"Art Unwin" wrote in message
...
On Aug 3, 4:51 pm, "Dave" wrote:
"Art Unwin" wrote in message
i don't know where you get your radios, but in all of them here the coax
connector is connected to the transmitter chassis which also has a lug
for
going to the safety ground.


By golly, I believe you are correct! I thought that was a bad
practice! Are all electronics at the same potential in the U.S. as
its container? So with a ground connection at the electrical supply
point and a provided connection for another generated "ground", is
that not an invitation for ground loops?

yes, it is. that is why your 'shack' ground has to be connected to the
service entrance ground.



Lightning tries to get to the true ground by the shortest route and
the requirement is for you to not be part of it's route, even if your
radio has to be the sacrificial lamb if you haven't pulled the plug.

there ain't no such thing as 'true ground', tis all relative.

H'mm, the ground point with the lowest potential relative to other
ground potentials in the same system is the "true" ground of the
system.

you have to define the 'system'. but there is always the possibility that
someone outside the 'system' can reach in and measure your 'ground '
connection relative to something else and measure a potential. potential is
what is relative, hook a 12v battery to a meter and set it on a table,
connect another 12v battery between a ground rod and one terminal of the
first battery... does the meter reading change? what is the potential of
the positive terminal of the battery? if you were sitting on the table and
couldn't reach the ground rod or 2nd battery terminals could you tell what
the voltage of the 2nd battery was?

Now can you tell me which "ground" is protected and from
what?

'ground' is not protected from anything. 'ground' is a convenient point to
connect everything and to make measurements from.

And RF ground; is that protected from lightning?

there is no such thing as an 'rf ground'. there is at best a low impedance
path for return currents to get back to the feedpoint... if that happens to
be through the earth (note for you, earth means dirt, not earth as in
electrical ground that we call it in the states) then we may want to call it
'ground' but it really isn't.

now, why do i say there is no such thing as an rf ground? consider what
happens around a 10m 1/4 wave vertical mounted on the ground connected to
the center conductor of the coax, with a single ground rod as the
connection for the shield of the coax. The current going into the 1/4 wave
vertical and the current coming out of the ground rod have to be equal at
the coax connection... so, current flows up the vertical, back through the
ground, up the ground rod and back to the coax... very simple... until you
take your rf voltmeter and connect it to the ground rod and stick the other
probe in the ground some distance from the rod, lets say about 1/4 wave away
(more or less, propagation velocity through the ground is slower so it would
be physically less than 1/4 wave in free space, etc, etc). what voltage do
you read on your meter?? it is difficult to calculate because of all the
interfaces and bulk conductivities, the sizes of the ground rod and probe,
etc... but suffice it to say, it ain't zero. why not, aren't both probes
connected to 'ground'?? well, yes, and no. because there is current
flowing through the earth trying to get back to that coax connection there
has to be a voltage difference. current in earth * resistivity of earth =
voltage between points on the earth, give or take a few units. now lets say
we add a radial wire to connect the ground rod at the feed point of the
vertical to the probe where you measure the voltage of the earth... does the
voltage go away? after all you just shorted out your voltmeter didn't you??
but no! there is still voltage! why you ask?? well, even though you have
made a lower impedance path there is still current on the wire, and since
the wire has inductance and capacitance the propagation time is not zero, so
there is still a voltage difference causing that current to flow along the
radial wire... why is it necessary to insulate the ends of raised radials if
they are a 'ground' plane under a vertical antenna?

lightning 'ground' is even worse because the currents are bigger... consider
a 100ka bolt with a rise time of 1usec hitting the ground and spreading out
at about .3c (approximately what i measured in one experiment for a buried
radial wire)... the potential difference between points just a few feet
apart can be huge, that is a common cause of lightning deaths, current hits
tree, goes through ground, up one leg, down the other, or up one leg and out
an arm that is closer to something conductive away from the tree... like a
golf club. the key with designing lightning 'ground' systems is to realize
that this potential exists and make sure you can handle it. the 'single
point ground' is the classic way because it makes the distance zero which
takes the rise time and propagation speed out of the equation.
unfortunately that is not always possible, so that is where other methods
are needed to keep voltages across protected equipment equalized.

Lightning is multi frequency oriented so it would appear to me it would
gyrate
towards a radiator.

i have never known lightning to gyrate. it takes all sorts of convoluted
paths, but none that i would call gyrating.


What about just one wire to the antenna and let the earth be the
return line to close the circuit!

certainly, and many have done that, and some have gotten burned. consider
this case... sit on your table with the 12v battery again, but instead of
the meter hook up a transmitter to it. assume there is one so-239 output
jack mounted on the radio's metal case and no other connections, just 12v
in, rf out. run a wire straight out from the so-239 center conductor then
touch the radio's metal case... what happens? if the power is high enough
you get burned... why? you are touching the radio case, isn't that
'ground'? you aren't touching the earth? so what is happening??? well,
you are now part of the return path for current that has to find a way back
to the chassis side of the so-239, and since you are likely much larger than
the case of the radio you make a more efficient collector.

change the setup, now run a wire from the case of the radio to the ground
rod where the 2nd battery used to be connected... now you have another path
for current to get back to the chassis side of the connector, from the
'earth' through the ground rod up the wire and to the radio... now touch the
case and what happens? well, if the earth to radio circuit is lower
impedance than your body to radio, more current flows on that side and you
don't get burned... not good enough? still getting burned??? get one of
those counterpoise resonator thingies... they are really just a small match
box for a long wire... run the wire around the floor and tune it up. why
does it work better? it provides a lower impedance path for collecting up
that return current and getting it back to the radio. no magic, it is just
providing the return current instead of letting your body collect it.

Seems like the definitions have gone awry without involvement of a chassis
ground

ah, chassis ground, yet another type of ground.

but then it is you that is electrically educated and the better judge.

one of the few things you have said that i agree with