnitude and phase of two 11mm H-field probes spaced 8mm apart (the probe handles are perpendicular to each other, but their sensing loops are parallel and coaxial)See the attached photo of one probe and the VNA plot.I am wondering why the phase plot shows values mostly close to 90º ?I thought that the voltages at the primary and secondary of an air-core transformer are supposed to be in-phase.

author: kg4arn
author_rank: Unknown
timestamp: August 03, 2020, 10:30:55 am
content: Flux leakage due to the low coupling coefficient???

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: August 03, 2020, 11:25:32 am
content: Yes, the coupling coefficient is definitely low and that explains the amplitude, but it doesnotexplain the phase between primary and secondary voltages.Unless, the VNA is not measuring voltages... ?Anyway, is the amplitude of incident signal at VNA's Port 1 measured before or after the 50Ω internal impedance of this port ?Also, why is the S21 magnitude increasing with frequency ? I expected the inductive reactance of the primary to allow less and less current into the primary as the frequency increases. Smaller current amplitude should mean smaller flux amplitude...and smaller induced emf in the secondary.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 03, 2020, 12:02:49 pm
content: Hi,What you measured is the expected resulted. The air-core transformer will have a very low magnetizing inductance, approximately 20nH.The source impedance and receiver impedance in parallel are 25The magnetizing inductance and this impedance forms a high pass filter.The corner frequency = 1 /(2 x pi x L/R)R= 25L= 20nH (estimate)= 1/ ( 2 x Pi x 20E-9/25) = 200MHz.Full modelThe transformer is modelled as two coupled inductors.Modelling ResultsThe Phase is 90 degreesAnd the amplitude is increasing with frequency.The model matches your experimental results.Regards,Jay_Diddy_B

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: August 03, 2020, 12:46:36 pm
content: Thanks,I am glad that theory and measurements agree.I am still puzzled where the VNA is measuring its voltage amplitudes. I have added a voltmeter symbol to the Receiver's side of your model, as I am pretty sure that is where the VNA measures the received signal's amplitude.Could you add a voltmeter on the Source side, too ?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 03, 2020, 01:03:42 pm
content: Hi,In the model I am plotting the voltage, where you have drawn the voltmeter.CouplingIn the model I can change the coupling between the two coils:and I get this result:The output voltage increases as I increase the coupling.What are you trying to do? It looks like you are are trying to calibrate H-Field probes?Regards,Jay_Diddy_B

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: August 04, 2020, 12:25:53 pm
content: Quote from: Jay_Diddy_B on August 03, 2020, 01:03:42 pmMy physical measurements show exactly the same relationship as I vary the distance between the loops.Quote from: Jay_Diddy_B on August 03, 2020, 01:03:42 pmTo do that, I'd need to have a source of constant amplitude and variable frequency magnetic flux. I do not know how to generate such constant magnetic field.Quote from: Jay_Diddy_B on August 03, 2020, 01:03:42 pmI am just learning. I am only 16 y.o.Quote from: Jay_Diddy_B on August 03, 2020, 01:03:42 pmI know, but the VNA is not plotting the received voltage. It is plotting a dimensionlessratioof two voltages.As far as I understand, in the VNA, the S21 magnitude measurement is aratioof the received voltage amplitude (VRx) to the Incident/Source voltage amplitude (VTx) ...in mathspeak: │S21│ = VRx/ VTx.That is why I am asking you to draw a voltmeter on the "Source" side, too.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 04, 2020, 02:19:50 pm
content: Hi,The H Field probes can be calibrated in a Helmholtz coil. These make a uniform magnetic field.Link:https://www.beehive-electronics.com/HelmholtzCoil.htmlThe setting you have on the VNA in your picture is LinMAG in mVS21 is dimensionless if you measure the ratio.Which VNA are you using?Jay_Diddy_B

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: August 04, 2020, 02:54:22 pm
content: Quote from: Jay_Diddy_B on August 04, 2020, 02:19:50 pmYes, but spatially uniform magnetic field is not the same as constant amplitude alternating magnetic field.Helmholtz coils provide only the former.Quote from: Jay_Diddy_B on August 04, 2020, 02:19:50 pmNo, │S21│ is in mU.Quote from: Jay_Diddy_B on August 04, 2020, 02:19:50 pmR&S ZVL3