Single Point Ground

Luis del MOlino · December 2014

Andrew and Howard, thanks for your comments.

Anyhow I wish to precise that you cannot have a common RF tension in any way in a copper bar connnecting all devices in a shack because this is a CC concept. Here in RF It's not a question of Ohm's Law but a problem of length. Remember RF tension changes with every meter on a conductor in HF, and changes in every centimeter in VHF+.

Lets say you have a table with 5 feet (1,5 meters) and a copper bar connecting all devices and a very short earht connection just under your own feet in the shack with 3 feets cable (almost 1 meter, the height of the table). That makes a total of 2,5 meters. This is a 1/4 of wave lenght on the 10 meters band. If you have a very good ground under your own feet with 0 V RF, you'll have a maximum RF tension at the other end of your fantastic common copper ground bar. And this high RF tension is just at your transceiver ground connection, and you could be in troubles in TX in 10 meters if you do not have placed a balun at your antenna to choke RF outside the coaxial cable.

The second precision is about the "common" mode current. Commonn to what? This "common" concept applies very well to 2 parallel cables that could carry diferential currents in opposite directions or common currentes circulating in the same sense. Or both at the same time.

But in a coaxial cable, if you have a current outside the coaxial cable, you do not have this "common" RF current circulating at the same time and the same direction through the inner conductor of the coaxial. So, they do not carry any currents in common. Only the diferential current that carries your RF to the antenna and a posible outside current if no balun is at the antenna connnection. So, we should better speack of "outside coaxial current" and we should forget that absurd word "common" to describe this fenomen so peculiar in coaxial cables, that seems to have three conductors instead ot the phisycal two it has.

Have a nice Christmas
73 Luis EA3OG

k3Tim · December 2014

Agree Luis that coax is 3 conductors at RF,

1 - center conductor
2 - inner shield
3 - outer shield

due to skin effect. Speaking of skin effect one could use copper foil instead of the water piping.

Back in the 90's I used a 1//2 inch / 12mm water pipe to form ground bus as being described here Was able to run a 1kw RTTY station w/o problems. Ferrite beads helped but some equipment had to have bypass caps installed on external leads.

The Flex 6500 and Flex Control seems bullet proof to RF as it is not having any problems even though the antenna setup here is not so good.
Can't say the same for the keyboard as its going bonkers when Xmt and ferrites helped but not cured.

Merry Christmas to all,

Tim