On 12/13/15 10:56 AM, Richard (Rick) Karlquist wrote:
You can estimate 20 nH per inch for these hook up
wires. On 80 meters, think of that as as about
a half ohm of inductive reactance per inch,
or 6 ohms of inductive reactance per foot.
Thinking of it that way, you can see why a single
10 inch wire in series with your ground connection
isn't a great idea. You might try modeling it
and see if it rises to the "noticable" level.
I use 1 uH/meter, which works out to about 25 nH/inch.. I doubt the
difference is important.
There's also the whole thing with series L (which can be "tuned out")
and the series R (which is plain old loss).
Putting strap, ribbon, or multiple parallel wires (that are close to
each other) reduces the R but doesn't change the L very much. The
magnetic fields couple between parallel wires or "filaments" within a
wide flat strip, so the L stays pretty much the same.
Strip has lower AC resistance (more surface area) than a round wire of
the same cross section.
So if you had multiple wires going out to the ring in different places,
you'll reduce both L and R (the magnetic field from the wires won't
interact).
This is why a single big wire works just fine for lightning protection
(AC loss isn't a big deal compared to the L, as long as the wire doesn't
melt).
But if the grounding system is part of the radiator (e.g. AM Broadcast),
then you want that big flat conductor to minimize AC resistance loss.
The fact that it's an OK lightning grounding conductor too is just icing
on the cake.
I have a 230 foot diameter ground screen with
wires spaced 3 feet apart in each direction.
When I first completed this, I placed a 40
meter vertical at the center junction, such
that there were four wires emanating from the
shield of the coax connector. These 4 wires
run 3 feet until they reached the next junction,
etc. The driving impedance of this was
70 ohms, despite being over a gigantic ground
screen. After adding 30 foot radials to tie it into
the ground screen 30 feet away from the vertical,
the impedance steadily decreased with the
number of radials until it reached the
theoretical value of 36 ohms. This is the
kind of weird stuff that happens if you deviate
from the classical radial paradigm.
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