Thursday, 6 November 2014

Shack RF&Safety combined earth

First of all, the subject grounding is controversial. Some say it's a need in your shack, some say the mains earth is good enough for safety. Since I use a safety transformer in the mains to supply 230V to my shack a mains safety earth is not working. So I made a separate earthrod outside the shack and connected it to the connectorpanel just behind the outside wall. Now this is just for safety, it's not a HF earth. However I am an experimenter and would like to test a HF earth, if it doesn't work I can simply disconnect and nothing is lost. 

I wrote about this subject before:

The RF surpression ground system myth
Making a second floor shack ground

The common way a RF or HF earth is made I think is explained in the figure above. To prevent earthloops everything is bonded to a single grounding point. The actual earth are the counterpoise wires that are 1/4 wavelength long for each band one or more (not really necessary if you use a balanced antenna). The star point could be connected through on of the counterpoise wires or better with a separate ground strip for safety.

Now in my case a problem with a earthloop was rising between the connectorpanel and the earthpoint in the shack which is best bonded to the last item in the coax chain, the antennatuner. Since the coax is earthed to the chassis of the panel a extra groundstrip would led to a groundloop. But I have the connectorpanel for safety, when the coax to the shack is unplugged the antennas and outside coax cables are grounded through the panel and I hope I'll not get dangerous high voltages on my equipment chassis when a lightning storm is nearby. So I made a two-way knife switch to choose between the shack earth and the panel. That way I will not create a earth loop. I don't really know if it is all worth the trouble, but at least it gives me a trusty feeling when the counterpoise is grounded for safety.

Now some of you will think....why a counterpoise? That's only working when you have some sort of  impedance problem with a unbalanced antenna. A simple balun grounded near or on the antenna works ten times better. Oh, yes indeed that's true. But actually my RFI is related to my balanced 84 Mtr horizontal loop fed by a 450Ohm ladderline which I have only up in the winter and is used on all HF bands through a Palstar antennatuner. Actually I don't need a counterpoise with 1/4 wave length wires, but that's what still had from previous experiments. The goal actually is to bond as much metal in the shack to the common grounding point to get a equal potential on all equipment when transmitting. More information about this can be found on the website of W8JI. Best would be a conductive grid on the floor below the carpet or so, I will make that in the future.

Just for the record, normally you don't need this. Most shacks don't need a HF earth at all. A safety earth is more then enough. However if you have a antenna very close to the shack it could help against RFI. In my case I do have the horizontal loop in winter very close to my operating position (2 meters distance) and have som RFI into the the headset, deskmike and computerscreen when operating on 17m and 15m. Very annoying and I hope this will partly solve the problem. Only time will tell.

Left to right: Knife switch in relation to the panel, Knife switch in shack position,
Common grounding point, large grounding strip.




4 comments:

  1. Hallo Bas, allereerst mijn complimenten voor de professionele aanpak en uitvoering van je totale aardingssysteem. Geweldig hoe goed je dit hebt bedacht en hebt uitgevoerd. Aarding is altijd een hekel item. Iedereen heeft vaak daar zijn eigen mening over, gebaseerd op ervaring en wat men geleerd heeft. Naar mijn mening heb jij het goed aangepakt. 73 Hans, PE1BVQ

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    1. Hallo Hans, ach ik weet het niet. Ik heb het zo aangepakt dat ik niet de problemen uit het verleden weer naar boven haal. Een aardlus is makkelijk gemaakt en maakt de boel alleen maar erger. Normaal is denk ik een aardsysteem helemaal niet nodig in de shack tenzij je een end-fed antenne gebruikt als basisantenne. Ik hoop dat het helpt. Tot nu toe geen RFI verschijnselen vernomen. Maar heb ook nog niet uitgebreid kunnen testen. 73, Bas

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    2. Nou een EndFed als basisantenne gebruik ik wel. Mijn enigste aarde in de shack is de randaarde van het huis. Wel heb ik in de coax een Line Isolator annex mantelstroomfilter opgenomen. Heb ik deze er niet tussen, dan is mijn RX-ruisniveau hoger en veroorzaak ik ook soms inspraak bij vermogens boven de ca. 70W HF. Ik zal toch eens mijn aarding nader onderzoeken. 73 Hans, PE1BVQ

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    3. Geen idee of een goede RF aarde bij jou zou helpen. Een kwestie van proberen denk ik. Mijn verstand zegt dat je met een endfed niet zonder kan. Niet dat je SWR beter word maar wel minder kans op rare effecten in de shack. 73, Bas

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Thanks for your comment. Bedankt voor je reactie. 73, Bas