03-07-2023, 07:29 PM
(02-07-2023, 04:08 PM)VK3RX Wrote: When I moved to the house here I noted that outside the room I'd selected to be the "shack" the copper mains water pipe came up from the ground and into a hole in the brickwork to feed the house water system.
So from a common point of the radio gear I ran a short heavy gauge cable out through the wall and clamped it to this water pipe.
Heavy AC 50Hz hum on TX audio from just about everything was the result.
So in a garden bed that ran along that wall I drove in a couple of copper earth stakes around a metre apart linked together with heavy gauge cable, connected the shack earth cable to one of the stakes and the AC hum issue disappeared.
My wire HF antenna is off the other end of the house so the shack isn’t close by. My only experience with RF getting in to mike audio was with another wire antenna that ran a tad closer, and that was solved by a tip from Heil Sound re earthing of Kenwood microphones.
See “RFI Removal from Microphone Inputs” here:
https://heilhamradio.com/faqs/
Hi
Unfortunately there are a few old myths floating around on the issue of earthing.
Some old notes of mine I dug up on earthing.
It is intended to provide some food for thought on the earthing issue and is not intended to cover all aspects of earthing.
1 An earth electrode does not behave like a simple resistor. It is more like complex web of many resistors connected to various different locations.
2 Earth resistance is measured with an earth resistivity meter (3 or 4 terminals, type dependant). A normal ohm meter is not suitable for this.
3 Connecting two electrodes in parallel will not normally halve the earth resistance. Two earth electrodes spaced their length apart reduces the earth resistance to about 60% of a single one. Closer electrode spacings result less of a reduction as they begin to act more and more like a single electrode. The old earthing practice of having an earthing bed of closely spaced electrodes (still found on some sites) was flawed as it does not provide the lower earth resistance envisaged.
4 Earthing installations normally cater for mains power and lightning requirements and not RF needs.
5 Local soil enhancement using water or soil additives will usually produce only minimal reduction in the earth resistance unless implemented over a large area. They are generally not worth the effort. If only localised they are in effect a poor substitute for a thicker electrode which does in itself not produce much of an improvement. The electrode diameter needs to be increased by a factor of around 20 to halve its earth resistance. However doubling the total length of the electrode is better but does not quite achieve halve the earth resistance. This all assumes consistent earth resistivity.
Mains earthing
The normal earth rod (13mm dia, 1440mm long) at a house premise is for mains earthing purposes. It relies on all the other earth electrodes in the surrounding area for it to work effectively. It forms part of the MEN (Multiple Earthed Neutral) system. It caters for 50Hz. It is not very effective for lightning protection where were the energy is in the form of a short pulse of energy mainly in the 100kHz range associated with a very high voltage and current. At HF in the MHz range it is even less effective. As the frequency goes up lead length and inductance becomes somewhat important. The earthing system may possibly even start to behave like the high impedance end of a half wave dipole!
Earth resistance is determined by a number of factors. The soil resistivity is a major player. In Perth (Sandgroper Country), the sand behaves more like an insulator than conductor and it is somewhat difficult to achieve a low earth resistance. The soil resistivity depends on the texture of the soil and this varies with depth and location. In some instances deep earth electrodes have even finish up partly in the air in voids underground!
A Perth site I was involved with in providing lightning protection had experienced repeated lighting strikes. These resulted in major problems in site operations and high operation costs. The new earthing system involved installing two loops in the ground one 3 Km long and another one 2 Km long inside it with 7 interconnections. All using bare 70mm2 copper cable. Connected to the loops were 59 electrodes (45 x 2.4M long and 15 x 14M long). Yes, it was a big expensive earthing system!
The measured earth resistances were as follows:
2.4M electrodes
Ranged from 6 to 980 ohms with the median value 395 ohms, average 406 ohms.
14M electrodes
Ranged from 6 to 650 ohms with the median value 180 ohms, average 228 ohms.
The few low resistance readings obtained are suspect and probably due to localised area effects.
As can be seen obtaining a low earth resistance in the Perth area is somewhat difficult!
So if you expect to get a good, low RF earth resistance using a short spike in the ground you will be sadly disappointed (unless it is in seawater)!
Someone did once say “life was not meant to be easy”.
73
Igor