Time-varying electric field measurement using dipole

[AliAttaran]

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

 

[Dale]

I am having trouble understanding

10sin(2pi*1KHz)

Is that 10 V? I assume there should be a *t inside the parentheses.

to each plate

How are you applying two voltages to each plate?

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.

 

[tech99]

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.

 

[AliAttaran]

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.

 

[tech99]

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.

 

[AliAttaran]

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.