[TowerTalk] Grounding help needed
Tue, 25 Sep 2001 10:54:47 -0400
> I thought that after a year or so of reading every TT post and the
> polyphaser book that I had this figured out. Then someone said that
> only the first few inches of ground rod made any difference to RF and
> it is better to bury the ground rod horizontally a few inches below
That's one for the books!
The skin depth on 160 meters in poor soil is 20 meters deep. That
means a ground rod 20 meters deep would still have the some
effect at the lower end, because noticeable current would exist at
that depth. (It tales a few skin depths for current to be negligible).
Skin depth is 5 meters with good soil on the 160-meter band.
At 10 Mhz in virtually any soil the full length of a ground rod six feet
long is useful.
The energy peak in lightning is at around 10 kHz.
From this I gather that lightning protection requires long
> vertical ground rods while RF grounding is better with shallow,
> horizontal ground conductors. Did I get this right?
Actually all grounding is better with a large spread-out area rather
than a single deep area, because the voltage gradient is reduced
over the entire large area. The goal is to make EVERYTHING
around your house rise at the same rate, rather than trying to
clamp your ground to the rest of the world's charge potential.
That is why all telco, CATV, and power grounds should be bonded
to the station ground and hopefully all will enter at about the same
area of the house. This is probably 90% or more of what reduces
damage in the house.
That does NOT mean you are better off burying a ground rod
"shallow" instead of deep when you only are talking about a single
rod. What it means is you are better off spreading out the system
with multiple rods spread over a wide area than just using one deep
A single ground rod eight feet deep and one inch diameter has a
resistance of 50-200 ohms on 2 MHz. Laid horizontally a few
inches it can be over 1000 ohms!
RF grounds and lightning grounds have a lot in common, with RF
you want a large "electrical mass" (low RF impedance) for the
system to push against. That requires a large surface area of
multiple conductors for a ground. At the same time, this reduces
voltage difference over that area when lightning hits.
73, Tom W8JI
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