Why Use an Earth Rod when a Disc will do?

Kelly Marie Angel:

Not very impressive 300 ohms is a bit high maybe if its buried deeper it will be better think ide stick to rods if given a choice

If it's a consistent Zs reading of say 300 Ohms, it will easily satisfy the requirements of Table 41.5 up to 100mA operating current R.C.D.s,  including the popular 30mA type. Of course I do not know if you radio people need a lower resistance to true earth for other reasons.


Z.

I wonder how well it responds to high currents like lightning surges. For comparison one of the problems of the Marconite concrete 'Ufer' electrodes can be cracking of the concrete after such an event.

To be honest of you are digging a hole you may as almost well dig a long thin one and put a rod in either laying flat or on the diagonal.

(or bent into a 'J to get a connection you can inspect)


Beware of very shallow burial, both drying out and freezing solid have the same effect of removing most of the conducting ions - you want to be deep enough it does not do that.


Radio earth arrangements fall into two cases - a sort of safety earth under  a balanced antenna where there is almost no ground current, and here an electrical grade earth electrode may do, or a for a monopole antenna 'working against ground' where for every amp of RF going up the antenna, an amp also comes out of the ground. In this second case  we do not especially need a low DC earth impedance, but a we need to keep the RF touch voltage down, so we do need a good capacitance to earth so a chicken mesh or radial pattern of wires that can even be insulated is often used as the 'counterpoise' to the active element. Long earth wires ( as in a significant fraction of a wavelength long)  - imagine the electrons doing a sort of click clack newtons cradle thing and bouncing back and forth - can form unexpected resonators, and that needs additional consideration as the current and voltage are not the same at different points on the line and so the effective connection impedance is (sometimes very) RF frequency dependant

Mike.

John Peckham:

I am a big fan of copper earth mats. Whilst you digging a hole a 900 x 900 copper earth mat might be a cheaper and better option.


JP

I've to dig a couple of trenches for swa supplies to a garage and workshop. Would you suggest I lay a tape in there whilst I'm at it? Anything special about terminating the tape? (I've never seen it, let alone installed it!)