Don - K4KYV wrote:
...SNIP...
> Unless you have very poor soil conductivity,
> the rf isn't going to penetrate the soil very far, and a ground rod driven
> below where the rf penetrates could just as well be made of fibreglass.
...SNIP...
I dunno Don. Fiberglass? (tongue in cheek of course).
When I measured between two 8' ground rods 10' apart, they behaved nothing like
a piece of wire 10' long. They were definately connecting to something. The
impedance changed smoothly over frequency.
And I'm not sure I buy into the 'penetrate the soil very far' either. "Very
Far" is clearly not a technical term, but the suggestion is that a few inches
is all you get. I'm not buying it. Compare for example the formula for a
transmission line pair. At 1.7" spacing you get about 475 ohms. Now compare
that to the formula for a single wire of the same diameter over ground. You
have to get to 45" spacing to get to 475 ohms. This suggest to me that the
'apparent ground' is down close to 4'.
I was just looking for my spreadsheet from a few years ago and cannot locate it
on this computer, but the gist of my noodling was to look at the Vp of a long
wire on the ground. The Vp becomes very low indeed (approaching 50%). I'll
keep looking for the spreadsheet, but the analysis indicates about 4' based on
my soils.
And again, looking at the formula for stripline over a conductor. I measured
the inductance of a about 450' of wire laying on the ground in a very large
enclosed loop. Using the length of wire, the diameter of the wire, and kept
plugging in a 'height' figure came up to close to 3'.
Basically, unless you can prove this 'thickness' issue in some way, I'm banking
on 3' - 4' apparent ground depths being relevant in my wet pasture soils of
central Minnesota. A 4' rod would just poke into it.
The frost issues in MN continue to be an unknown. I read and hear lots of
speculation but no data! Although I do remember as a kid, running my HW16 and
touching my bare feet to the ground wire and getting one heck of a wallop! The
rod was frozen. That was an 8' rod and all it needed was some hot water
soaking it for a while to bring it back to life.
Tonight, I wound up a coil that should resonate a 21' piece of top fence rail
on Topband. I intend to do what W8JI suggests and document my ground system.
I may also drive some 4' pieces in and measure those too. Next winter, I'll
dig down through the drifts to find the buggers and measure them again after
the frost is at the 5' - 6' levels. At least I'll be able to report here some
evidence of what happens on the tundra. If it is true that you don't need 8'
rods, then I can cut 'em in 2 and buy twice as many for the same $$ ; )
Ford-N0FP
ford@cmgate.com
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