Topband: Two Wire Beverage Query...

[Ford Peterson]

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 at cmgate.com