RF Grounding the Shack – Part 1

RF Grounding the Shack – Part 1

July 17, 2018 3 By Andrew

So now I am an Intermediate Licence holder, or will be as soon as the letter arrives, I need to think about my shack RF earthing even more. Moving from 10 watts to 50 watts with a poor earth is going to cause more RFI (RF Interference) and seeing as the bed side lights already turn on when I try and tune the antenna up at anything more than 5 watts, I must address the issue.

The main problem I have is that my shack, is on the first floor of my house. With no real way of getting down on the ground floor I need an alternative. Strapping an earth to the radiator is a big no no, so that's out...

So time for googling... (I've come to realise that being an adult is basically googling how to do things...)

BTW, I am NOT connecting the RF earth to a plug... But I bet you opened this to write in the comments. NOO DONT DO IT.... so it worked in much as it made you read this!

I've come across two options to try...

Option 1:  The RF Suppressor Ground System

This is the modern version of an ingenious device developed and introduced by several hams in recent years, notably by William Chesney/N8SA (See http://www.hamuniverse.com/grounding.html) who published the article in 2003. This grounding system addresses both the electrical ground and RF grounding requirements in Ham radio. The device is intended for long grounding wires. The grounding device utilizes a coaxial line where the ground wire is enclosed by a shield, such as RG-8 transmission line, to prevent the buildup of high voltage standing wave near the station equipment. This ground line is not length sensitive and can be used at any length without concern. It will keep out RF away from the shack. The wiring setup of this practical grounding system is shown on the right

 

Now the information I have found states that the coax can be any length. I simply short the centre and the braid at one end outside and connect to the earthing rod. The other end in the shack is shorted using a capacitor. Of course the capacitor value is selectable depending on the lowest operating frequency band and length of Coax. The correct value is selected until RF disappears in the shack (at the lowest band). Or, when your lips doesn’t get to be burned or electrocuted (when touching the metallic mic case) as you speak or transmit. However, YOU MUST USE A HIGH VOLTAGE CAPACITOR RATING, about 1KV minimum, but the higher the better. Otherwise, ZAPPP!!!, this capacitor will explode if a surge of high voltage standing wave will develop instantaneously at or above 500 volts at this terminal.

So this is one option....

 

Option 2: Use of an artificial ground or tuned counterpoise

A tuner for a counterpoise or other un-grounded grounds.

This neat little box was designed specifically for use with a counterpoise (i.e., a wire or wires substituted for a ground system).  There are commercially available options such as the MFJ-931 which tunes the counterpoise to series resonance which in turn, translates into improved ground efficiency and improved antenna performance.

The usefulness of the MFJ-931 is not limited to resonating a counterpoise. It can be a great help to those of you with radio shacks on the second or higher floors of a building. For you, and even many ground floor installations, a very short ground wire is not possible. The MFJ-931 helps solve the problem of long ground runs by tuning your ground system to series-resonance. The impedance of a series resonant tuned circuit drops to a very low value when resonance is achieved. In effect, all the reactance is 'tuned-out' of your ground system.

 

Ignoring the left hand part of the circuit which is a meter. This is basically an inductor and capacitor in series. 

OK, but I have a home made Tuner I picked up for £10 at the Ipswich rally.... That has a capacitor and rollarcoaster inductor inside... We can try that!

As this option seems to allow for tuning or de-tuning of the grounding system, I am going to try option 2 first. 

The theory as I understand it is that normally a long wire has an impedance to the RF and therefore a high voltage can form so your ground is not at ground causing issues. The tuner/detuner causes the ground impedance to drop at the resonant frequency and thus presenting itself as a very short wire with little voltage present at the radio end....

I am going to test this when I have more time and will report back on the results. Watch out for part 2 coming soon...