[TenTec] Station Grounding

[Bob & Linda McGraw K4TAX]

Jerry's comments are well stated and accurate.

All.......take heed.

73
Bob K4TAX

Dr Gerald N Johnson electrical engineer wrote:
> 
> I disagree.
> 
> 1. The NEC is based on presuming that the MINIMUM ground conductor be
> only large enough to carry any current the hot conductor would and so
> trip the protection with damaging the ground conductor. Its not
> anticipating ground currents from outside sources, such as RF or
> lightning. And the NEC is the MINIMUM for safety, not necessarily for
> good engineering. There have been time periods where the NEC accepted
> safety ground conductors smaller than the phase wires. E.g. 14 in 12
> romex.
> 
> 2. Length and width are both important for an RF ground. Looking at a
> ground wire as a transmission line, the impedance seen at the radio end
> is Z0 * tangent(length in wavelengths). Which means that unless exactly
> at 1/4 or 3/4 wavelength (or other odd multiples of a wavelength) the
> impedance is directly proportional to the CHARACTERISTIC impedance of
> the transmission line. A flat sheet has a much lower characteristic
> impedance than a round wire, hence for all practical purposes and except
> at exactly the 1/4 wave resonance, the impedance to ground is smaller.
> 
> We accept the small wire in the open wire transmission line because the
> characteristic impedance of the line is high and so except at the dipole
> load, the transmission line currents are small and so the loss is small.
> We correct for the changes in impedance using a tuner.
> 
> 3. Both the effective conductance AND the current carrying capabilities
> of the lightning ground are very important. Lightning currents can be a
> few to several kiloamps. It can require more than a wire to handle them.
> I've seen wires from equipment and houses after lightning where the
> lightning had melted the surface and the magnetic fields had expelled
> that melted copper out in a fan showing the magnetic field's shape when
> the copper then froze preserving that shape. My comments in part 2 for
> characteristic impedance apply to lighting protection.
> 
> The trouble with depending on grounds for lighting protection is that
> the earth, wet or dry dirt is a rotten ground. So the ground rod (and
> any rod under 8' long is probably more decoration than safe ground)
> supplies much local resistance to the ground system. Multiple ground
> rods are important, and series isolation is important. I prefer to use
> several ground rods for the antennas (solid guy wires to screw in
> anchors are a benefit to spread the grounds out) outside the shack, then
> only a single wire to a patch panel connecting antennas and ground to
> inside and when there's lightning about, I disconnect ALL including the
> ground and maintain a significant (feet) air gap. Times when I have not
> done that and have been hit, there's been damage all over the house.
> Transmission lines wound into inductors between the tower grounds and
> the interior grounds may have some benefit in diverting lightning
> currents to the earth, but when a lightning strike can be 10,000 amps,
> cutting it down by a factor of say 10 still leaves a current to the
> radios capable of severe damage. And small air gaps are not much
> benefit. If there's 10 KA strike to a 10 ohm ground system (and 10 ohms
> for ground resistance is very difficult to achieve), there's still 100
> KV voltage difference to other ground systems (e.g. the power line and
> phone line) and that can jump 6 or 8 inches... So any isolator that
> depends on a shorter air gap is false protection.
> 
> RF filters in power lines do nothing to dissipate peak energy, however
> they do integrate short pulses into longer pulses of lower amplitude.
> The lower amplitude pulse after the filter contains virtually the same
> destructive energy. Because of that, MOV protective elements that are
> based on voltage alone need to be on the line side of power line
> filters. This is inconvenient with the filters that have an integral
> line cord connector. I have conducted experiments using line impulse
> generators to prove that a power supply protected by MOV outside the
> filter would be protected from the magnitude of pulse that would
> perforate the transformer insulation without the MOV and filter
> combination.
> 
> 73, Jerry, K0CQ
> 
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