## Wednesday, September 17, 2008

### GROUNDING REVISITED - Bob, Week 17

This evening we will discuss one small but important aspect of grounding. Recall from our earlier segment that within your shack you want to have short very low impedance connections from each piece of gear to a single ground point also called a bus. That ground bus should be an extremely low impedance point like a copper or aluminum plate or a copper pipe or rod.

The aspect we are going to discuss is the path from the shack's ground bus to the actual earth ground. For purposes of tonight's discussion, I am going to call this the Main Ground Wire. You will recall that like all ground paths you want the main ground wire to be very low impedance and short. One last review before we get into tonight's discussion is that the ground wire has a small amount of inductance which will shorten the electrical length of the wire when compared to the physical length. And by physical length I simply mean the length as you would measure it with a tape measure. So on to our discussion.

The best place to have your ham shack is in a basement; next is the ground floor. One of the advantages of this is to have a very short run from your shack's ground point to the earth ground. Not everyone has this luxury, of course, and has to have a longer main ground wire. With that comes a possible problem. That problem is a resonant ground wire.

I have to jump ahead on Lee's impedance series a bit to explain this part. I will only briefly touch on impedance matching. After Lee completes his series we will be ready to discuss it in detail. The reason impedance matching at junctions is so important is that when there is a mismatch some of the radio wave going through the wire will be reflected back from the junction. This is what causes standing waves and is the reason for measuring the standing wave ratio (SWR).

In our case the impedance mismatch is between the ground rod and the earth. This cannot be avoided. It sets up the entire length of the main ground wire and the ground rod as a potentially resonant circuit element. In fact, it acts very much like an antenna element. In this case what is driving the ground wire turned antenna element is the current that should be flowing to the earth. Just like when we were designing our quarter wave and half wave antennas, if the length is just right the ground wire will resonate. This will severely reduce the current flowing to earth from the main ground wire and will cause RF voltages to appear at the shack's ground bus. And these RF voltages can cause RF burns to you, the operator.

The actual lengths to be concerned about are odd multiples of a quarter wave of your operating frequency. Because of the inductive shortening the physical length will be a little shorter than a quarter wave.

Here is the General Class test question with the correct answer given:

G4C05 (D)
What might be the problem if you receive an RF burn when touching your equipment
while transmitting on a HF band, assuming the equipment is connected to a ground rod?
A. Flat braid rather than round wire has been used for the ground wire
B. Insulated wire has been used for the ground wire
C. The ground rod is resonant
D. The ground wire is resonant