# Science experiment, low rf noise problem

#### Jm_electrons

Joined Nov 25, 2018
10
Hi everyone
Making a parallel plate capacitor 10x10cm with 10mm teflon in between as part of a science experiment.
200pF capacitor is charged to 4V, tiny amount of charge.

The plan is to take it to a desert (dry air and lower rf noise), sit it on an ultrahigh resistance mount, charge it to 4V and then measure plate to plate voltage with an instrumental amplifier.

Q. The plates will act like an antennae to low frequency radiowaves in the air, will the low frequency rf noise be higher than the 4V dc signal? Looking for any ball park number.

To ask a similar question, if i had a piece of straight wire 10cm long located away from a city / power grid. What peak to peak voltage noise would you expect to see? Microvolt? Millivolt? Volts? How about noise in an urban area?

#### nsaspook

Joined Aug 27, 2009
7,753
What's the theory here?

The rf noise signal will likely be in the low micro-volt range.

#### Jm_electrons

Joined Nov 25, 2018
10
What's the theory here?

The rf noise signal will likely be in the low micro-volt range.
Thanks, basically i'm trying to measure the earths electric field of about 100V/m using a novel method:
1. Electrically isolate an ultra low leakage plate capacitor.
2. Electrically short it, sit it flat on an ultra resistive mount. Plates get different charge from Earths electric field.
3. Remove electrical short. Flip capacitor upside down. Charge on plates want to discharge but they can't go anywhere.
4. Measure voltage across capacitor plates and deduce electric field.

Any major problems with this? Will the noise really be small compared to the 4V signal? It's going to be hard to ensure the capacitor won't leak. Maybe something like electrostatic charge of the wind or ultraviolet sunlight inducing photoemission of electrons from the plates could cause discharge.

Why am i doing all this? All a part of my master plan to take over the... oh um.. i forget what i was saying.

#### Jm_electrons

Joined Nov 25, 2018
10
That's exactly where i got the idea from haha! That approach uses the actual ground in the circuitry, im not sure if that'll make things better or worse noise-wise? Hmm will try it.

I made a mistake, the capacitor is 0.1nF at 1V. A trillion ohm impedance amplifier would discharge the cap in a minute.

#### nsaspook

Joined Aug 27, 2009
7,753
That's exactly where i got the idea from haha! That approach uses the actual ground in the circuitry, im not sure if that'll make things better or worse noise-wise? Hmm will try it.

I made a mistake, the capacitor is 0.1nF at 1V. A trillion ohm impedance amplifier would discharge the cap in a minute.
I do have a crystal ball and sometimes, it even works.