11 July 2019 – RF and Earthing by Richard, G3Nii

It became apparent right from the start that Richard’s talk was not for the un-initiated. Quoting directly, Richard said:  The earth provided by the electricity supplier is intended as a protective part of the circuit to reduce SK membership, as well as protecting devices and equipment attached to the supply.  It is a very important part of the circuit that has to comply with various parameters as set out in the 17th Edition of the IEE Regulations, BS7671. The 18th Edition is now replacing the 17th, this may result in further adjustment of the earthing requirements. Over the years the way in which the earth connection has evolved and is provided has seen changes and indeed variations for different types of consumers, e.g. industrial, farming, those supplied by over-head lines and domestic users.

Richard continued with a description of the mains supply from the sub-station to the supply into the property.  He described various methods of earthing used in the past and finished with the TN-C-S system (PME) used in the majority of properties today that offers the biggest challenge to incorporating an RF earth and where extreme care needs to be taken.

Richard then pointed out a potential problem which was highlighted in an article by Peter Chadwick, G3RZP, well known for his many presentations at RSGB conventions, highlighted in a RadCom article in 1980 the possibility of a failure of the neutral  of a PME supply system  that could lead to a disastrous situation arising, a situation that can and has arisen in the past. So although a rare possibility of it happening take careful note of the consequences should it do so and the procedures necessary to avoid a catastrophe.

Many Radio Amateurs will have their RF earth attached in some way to the general earth wiring of their house mainly due to their not understanding any of the  limitations or requirements associated with doing so. It is a very dangerous path they tread.

Richard continued; So what’s to do? Do you really need an RF earth ? Insulated radials, above ground, or a counterpoise arrangement for those antennas needing an earth, both would work well. Many claim that the counterpoise is quite superior to radials  But, if you have a tower you will no doubt want to protect against high static charge build-up and electric storms thus needing an earth point close by. However, consider a lightning strike yielding 100MV and 200kA, this obviously will destroy everything in its path. A near discharge could be tolerated by a good supplementary earth point, but don’t rely on the PME earthing for this purpose.

Other things that could be done included completely isolating the shack, but that must comply with regulations, or a large isolation transformer, apart from the cost, might an answer.

More info can be obtained from the RSGB. Search for their EMC07 Advanced Leaflet titled Earthing and the Radio Amateur.

 

 

 

 

 

 

 

 

4 July 2019 – This year’s club project

Richard, G3NII explained the main purpose of the latest club project, an HF choke, is to stop common mode currents flowing on the outer skin of coax feeder thereby stopping radiation from the feeder, stopping RF from getting back into the shack and also to lessen QRM pick up on the coax feeder. Thoughts of where if should be placed indicated that since this was adding impedance to the circuit, then positioning in the shack would be worth considering – BUT, make sure the feeder is not an odd number of 1/4 wavelengths long, any number of 1/2 wavelengths long would be ideal.

Richard pointed out that the project required 12 turns of coax on the toroid. You have the choice of 5 turns then cross-over then 6 turns, alternatively 6 turns then cross-over then 5 turns. The cross-over counts as one more turn.

The kits, which included all materials required, including the box,  were handed out and members started the winding and fixing. The evening ended with a Q&A session about the choke and other antenna configurations.

The kits were supplied to members only

Winding details