Examples of Work on Electronics
Amateur Radio Station -- K1TMA
In the last month’s issue we were discussing the Bus Bar Grounding System and its relationship to the System Ground. Then, we started to discuss the purpose of the Bulkhead Grounding System.
Bulkhead Grounding System -
In any effective radio communication station there must be filters -- such low pass filters and band pass filters -- and surge protectors, RF-lightning arrestors, AC-line lightning arrestors, line-noise filters and DC and AC power distribution panels. We would ordinarily place these accessory devices on a Bulkhead Ground; then connect it to the Bar Bus Ground which goes to the System Ground.
The Bulkhead Ground is made up of all of these devices and components on a piece of metal usually not much more than 12 to 36 inches long by 12 to 14 inches wide and 1/8 of an inch in thickness. It can be made of copper or brass but usually aluminum.
Usually, we place the Bulkhead Ground on a
piece of wood, such as a 1/4 inch AC plywood, a little bigger than the
Bulkhead Ground, either on the back of the radio desk or immediately on
the side of a wall in back of the radio desk. From there, we go outside
to the System Ground. Everything else -- Foundation Ground from the tower
and all of the guy wire
Before We Go Any Further -
It has been brought to my attention, in response to my last two articles on grounding systems, people have a problem understanding the lost of a path to ground, in the grounding system, and the relationship between receiving noise and ground.
So I have decided to stop long enough to focus
more attention on these two important subjects.
Lost of RF Path to Ground -
As I stated in Part II, when the ground cable is so long, or long enough to resonate with the operating frequency of the station, it will take on the characteristics of an “isolation transformer.” It means the impedance will become so great at resonance (impedance means resistance in an AC circuit), the RF circuit will not have a path to ground. Think of this impedance as an enormous “lump” of resistance between earth ground and the station. The RF circuit will then “back up” and start searching for a path to ground through your radio equipment, the electrical ground, microphone, headphones, etc. You will feel a “tickling sensation” in everything made of metal when you touch the metal parts of your microphone, your rig, etc., and your meters will move or jump erratically with your touch.
Removing that Impedance or “lump” -
We have two common ways of removing that impedance from the ground cable: First, we can install a resonate circuit, made up of a variable capacitor and variable inductor, in series or parallel to ground and, through a manipulation of the variable capacitor and inductor, cancel out the impedance.
Secondly, another common method is to install ground radials around the grounding rod(s) with a physical length (resonance) to cancel out the resonance of the ground cable between the Bus Bar Ground and the System Ground (location of the grounding rod(s) immediately to the station).
When we cancel out the resonance the high impedance (AC resistance or “lump“) disappears and then we restore the RF path to ground.
Without the costly electronic instruments, it takes time and patience to experiment with both methods.