[TenTec] Field day antennas

[Jim Brown]

On Thu,4/16/2015 8:48 AM, Don Jones wrote:
> Sorry for taking to the field or sharing here what works for me in my 
> EMC/Lightning Lab on a daily basis. We make tens of thousands of 
> dollars every year off those who fail to ground properly. That figure 
> does not include the cost for damage to test samples and careers. Over 
> the years I've had several know it all's take up software engineering 
> instead.
> It is my experience that the remote ground rods help in several ways, 
> including CM suppression.

The ONLY benefit of a connection to earth is to provide an alternate 
path for current on the chassis that chassis that should not be there in 
the first place. The earth is NOT a sump into which noise is dumped.

An example of when a ground rod can help is a generator, like a Honda 
2000i, that puts out common mode noise at HF. Bonding the generator 
chassis to a ground rod gives the current somewhere else to go other 
than the power cable. BUT -- that function is also easily served by a 
common mode choke formed by winding the power cable multiple turns 
around a suitable ferrite core. Indeed, the combination of rod and choke 
would work better than either alone.

And here's the other factor -- soil conductivity varies widely from 
pretty good to really bad. Here in the Santa Cruz Mountains, I've 
measured hundreds of Ohms between two 8 ft rods driven their full length 
about 8 ft apart. The top soil here is pretty loamy -- tall redwoods 
have been shedding lots of duff for centuries -- but underneath is lots 
of granite. The rod-to-rod resistance is far lower than the rod to earth 
resistance. The resistance between the two ends of my 550 ft Beverage is 
almost 50K Ohms. In sandy soil, it would be even higher.

A major cause of this fuzzy thinking is that we use the word "ground" in 
so many different ways, including circuit common.

When we connect the shield of a cable to the shielding enclosure of a 
piece of gear, we are NOT "GROUNDING" it, we are BONDING it to the 
shielding enclosure, making the cable an effective shield. It does not 
matter whether the equipment has a connection to earth. Without that 
shield bond, the cable is not shielded. If the cable shield goes inside 
the equipment and then to the chassis, we've got a classic "Pin One 
Problem," which conducts signals into and out of the box. When we bond 
the shield to the shielding enclosure, shield current goes to the power 
system green wire -- it stays outside the box.

Neil Muncy, ex-W3WJE (SK) taught EMC classes for many years, and 
published landmark papers on EMC. He discovered the mechanism we call 
Shield Current Induced Noise and also what we call The Pin One Problem. 
In his classes, he emphasized that the only reason for an earth 
connection is lightning and electrical safety. And he told his classes, 
"get yourself a six-pack, a comfortable chair, and a pair of really good 
binoculars and set yourself down at the end of a runway at your major 
airport, and CALL ME COLLECT when you see one of those jets taking off 
trailing a ground wire."

I suspect that this "failure to ground properly" you're talking about is 
really "The Pin One Problem" -- connectors mounted to a PC board but not 
to the shielding enclosure. To get to the shielding enclosure, shield 
current must travel along the equipment common bus (or "ground plane), 
where it is then injected into gain stages at the whim of the circuit 
layout specialist.

Remember that on that "ground" layer, at all frequencies above audio, 
the current of a circuit trace returns in a narrow region directly under 
the trace, forming a transmission line. AND -- if that "ground" layer 
happens to be broken under a trace (to add a trace that wouldn't fit on 
another layer, for example), that return path is broken, so the return 
current flows all over the box, creating a large magnetic loop and 
antenna that couples into circuitry at random.

73, Jim K9YC
Vice-Chair Working Group on EMC,
Standards Committee of the Audio Engineering Society