How Does an RF Probe Work without Ground? - Page 1
Forum section: electronics / beginners

author: NY2KW
author_rank: Regular Contributor
timestamp: September 11, 2024, 01:28:51 pm
content: I have used this kind of setup for years but recently when asked, I wasn't certain how it worked. For example I was using a handheld battery operated spectrum analyzer like the TinySA with a typical 10x oscilloscope probe tip --- hook removed, ground barrel covered in shrink wrap and only the tip exposed. I am monitoring an RF HF signal (range 1 MHz - 30 MHz) at various stages in a vintage vacuum tube transmitter. The transmitter has BNC connector test points between stages (oscillators, mixers, preamp, driver) and I connect the probe tip directly into the BNC center conductor. There is no direct ground contact from probe barrel ground to the BNC ground. I am not interested in absolute values just relative signal voltages for alignments.My initial thought was that the probe ground barrel, while covered to prevent a DC short, was completing the RF circuit through the probe by induction between the close proximity of prober barrel ground and BNC connector ground. However, I was challenged and was told to repeat the exercise with a single long wire attached to probe tip and BNC center thereby removing any close proximity coupling between the probe ground and BNC ground. I was surprised that the relative signals looked the same.What is the correct explanation?Jerry

author: Zero999
author_rank: Super Contributor
timestamp: September 11, 2024, 01:34:41 pm
content: It's caused by capacitive coupling.Look up self-capacitance.https://en.wikipedia.org/w/index.php?title=Capacitance&useskin=vector#Self_capacitanceBasically both circuits have some capacitance. Electrons move to and fro between the probe and circuit being tested.

author: iMo
author_rank: Super Contributor
timestamp: September 11, 2024, 02:12:34 pm
content: As a HAM operator you know well how the RF signal propagates - via elmag fields.Any RF currents generate elmag fields which propagate in the space, and your probe's pin works like an antenna receiving the RF.Electrons do not move between the source and load in the "capacitive coupling", but the elmag fields do..

author: Zero999
author_rank: Super Contributor
timestamp: September 11, 2024, 02:32:14 pm
content: Quote from: iMo on September 11, 2024, 02:12:34 pmI can see this turning into a big thread, if we're not careful. . .If that's true, then why doesn't the probe pick up the same strength signal, irrespective of whether it's electrically connected to the device or not?It depends on the frequency/wavelength. At much higher frequencies, indeed, it's true, but the original poster is working with relatively low frequencies here. The wavelengths the original poster is working with are between 10m and 300m, which is much bigger than his test equipment. Capacitive coupling better explains this, than waves, in this case.

author: NY2KW
author_rank: Regular Contributor
timestamp: September 11, 2024, 02:56:11 pm
content: Thank you. I found references explaining the self-capacitance principles of a single copper wire... spheres within spheres.Jerry

author: Zero999
author_rank: Super Contributor
timestamp: September 11, 2024, 03:04:18 pm
content: I originally thought of this, when I noticed my TV CRT fly-back transformer driver could still give me a shock from one wire, even when powered by a lead acid battery completely isolated from earth. I also noticed sparks would fly to a spoon tied to a hook, via nylon fishing line. Connecting the 0V of the fly-back's output to a large metal object would increase the shock, because it would increase the self-capacitance.

author: radiolistener
author_rank: Super Contributor
timestamp: September 11, 2024, 06:15:20 pm
content: Your vacuum tube transmitter works with high voltage. High voltage creates high electric field strength around source in the space. When you put your probe in this electric field it sense it due to capacitive or inductive coupling and due to high impedance on probe input (or high sensitivity for low impedance input).Technically it works the same as when you connect incandescent lamp to the mains through capacitor or through transformer. There is no direct connection, but AC current flows.Also you can use neon lamp to detect high electric field strength in the space. At some voltage threshold it starts to glow.