On 7/24/16 10:08 PM, Jim Brown wrote:
I noticed in one of your slides that you said to NOT run the tower
grounding system back to the house AC input ground if it is too long
(but to depend on the antenna coax sheath). How long is too long?
Note the slide(s) showing that 1) lightning is NOT a DC event, it is an
RF event so that 2) the inductance of a long bonding conductor dominates
the resistance of the conductor. The logic here is to give lightning at
the tower a low impedance path to earth at its base, do the same at the
house. And, of course, follow proper bonding at the house.
It is about 40-50' from my shack wall to the tower base, but probably
100' from the tower base to around the other side of the house to the
AC input of the house (house ground rod). I am planning on putting in
about 9 ground rods around the base of the 73' tall crank-up tower (3
off of each leg in a "Y" configuration). The first rod off of each Y
leg will have a wide copper strap connected from the ground rod to the
tower leg in a gentle arc with no sharp turns. Most installations I've
seen have maybe a 3" or 4" wide piece of copper. All three Y base leg
ground rods will be cadwelded to #2 copper stranded in a ring around
the tower. It was this ring that I was going to run to the AC house
ground rod. Finally, the other 2 ground rods in each Y configuration
were planned to be cadwelded to the base leg of the Y with #2
stranded. Any comments? PS - We have heavy clay soil here in NC.
Gentle arcs don't change the inductance much as compared to a sharp
right angle bend. It's all about the length of the conductor. if you
have a 1 foot radius 90 degree arc, it's about 1.6 feet long. If you did
the same thing along the sides of a square it would be 2 feet long. If
you cut across the middle with a 45 degree, it would be 1.4 feet long
(which would be the lowest inductance).
The reason for not having sharp bends in *lightning* grounding cables is
to reduce the field concentrations with small radius of curvature.
Small radius of curvature increases the field at the curve which
increases the likelihood of a flashover from the conductor to something
else.
For reference, a good rule of thumb is 1 microhenry/meter (or 1/3
uH/ft). At 1 MHz, this works out to 6.28 ohms/meter impedance. That
completely dominates over the AC resistance, even if you were using AWG
30 wire-wrap wire.
You DO want low resistance bonds, to reduce the heating from the
multi-kilo-amp currents (that last a short time - 50 microseconds)
All sounds fine. I'd say the tower is probably close enough to the house
that the tower ground should be bonded to the house. But remember, it's
inductance, so a pretty high Z at 1 MHz, which is where the energy in
lightning is roughly centered.
What you want is everything that is "near" each other connected with
good conductors that are similar in length (so the inductance is
similar, and the voltage rise from the pulse is similar).
If you have one piece of gear with a 10 foot cable to the lightning
impulse. And another piece of gear on the bench connected with a 100
foot cable to the lightning impulse, and then you interconnect the two
with a short jumper, you can see that there might be a problem.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|