[TowerTalk] Radials, lightning ground, RF ground
n1lo at hotmail.com
Wed Nov 23 09:44:56 EST 2005
Let me try to help clear the confusion. to the best of my understanding.
There are three separate entities to consider:
1) RF COUNTERPOISE/GROUND SCREEN. A set of radials or mesh to help an
antenna (usually a vertical) radiate more efficiently with fewer ground
losses is an RF counterpoise or ground screen. These can be light gage wire,
bare or insulated, elevated, laying on the ground surface, or buried. The
maximum gap between conductors should be no more than .05 wavelengths for
good efficiency. These 'radials' (in the case of a mesh, they're not even
radial) may or may not make a decent connection to a volume of earth.
2) An RF GROUND, is a connection to earth; that is, soil, typically. A basic
RF ground is typically one or more ground rods connected together, driven
vertically, or laying horizontally in trenches (in rocky soil). Horizontal
buried copper strap can also act as an RF ground. One or two ground rods,
typically connected with #6 bare copper, or lighter wire, can act as a basic
RF ground for your radio, and can sink a small amount of current to earth.
3) A LIGHTNING GROUND, is an RF ground to protect a tower or mast that has
typically at least 6 ground rods, connected with #6 or heavier copper wire,
mechanically clamped or CAD-welded together, that makes a connection with a
large volume of earth and:
can distribute and sink a large current (from a lightning strike) to that
A general rule of thumb is that a single vertical ground rod can reasonably
connect to a volume of earth approximately equal to a sphere having a radius
equal to the depth of the ground rod. More than one ground rod in the same
sphere is redundant. This is why ground rods in a LIGHTNING GROUND system
are spaced twice their depth: to help connect to the maximum volume of earth
without overloading that earth with charge. When there is an insufficient
volume of earth to sink the current from the lightning strike, that energy
will seek other paths that can be destructive.
Now, you see, that insulated or elevated wires in an RF counterpoise or
radial system, cannot sink any current to earth, and cannot act as an RF
ground or lightning ground.
AN RF counterpoise/ground screen having light gage bare, buried wires or
mesh, can be both an RF counterpoise AND and RF ground, but would be burned
up by lightning strike current.
A heavy duty RF counterpoise made from #6 bare, buried copper radial wires,
such as already described here, used on commercial broadcast installations
(very costly), can fulfill all 3 functions: RF counterpoise, RF ground, and
Hope this helps.
I am now officially confused. Suppose I built a vertical and ran 40 0.2
wavelength rf radials of insulated #14 wire along the top of the ground.
Suppose I had a single ground rod near to and connected to the radials near
the vertical. How would you augment this installation to provide good
lightning protection? Wouldn't it still require a handful of heavy gauge
radials with ground rods along their length? Dudley - WA1X >>
>A quote from Polyphaser: "A good lightning ground is also a good RF ground.
>But a good RF ground is not necessarily a good lightning ground."
>The RF ground and lightning ground need not be treated separately.
>Heavy radials are not necessary if there are more than a few. The lightning
>energy will be divided among all so no single radial will carry high
>Insulated wire radials will dissipate the higher frequencies involved in
>lightning but bare wires will be required for the low / DC component of
>lightning. The insulated wire will arc to ground as voltage rises but bare
>wire will help keep that arc point lower.
>Arcing to ground with bare wire still happens at times in some soil types.
>Some broadcast stations employ ground rods along the radial lengths when
>difficult soil conditions are present for lightning dissipation.
>A properly installed radial system can be both an excellent RF ground and
>lightning ground as a large portion of lightning energy is in the RF
>As a mater of interest, you do not have a good lightning ground unless it
>a good RF ground in addition to a good DC ground.
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