Counterpoise?

[bigdonho]

I have an antenna site that is in wetlands. The ground system has probably corroded away. When the tower was replaced, a new ground system consisting of three radials and 12 ground rods driven around the tower was installed. The impedance on the tower seems to vary with the state of the tide and the amount of rain that has fallen. One suggestion to stabilize the impedance is to use a counterpoise. Any of you engineering types have any thoughts on this?

 

[Kelly A]

I have an antenna site that is in wetlands. The ground system has probably corroded away. When the tower was replaced, a new ground system consisting of three radials and 12 ground rods driven around the tower was installed. The impedance on the tower seems to vary with the state of the tide and the amount of rain that has fallen. One suggestion to stabilize the impedance is to use a counterpoise. Any of you engineering types have any thoughts on this?

If the existing ground radials are above ground, then technically it's already considered a counterpoise ground. I assume this is a single tower ND?
Unless the ground is saturated with salt water, the original ground system is probably still intact. I've seen original ground systems in swampy, marshland that's still intact and functional after sixty years of use.

 

[bigdonho]

Yes, it is a single tower ND. And, yes, the ground is saturated with salt water.

 

[Radio Bob]

In Florida I found a ground system buried in salt water detreated and eventually disconnected from the transmitter about every five years unless buried deep in the muck

 

[Kelly A]

Yes, it is a single tower ND. And, yes, the ground is saturated with salt water.

I would get hold of an FIM-21 and and map, then run some radials out 2km 360 degrees around the site to see what the field looks like now. If you did the calculation and the efficiency appears close to what was licensed, then I wouldn't worry about it. If you see big dips in the pattern that can't be explained by re-radiation from utility lines or freeway overpasses, then you might have something wrong with the ground system in that particular direction.
The other simple tell that your ground system isn't working could be measured as base impedance. Assuming the site was built and tuned for 50 ohms J0 on the transmitter side of the TX line going back to the transmitter building, then your base impedance should be pretty close to 50 J0. If the tower appears as a higher impedance than 5 or 10 (55 or 60) ohms or higher toward the TX, then your ground system could be partically disconnected or missing in some areas.

 

[davideduardo]

If the tower appears as a higher impedance than 5 or 10 (55 or 60) ohms or higher toward the TX, then your ground system could be partically disconnected or missing in some areas.

I saw one case of a station located in salt flats. They had intelligently used a counterpoise ground system to avoid buried copper being devoured. What they did not realize is that gradual tower vibration and wind movement would grind through the galvanization of the section "feet" allowing higher resistance oxidation.

The base impedance was changing between windy days, humid days and non-windy days. At times, the transmitter did not like that at all and it would go off the air. On top of that, it was an AmpliFuzz.

I recommended tower replacement, but the owner did not want that. So the alternative was to run copper up the sides of the tower tied with copper straps. Not my favorite, but it worked.

 

[Kelly A]

I saw one case of a station located in salt flats. They had intelligently used a counterpoise ground system to avoid buried copper being devoured. What they did not realize is that gradual tower vibration and wind movement would grind through the galvanization of the section "feet" allowing higher resistance oxidation.

The base impedance was changing between windy days, humid days and non-windy days. At times, the transmitter did not like that at all and it would go off the air. On top of that, it was an AmpliFuzz.

I recommended tower replacement, but the owner did not want that. So the alternative was to run copper up the sides of the tower tied with copper straps. Not my favorite, but it worked.

Yeah the old Ampliphase line wasn't very tolerant of big impedance swings. When everything was dialed in and tuned up right? Very few transmitters could modulate as well. They were just pretty finicky. A consulting engineer friend of mine once dubbed me the 'Funnyphase Whisperer'.

I did something similar to an old tower in the Seattle area. It was (still is) a three legged self supporter. During a tower inspection for the insurance company, the crew discovered some corrosion right at some of the leg joints, and it looked like some of the paint had burned around those joints. We pre-fabricated some copper jumper straps around each of the leg joints to bond the section above. The other thing I did, was feed all three legs from a common center point feed under the tower. I purchased 20' lengths of 1" copper water pipe attaching each leg to the center where they met the feed point via a bowl insulator. Ended up doing the whole fabrication over several days working on a tall fiberglass step and extension ladders while the station was on the air at full power. I climbed up the tower and suspended the copper pipes on ropes, then did all the attachment from the ladders on the ground. Last I checked, it's all still in use.