[OZAPRS] rf earth

[Ray Wells]

Ben,

As with earthing for any purpose, aim for the lowest possible impedance. 
A single stake, or even multiple stakes, would rarely qualify as a 
"good" earth, particularly in typical soil, whatever that is!

Techniques that we used at the country council for both electrical 
safety and lightning protection earthing include laying 25mm x 3mm 
copper strap in shallow trenches or, more commonly, a drilled 50mm hole 
as deep as possible to carry a heavy (typically >35mm²) copper 
conductor. The hole was then pumped with a Bentonite/clay slurry to 
ensure good contact with both the conductor and the sides of the hole as 
the Bentonite expanded. At one remote radio site I looked after, we had 
earthing points at the base of the 63m tower and at each of the 6 guy 
points. We used the drilled hole method and each hole went down 80ft. 
The individual earth points were tied together with heavy conductor in 
shallow trenches. We achieved about 1 ohm and that was in poor, 
rocky/gravelly ground. We also had an earthing gang and the equipment to 
be able to carry out that work.

All of the above is fine for electrical safety and lightning protection 
but *may* be useless for RF where resonant lengths can come into play. 
If the length of conductor to the first earth point is an odd multiple 
of a quarter wavelength, from an RF perspective, the earth is virtually 
useless at that frequency. The gauge of conductor in use being 
irrelevant in that case.

In broadcasting service, a typical earth mat under a vertical radiating 
element comprises buried conductors, each a quarter wavelength long and 
spaced every 2.5 degrees around the vertical.

Since few of us are able to achieve effective RF earthing, my advice is 
to use the SIAS (Suck It And See) method using resources that fit your 
circumstances. Regardless, I believe that multiple earth stakes and 
bonding everything together with heavy conductor is sound methodology.

Ray vk2tv



Ben Lindner-VK5JFK wrote:
> Hi All,
>
> I intend on using shed as an RF earth since it is all bonded together 
> and is in the ground with steel "C" section.
>
> Is this a good idea or is it better to use earth rods.
>
> thoughts or suggestions would be much appreciated
>
> cheers
> Ben
> VK5JFK
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