Dave (and anyone else interested),
I have heard others voice comments such as you make with respect to lengths of
grounding conductors and their subsequent self inductance. As I stated in my
prior post, my guidance for my station's grounding scheme was MIL-HDBK-419A, a
juicy little tome in 2 volumes and available as a free PDF download. It is (or
was) used as the standard site grounding scheme for most military fixed
installations. The key features are complete ground rings around each building,
tower, or fixed asset, and bonding of all RF system grounds to the main site
power distribution ground.
The installation I described in my prior post is based around a house that is
about 54 feet wide and about 35 feet deep. When only the basement walls and
concrete footings were in place, I installed 14 ground rods (each 8 ft long by
5/8 in diameter copper plated steel) around the outside of the footings and
foundation, with most of them Cad-Welded to the more than 200 feet of 00 gauge
bare copper stranded wire that is buried about 1 foot beneath the backfill soil
around the foundation. (I ran short of Cad-Welds and had to use compression
clamps to bond a few of the rods to the wire.) That wire forms a complete loop
around the foundation. The ham shack is at the north end of the house and the
electrical panel is at the other end. Thus, it was very important that I
installed a short jumper wire between the power panel grounding conductor and
the loop. The NEC defines that as an auxiliary ground connection. Each of my
3 towers has a full ring ground around its base and those ring grounds connect
to the main loop.
Now, a final word about ground rods: Prior to coming to Rockwell Collins, I was
chief electrical engineer for Lindgren RF Enclosures, Inc., a major
manufacturer of RF shielded enclosures for military and numerous other
applications. Those Faraday cages were equipped with power and control line
filters as needed for uses of the chambers. The filters were designed with
conductive cases that were directly mounted to the wall and/or ceiling of the
enclosure. Since the filters pass all impinging energy to the case, it is
important that the filter case (and the shielded enclosure) be well grounded.
It is also essential that power filters (for 50 or 60 Hz applications) have a
low impedance path to the power panel so as to minimize shock hazards.
I got a phone call from an industrial customer who complained that he was
getting shocked anytime he touched the enclosure and an external grounded
object (such as a water pipe or metal building column). He told me that the
enclosure had been stalled about 3 or 4 years ago on the factory floor. Two 8
foot ground rods were welded together and passed thru the concrete floor. He
had faithfully poured salt water down that hole to maintain conductivity. I
suggested that he check the rod and sure enough, only the top 3 feet of that 16
foot rod still existed.
There is a solution for that problem. When drilling or setting a ground rod
into an area of low electrical conductivity, use a material called Bentonite
Clay. I got that info from a California company that specializes in difficult
grounding situations.
73, Dale
WA9ENA
-----Original Message-----
From: David Eckhardt <davearea51a@gmail.com></davearea51a@gmail.com>
Sent: Jan 23, 2025 3:29 PM
To: <jim@audiosystemsgroup.com></jim@audiosystemsgroup.com>
Cc: <rfi@contesting.com>, Ham-Antennas
<main@ham-antennas.groups.io></main@ham-antennas.groups.io></rfi@contesting.com>
Subject: Re: [RFI] FW: RF noise and ground
The average ham's concept of "ground" is usually completely off the wall
and absolutely incorrect. As Jim stated, "ground" (the Brit's got it
right using the term "earth!) is not a sump/sink for everything bad
associated with RF and/or RFI.
How many time do we hear or read as a "solution" to RF-in-the-Shack:
"Connect a ground"? BS!! What if your "ground" *(earth) conductor is
33-feet to your "ground" (earth) connection? That's a quarter wave on
40-meters, so you have a floating - open circuit - to your "ground" (earth)
connection. Also, its an open circuit at odd harmonics of 7.15 MHz. Do
you have a reliable "ground" (earth) connection? Absolutely not!
In addition, "ground" rods are NOT sufficient for lightning remediation.
They are highly resistive to the preverbial "RF garbage sump/sink" that
most hams believe a single "ground" (earth) rod provides. Further, for
them (notice plural) to remain effective over time, they must be
periodically treated with a solution of CuSO4 (blue vitrol) or NaCl (garden
variety salt).
The absolute most important item to address in lightning remediation is
that everything of concern (electronics) floats up and down in potential
together. That requirement eliminates a "standard practice" of connecting
everything together with copper wire, even of heavy gauge copper wire. When
10-thousands of amperes are encountered from a lightning strike, even
nanoHenries of inductance forms a reflective circuit element! Look at the
feed coax at the very base of any AM BC transmitter. The coax cable is
usually dressed in a large loop before it enter the tower. This loop
introduces enough +jX (inductive reactance) to keep "most" of the
potentially damaging elements of the strike out of the matching network and
transmitter. The requirement I described requires EVERYTHING of concern
sit on a large highly conductive sheet and each piece of equipment of
concern be BONDED to that highly conductive sheet. Doing so results in no
potential differences (high voltages) between the electronics placed and
bonded to that sheet. A connection from that sheet to "ground" (earth)
then bleeds off any charge the sheet may have accumulated.
Thanks, Jim, for motivating me to write this email!🙂😀😉
Dave - WØLEV
On Thu, Jan 23, 2025 at 7:15 PM Jim Brown wrote:
> On 1/23/2025 4:30 AM, nlsa@nlsa.com wrote:
> > Question: I’ll bet that my noise floor could be improved by a better
> > RF
> connection to Earth, but how can I do it? What mode does EMI take in this
> situation? What is “ground” in a non-metallic enclosure?
> >
> The earth is not a sump into which noise is poured. The ONLY reason for
> an earth connection, and it is ABOUT LIFE OR DEATH, is LIGHTNING
> PROTECTION, and it is required by LAW. So is the bonding of everything.
>
> I would not expect any noise reduction at VHF from an earth connection,
> but proper grounding and bonding is required by Electrical Codes, which
> are LAW, and for human safety.
>
> 73, Jim K9YC
>
>
>
> _______________________________________________
> RFI mailing list
> RFI@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
>
--
*Dave - WØLEV*
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|