Time-varying electric field measurement using dipole

AI Thread Summary
The user is measuring the time-varying electric field of a parallel plate capacitor but finds their results significantly lower than expected. They calculate a peak electric field of 200 V/m based on their setup, but their dipole measurement yields only 2 V/m. Discussions highlight that the dipole's high impedance may be causing signal loss and suggest that the voltage between the plates is indeed 20 V peak, making the calculated electric field reasonable. Recommendations include testing at higher frequencies or using a diode for better measurements, though the user cannot use a diode due to the nature of their signal. Overall, the discrepancy appears to stem from the dipole's characteristics and measurement setup.
AliAttaran
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Hello,

I am trying to measure the time-varying electric field of a parallel plate capacitor using short dipole antenna, but my numbers are off by almost 100 times. I have a parallel plate capacitor, A=23cmx11cm, d=10cm. I apply 10sin(2pi*1KHz) and 10sin((2pi*1KHz)+180deg) to each plate. I am using a L=2cm dipole. So based on the book, peak electric field is E=(10+10)/d=200V/m. Now measured electric field using dipole is E=V/L=40mv/2cm=2V/m. I need help to understand the reason?!

Thanks
 
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I am having trouble understanding
AliAttaran said:
10sin(2pi*1KHz)
Is that 10 V? I assume there should be a *t inside the parentheses.

AliAttaran said:
to each plate
How are you applying two voltages to each plate?

AliAttaran said:
So based on the book, peak electric field is E=(10+10)/d=200V/m
I am not sure if this is the applicable formula, but 200 V/m seems way too high for a 10 V signal measured 10 cm away. Are you sure about this formula? It seems more likely that the d in this formula is the thickness of the capacitor and the E-field is the E-field between the plates. Do you have an online reference for this formula, it just doesn't seem right to me.
 
Last edited:
AliAttaran said:
Hello,

I am trying to measure the time-varying electric field of a parallel plate capacitor using short dipole antenna, but my numbers are off by almost 100 times. I have a parallel plate capacitor, A=23cmx11cm, d=10cm. I apply 10sin(2pi*1KHz) and 10sin((2pi*1KHz)+180deg) to each plate. I am using a L=2cm dipole. So based on the book, peak electric field is E=(10+10)/d=200V/m. Now measured electric field using dipole is E=V/L=40mv/2cm=2V/m. I need help to understand the reason?!

Thanks
The voltage on each plate seems to be referenced to ground, so there is 20 volts peak between the plates. The spacing is 0.1m so the field strength looks correct at 200v/m.
I notice that the test dipole is very short. It will have an immensely high impedance. Does your measurement system have an even higher impedance so it can deal with this? Does the connecting lead avoid pick up? Is the test device balanced with respect to ground? Basically, I think the test dipole is a problem.
 
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tech99 said:
The voltage on each plate seems to be referenced to ground, so there is 20 volts peak between the plates. The spacing is 0.1m so the field strength looks correct at 200v/m.
I notice that the test dipole is very short. It will have an immensely high impedance. Does your measurement system have an even higher impedance so it can deal with this? Does the connecting lead avoid pick up? Is the test device balanced with respect to ground? Basically, I think the test dipole is a problem.
Yes you are right, even though, I have connected it to 1Mohm terminal of the osc, but dipole has higher impedance, therefore I lose the signals.
 
AliAttaran said:
Yes you are right, even though, I have connected it to 1Mohm terminal of the osc, but dipole has higher impedance, therefore I lose the signals.
If you raise the frequency to, say, 10 MHz it might work. It is also possible to place a diode at the dipole centre with two resistors connecting it to the cable. . This allows quite good RF measurements at UHF. You need to calibrate the diode with a signal generator, maybe at a lower frequency if necessary.
 
tech99 said:
If you raise the frequency to, say, 10 MHz it might work. It is also possible to place a diode at the dipole centre with two resistors connecting it to the cable. . This allows quite good RF measurements at UHF. You need to calibrate the diode with a signal generator, maybe at a lower frequency if necessary.
I did different measurement in higher freq and I did not have any problem. since the nature of the signal is important for my probe, I cannot use diode to rectify the signal, Thanks.
 
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