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RF arcing at tower top.

C

ChiefEngineer

Anyone have this issue:

Class A FM (less than 3 kw) has arcing at tower top. In the investigation process it doesn't take much rf to arc at the tower top. Discovered this with a wireless ISP. Burn marks on their equipment and rf arcing that strips paint off the equipment. No lightning. I can watch this from ground. Typical arcs from one foot to the tower.

No vswr noted. CCA transmitter. Not noted in the past. I used to climb to tower top before my blindness became a problem and never had this happen. I have never been impressed with the signal from this station.

At some point the NARDA is not registering a problem and workers have gotten rf burns when they run a long screwdriver out from the tower.

If I have this much rf arcing to ground, how much am I loosing that isn't going to listeners? Any possible causes? This is something I have not seen.

Years ago when there was a question of FM antennas and bays I recall Tom Silliman riding up on a motorcycle (Harley of course) and using a long screwdriver to arcover from each element. In this case the arcover isn't in the area of the antenna buit can be anywhere in the top 2/3 of the tower.

I feel this is indicative of a problem but can't place my finger on it. If it were true rf overload from a less than 3 kw transmitter it should be at the antenna.

Ideas appreciated.
 
I had a tower once with a STL issue...when the wind blew a certain direction, the STL signal would be overwhelmed with crackling and popping. The ground strap that bonded the STL coax to the tower had broken. When the coax strap was fixed, and all STL and FM antenna components that were supposed to be at ground potential were checked for proper bonding to tower steel, the problem went away and has remained away for almost 10 years. I guess that's where I would start.

Of course the tower guys wouldn't get RF burns if the FM were shut down while they were up in it. ;)
 
Is the arcing continuous or just an occasional arc?
Are you seeing arcs from your transmitting antenna to the tower?
Is the tower grounded?
Is it an AM tower with other antennas on the structure?

More information would be helpful.
 
Most FM elements are common with the tower at 1/4 wavelength, regardless of being a hot tower or not. As you indicate, more information is needed.
frankberry said:
Is the arcing continuous or just an occasional arc?
Are you seeing arcs from your transmitting antenna to the tower?
Is the tower grounded?
Is it an AM tower with other antennas on the structure?

More information would be helpful.
Click to expand...
 
ChiefEngineer said:
Anyone have this issue:

Class A FM (less than 3 kw) has arcing at tower top. In the investigation process it doesn't take much rf to arc at the tower top. Discovered this with a wireless ISP. Burn marks on their equipment and rf arcing that strips paint off the equipment. No lightning. I can watch this from ground. Typical arcs from one foot to the tower.
/snip/

I feel this is indicative of a problem but can't place my finger on it. If it were true rf overload from a less than 3 kw transmitter it should be at the antenna.

Ideas appreciated.
Click to expand...
Is this tower anywhere near an AM station? Our tower has a detuning skirt to minimize the effects upon an AM DA down the road, and tower climbers have reported feeling hot spots on the skirt. Is your tower "receiving" energy from some other nearby broadcaster?
 
frankberry said:
Is the arcing continuous or just an occasional arc?
Are you seeing arcs from your transmitting antenna to the tower?
Is the tower grounded?
Is it an AM tower with other antennas on the structure?

More information would be helpful.
Click to expand...

The blue line from anything metal to ground is continual. In that the antenna is not arcing to anything. It is only when a new piece of metal with a new ground potential, ie service guy's screwdriver, is introduced into the field and placed within a foot of the tower. Quite the sight.

This is specifically a new issue. It never did this before. I don't know how new as this is the first antenna work since 1 year ago and at that time we killed rf when the crew with a Narda indicated it was needed. They were well above the arc level when this took place. Top 2/3 of a 200 foot tower.

No arcing from antenna to tower. Tower is grounded and I checked the bonding. This is not an AM site. A single FM at top of the tower, side mounted, with a 2 bay SWR antenna.

The current antenna was in service, removed when a bullet opened a hole in the inner bay and water filled it. This was repaired and the antenna was placed back in service.
 
DTV-Chief said:
ChiefEngineer said:
Anyone have this issue:

Class A FM (less than 3 kw) has arcing at tower top. In the investigation process it doesn't take much rf to arc at the tower top. Discovered this with a wireless ISP. Burn marks on their equipment and rf arcing that strips paint off the equipment. No lightning. I can watch this from ground. Typical arcs from one foot to the tower.
/snip/

I feel this is indicative of a problem but can't place my finger on it. If it were true rf overload from a less than 3 kw transmitter it should be at the antenna.

Ideas appreciated.
Click to expand...
Is this tower anywhere near an AM station? Our tower has a detuning skirt to minimize the effects upon an AM DA down the road, and tower climbers have reported feeling hot spots on the skirt. Is your tower "receiving" energy from some other nearby broadcaster?
Click to expand...

The closest AM is probably WSVX 250 watts at 15 miles. We are out of their pattern well. I was their chief years back. No de tuning skirt. A local municipality built a new water tower about 1000 feet away but I don't see this as an issue.

Verizon installed a new cell site about 1000 feet away but I have never had this experience with any other cell sites.

The antenna (2 bay) is side mounted on the tower and the inner bay has short brackets that connect to the tower leg (42inch face). Standard SWR mounting. As I recall they have muffler type clamps around the inner bay and have a cushion inside where the inner bay rests. Clearly a ground connection however.

Will weathering cause the stainless steel clamps to open and could a ground difference in potential trap rf on the tower? The station is 2700 watts. Verified Bird thruline in the transmission line. The reflected on the CCA output and the Bird are minimal.

CCA transmitter with solid state IPA feeding the tube box. Changed the IPA out a few years back. It is still resident in the transmitter with a hole where the tube was previously for air flow. Stack temps are not higher than normal. Still generates about the same amount of heat as when it had the IPA driver. IPA change out was at least 3 years ago.
 
"The blue line from anything metal to ground is continual. In that the antenna is not arcing to anything. It is only when a new piece of metal with a new ground potential, ie service guy's screwdriver, is introduced into the field and placed within a foot of the tower. Quite the sight."

If the service guy is hanging on the top 2/3 of the tower, he is not at ground potential.
If his screwdriver is arcing to the tower, there is some SERIOUS RF being induced from somewhere.
Not necessarily an AM station.
What high power transmitters are located in the area? Any shortwave stations nearby?
 
I've read through this a couple of time, but must be missing something.

Does the arcing occur only when someone is on the tower near the antenna? Or does it flash from the antenna to the tower all the time?

If it happens all the time, does the arc go away when you turn off the transmitter?

I've had a tower guy tell me that his belt buckle arced to the tower when climbing near a hot, but power-lowered FM antenna.

I don't see the puzzle here.

But having said that. I would suggest you check the grounding kits on the transmission line to the FM antenna to make sure they are in good shape. Same for whatever the antenna is arcing to. A ground differential is about the only thing that would cause this condition. But never, say never. Could be wrong.
 
Like Stan says, "I don't see the puzzle here". If you are close to the antenna, the field is going to be very high. I have seen several instances where things not tied down in the near field of the antenna bays will develop corona, even to the point of being visible on the ground. Its external to the antenna so would not show up on the reflectometer on the transmitter to any real magnatude. I recall seeing a class "A" that they couldn't understand why the wrap-lock conduit tie downs near the antenna were breaking and falling off. There again if I am understanding correctly what you are saying its not uncommon at all. 1/4 wavelength at 100 mc is about 2 1/2 feet.

Good luck and best regards,
w/
 
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