[TowerTalk] RF Ground is a Myth
jim at audiosystemsgroup.com
Mon Jan 19 13:59:04 EST 2015
The real issue is that the concept of "RF Ground" is a myth and the
result of fuzzy thinking. Part of the reason is what Jim has addressed
below. The other reason is simply that a connection to earth does NOT
make TX antennas work better, and is NOT part of a solution to hum,
buzz, or RFI. The earth is NOT a sink into which noise and RF is dumped.
The ONLY reasons for an earth connection are to sink lightning current
and other equipment-related surge currents on the AC line.
My late colleague, Neil Muncy, ex-W3WJE, taught classes on power and
grounding for many years to audio professionals, and I took over those
classes when he no longer had the health to do them. He is also the guy
who alerted the world to "The Pin One Problem" back in 1994. He gave one
of my favorite teaching examples. He would say to a class, "park
yourself at the end of the runway of the nearest major airport with a
good pair of binoculars, and call me collect when you see an aircraft
take off trailing a ground wire."
73, Jim K9YC
On Mon,1/19/2015 9:15 AM, Jim Lux wrote:
>> Are there different answers depending on why we have the ground rod?
>> (RF ground, power line ground, or lightning protection)
> ground rods make terrible RF grounds, in general (where RF is HF and
> up): skin effect means that wires and rods have high ac resistance.
> (skin depth in copper at 10 MHz is about 0.8 mils/0.02 mm.)
> They also have significant series L (1 microhenry/meter for a wire..
> so a 30 foot run to the rod is a 10 uH inductor, that's 600 ohms
> reactive impedance.
> Rods are really for electrical safety ground and/or lightning ground.
> And they don't work all that well for that, unless deployed in large
> numbers. The advantage of a rod is that it's easy to install by
> driving, but as an electrical connection to the earth, it's just not
> that wonderful: the surface area is quite small (8 foot rod, 1" in
> diameter is only 300 square inches. You could probably do better,
> electrically, by burying a 1 foot square plate (288 square inches).
> Rods are also used in phone and power line applications.. you drive a
> rod at every pole (or wrap the ground wire around the foot of the pole
> when planting it). Even if any one rod has crummy characteristics,
> there's lots of other rods in the circuit to help establish the common
> voltage reference and provide a fault current return. I've had telco
> installers drive a new rod next to the existing rods on the general
> principle that at least they knew the new rod was in good condition:
> faster to just do a new rod than to test the existing one
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