Common RF / AC ground?

Status
Not open for further replies.

wpri268

Newbie
Joined
Jul 15, 2014
Messages
1
Location
Central MA
I question if I have a ground loop problem between the AC ground and a vertical antenna radial system that may be causing some RFI.

My AC safety ground consists of two 8' rods one vertical, one horizontal driven into the earth. The horizontal one is several feet below grade and both are nearby the breaker panel.

The radial system for the transmit antenna is made of thirty, 60ft long #12 buried wires fanned out in a semicircle.

My receive antenna is two ground independent pennant antennas galvanicaly isolated from the coax feedlines via transformers. The coax outer braid however is common with the radial system at its center where it meets the vertical antenna. Coax from both antenna systems runs about 90ft on top of the ground to receiver and transmitter.

There is no direct connection between the radial system and AC ground except through the coax outer braid and grounded chassis of receiver and transmitter. This seems to me to create a large ground loop. Should I connect the radial system to AC ground directly? Otherwise, should I isolate the RF and AC grounds completely even though this is not recommended for lightning protection?
 

WA0CBW

Member
Joined
Dec 8, 2011
Messages
1,448
Location
Shawnee Kansas (Kansas City)
The NEC (National Electrical Code) states that all grounds must be connected together. So any external grounds such as a ground rod outside where the coax enters the building or your RF ground consisting of many radial wires should be connected to the SINGLE POINT GROUND that is your main electrical ground. They also need to be "home runs" to that single point ground. Many times a ground loop can be caused by "daisy chaining" grounds from one to the next. This affects the likelihood of a "potential difference" between the beginning and end ground point increasing the possibility of an arc or flash over between these points. The result of daisy chaining would not be RFI.
As for the horizontal and vertical safety ground rods, a vertical ground rod has a "sphere of influence" that is equal to its length. So a vertical 8-foot ground rod has an influence of an 8-foot diameter circle. Ground rods are then typically placed no closer than 16 feet apart. Placing them closer has little effect on the total effectiveness of the ground.
See Sections 800, 810 and 820 of the NEC for additional information about grounding.
BB
 

majoco

Member
Joined
Dec 25, 2008
Messages
3,791
Location
New Zealand
The NEC (National Electrical Code) states that all grounds must be connected together.
Electrical grounds - yes. But what if my antenna farm is not connected to the chassis of any of my radios due to baluns and other isolating devices to keep electrical noise out of my inputs. Then I reserve the right to have an RF ground system, counterpoise or whatever that is not connected to the house electrical ground.
 

WA0CBW

Member
Joined
Dec 8, 2011
Messages
1,448
Location
Shawnee Kansas (Kansas City)
As one who has installed many cellular and public safety communications RF sitesfor 35 years I can say that the NEC considers a ground anything that is connected to ground whether you call it an RF ground or a counterpoise. I would suggest that your effort to keep electrical noise out is what is causing the noise due to multiple grounds at different potentials. The reason for the single point connecting ALL grounds is to prevent a potential difference between the connections. If there is a potential difference then current flows and can cause ground loops and noise and in the case of a nearby strike flash over between the two dissimilar grounds. When using a ground radial system common practice is to extend one of the radial wires so that it connects to your single point ground. You can find additional information concerning this in any of the current grounding standards such as Motorola's R56 manual, the ARRL's three part series on grounding and the appropriate NEC sections.
 
Status
Not open for further replies.
Top