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Artificial Grounding

Mar 20th, 23:45

tedsiok

Joined: Dec 29th 2013, 10:59
Total Topics: 0
Total Posts: 0
Ted, KB1WCV here. My shack is, out of necessity, on the 2nd floor. My ground will be 19’ straight down to the ground rod from the ground buss. I have a Kenwood TS-940 with a 250 Watt PEP max output. I would like to have a 600 Watt amp eventually. I don’t care to have anything over that wattage. It seems that the artificial grounding units, such as the MFJ-934, handle only up to 300 watts. Why is this? What am I to do when I get my 600 watt amp? Any advice or experience you have to share would be appreciated. Thank you.
Mar 21st, 13:15

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

The purpose of an artificial ground is to put as much current into a counterpoise or radial wire, in an attempt to minimize the current on the station chassis and ground return wires--you don't want a lot of common mode current on the mic ground, for instance. But, if that wire is high in the air, you can expect it to radiate. Thus, for RF safety reasons, you only want to do this at low power levels. But, it can work well for QRP or low power stations who don't have the resources for more a better station.

In the old days, a short ground wire was an inexpensive way to decouple feedlines from common mode shield currents on the popular 40/80 meter bands. Choke baluns weren't practical on these bands, and ferrite baluns weren't readily available.

What you should do today is to either use balanced antennas, such as dipoles with a proper balun, or verticals with adequate radial systems to decouple the feedline. If necessary, you can decouple the feedline by putting additional radials at the single point grounding entrance to the station. If you run coax to single point ground with good RF grounding from the antenna, and then back up to the shack, you should have minimal issues with a 2nd floor station. I'd suggest locating the single point ground where it is line of sight to the service entrance, to minimize the hazard from lightning.

It isn't obvious, but grounded grid amplifiers can have severe common mode issues if the input isn't properly tuned--the input is in series with the output circuit. Trying to fix an input tuning issue with an external tuner can result in unusually large current pulses on the connecting cable between the exciter and amplifier.

Zack Lau W1VT
ARRL Senior Lab Engineer


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