Artificial RF ground "device" - help understanding - Page 1
Forum section: electronics / rf-microwave

author: A.Z.
author_rank: Super Contributor
timestamp: March 27, 2021, 02:04:56 pm
content: Hello all, I was looking around on the internet and found a page describing a "device" which, according to the author, allows to have good RF grounding even if the shack is far away from a good ground stake, the URL of the page ishttps://helpdesk.flexradio.com/hc/en-us/articles/204779159-Grounding-Systems-in-the-Ham-Shack-Paradigms-Facts-and-Fallaciesand the device I'm referrring to is described at the section titled "alternative 2 - the rf suppressor ground system" showing a "device" built using a run of RG8 coax and a capacitor (depicted here)https://helpdesk.flexradio.com/hc/en-us/article_attachments/202776025/Ground12.jpgnow, the author states it works very well, but after reading and rereading the description and the (supposed) details a lot of times, I stll can't understand how (and IF) that device should workWould someone here, much better knowledgeable than me (well, that's easy), help me understanding if and how that "device" is supposed to work ?Note:found an archived copy of the hamuniverse paper referenced at the above link herehttps://web.archive.org/web/20070403070509/http://www.hamuniverse.com/grounding.htmlbut that doesn't explain how that run of coax plus a capacitor should work so I'm still confused

author: fourfathom
author_rank: Super Contributor
timestamp: March 27, 2021, 05:20:11 pm
content: if it's any help, I also can't see how that coax plus capacitor is any different than a long fat ground wire. And if it happens to be a quarter wave (or odd multiple) in length then whatever that ground-stake impedance is gets shifted to something probably close to an open-circuit. The capacitor is like a short at RF frequencies and they might as well have just connected to the coax braid at each end.I would love to have someone explain otherwise.

author: A.Z.
author_rank: Super Contributor
timestamp: March 27, 2021, 05:24:51 pm
content: that was my reasoning too, but the folk who wrote that isn't a newbie, so there must be something I missed... or maybe there's some sorcery involvedreally, I got my brain boiled trying to understand how that is supposed to work

author: A.Z.
author_rank: Super Contributor
timestamp: March 27, 2021, 05:26:44 pm
content: although (thinking loud) the distributed capacitance of the coax and CM currents may play a role there... scratching my head !

author: Bud
author_rank: Super Contributor
timestamp: March 27, 2021, 07:47:44 pm
content: The description of "theory of operation" given in the article makes no sense. You have to approach it in any case from transmission lines theory. Using a length of a coax shorted at one end will make it an impedance transformer on the other end , so the impedance looking into the open end of the coax will depend on the coax length and operating frequency. This is easy to calculate using many transmission lines calculators. Now, once the open end impedance is known, calculate the combined impedance of it with the capacitor's impedance in parallel, again at the operating frequency. This will give the equivalent impedance of this contraption at the point where it connects to the transceiver's grounding wire.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 27, 2021, 07:49:23 pm
content: Or as the memes would tell it:The real root problem is, why are you using an antenna and feedline, that brings all that damn RF current into your shack in the first place? Ground that damn antenna, use a balanced one, whatever. Earth ground sucks beside, so plan on using several ground rods, along the feedline, perhaps with ferrite beads on the line between ground points. This makes an impedance divider. The ground points have notable impedance, but by chaining enough together, with some added impedance inbetween (ferrite beads are in the ballpark of 100 ohms, not a lot for filtering purposes but if your ground is tens or ones of ohms, that's solid attenuation right there), you can get SFA common mode into the shack.Same thing applies between equipment, there's no reason for equipment to suffer from the current loops indicated. If they have common mode emissions,they are malfunctioning and need to be serviced. Those current loops can still be a problem for more mundane reasons -- mains frequency and DC ground loops, affecting audio quality etc., which are best dealt with through standard means (at worst, isolation transformers, but from a design perspective, one should simply be using differential methods; you can have the best of both worlds, even with coax, it doesn't need to be all STP cable and balanced signals).Tim

author: fourfathom
author_rank: Super Contributor
timestamp: March 27, 2021, 08:07:15 pm
content: Quote from: Bud on March 27, 2021, 07:47:44 pmIf the goal is to reduce impedance looking into the shack-end of that shorted coax, wouldn't we need a series reactance to cancel the reactance looking into the coax? A parallel reactance isn't going to do much but potentially increase the combined impedance.I had just glanced at that capacitor and saw 0.001uF to 0.1uf -- and was focusing on the 0.1uF value (which would have an Xc of 0.53 Ohms at 3 MHz. That isn't going to give much impedance transformation range. But I admit, the 0.001uF value will perhaps provide some transformation with the effective shorted coax reactance.The problem is that this will hardly provide a multi-frequency Z cancellation when using a fixed capacitor, and there will be some frequencies (and/or cable lengths) where no value of shunt capacitor will work since the impedance looking into the coax will be capacitive and an inductor will be needed rather than a capacitor.At least that's the way I see it.

author: Bud
author_rank: Super Contributor
timestamp: March 27, 2021, 09:01:43 pm
content: Yes, this is voodoo solution, it does not seem to have a technically sound base.

author: A.Z.
author_rank: Super Contributor
timestamp: March 29, 2021, 04:07:08 pm
content: First of all, thank to all those who answered !That being said, I believe that I'll put that "device" into the "snake oil" closet together with some "magic antennas" and similar stuffThanks again !

author: radiolistener
author_rank: Super Contributor
timestamp: March 29, 2021, 04:44:27 pm
content: If ground affects your radio performance, it's just a good indication that your antenna is sucks.Just use good and properly designed antenna and ground won't really matter on your radio performance.In other words, don't use kludges with grounding and voodoo rites, just fix your antenna

author: A.Z.
author_rank: Super Contributor
timestamp: March 29, 2021, 05:10:45 pm
content: no, luckily I've no problems, I was just curious to understand how that "device" was supposed to work

author: radiolistener
author_rank: Super Contributor
timestamp: March 29, 2021, 06:38:13 pm
content: Quote from: A.Z. on March 29, 2021, 05:10:45 pmAt a glance it propose to use safety ground as a part of antenna. This is a bad solution.In practice it will suffer from noise interference.If you try to use such configuration for transmission it can make high RFI for electronics equipment in your home and for your neighbors.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: March 30, 2021, 04:03:18 pm
content: We also talked about this some months ago here too:https://www.eevblog.com/forum/rf-microwave/grounding-the-shack/msg3180758/#msg3180758

author: A.Z.
author_rank: Super Contributor
timestamp: March 30, 2021, 04:08:52 pm
content: Quote from: pardo-bsso on March 30, 2021, 04:03:18 pmwhooops, forgot about that ! My poor brain is probably becoming fried

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: March 30, 2021, 05:12:03 pm
content: Quote from: A.Z. on March 30, 2021, 04:08:52 pmHa, our brains here too.I didn't notice that it was your post back then