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1-5 of 5 messages
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Grounding system design questions
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by KC9GGH on November 27, 2005
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Windom antenna at a 30' height. It will either be a Carolina Windom or a Buxcomm Windom.
Use 1/2" or 3/4" conduit strapped to a tree as a mast to support the balun and coax feed line. Antenna will be supported at 3 other points by ropes hanging from trees using insulators. Will provide for flexing in wind.
Plan on using two I.C.E. lightning arrestors with single point grounding on the coax 1)at the base of the tree and 2)at the point where the coax leaves the ground and goes up the side of the house to the attic and then to second floor ham shack . Both devices will be tied into my home's ground system. There will be 50' buried cable in between the I.C.E. devices.
Ground conduit to same point that the coax is grounded at the base of the tree.
Installing whole house lightning suppressor at the breaker panel.
Questions
1. Is conduit best for mast? I was considering treated wood as a mast but I read that only conductive towers or metal poles should be used for mounting antennas.
2. How much separation should there be between the conduit and the coax feed line?
3. Should ground for coax be connected to a rod different from the coax? Would still be tied in to system for single point grounding!
4. I was reading on the Polyphaser website that the coax should be grounded at the top of the tower (I assume my conduit qualifies as a tower) and at the base of the tower. I had not planned on grounding to the top, is this necessary?
5. Do I need to ground equipment in the shack or is the house ground sufficient and protected by the whole house unit?
6. Am I overlooking anything?
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RE: Grounding system design questions
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by K5LXP on November 27, 2005
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1. Is conduit best for mast?
No, not really, but it's better than wood, especially for that length.
> I read that only conductive towers or metal poles
> should be used for mounting antennas.
What are trees for then?
2. How much separation should there be between the conduit and the coax feed line?
Pretty sure for a windom the coax is serving as a transformer before the balun. I would keep it as far away as practical, a foot would be a good start.
3. Should ground for coax be connected to a rod different from the coax?
I don't understand the question. Ground rods should be as close as practicable to their associated arrestors.
4....the coax should be grounded at the top of the tower ...I had not planned on grounding to the top, is this necessary?
I don't think most of what you are doing is necessary. One arrestor at the entrance of the building, connected to it's own ground rod would be more than enough for what you're trying to do.
5. Do I need to ground equipment in the shack or is the house ground sufficient and protected by the whole house unit?
I don't believe in grounding equipment in the shack.
6. Am I overlooking anything?
If you're going to go through all of this trouble for a windom, make it worth your while and get it up higher.
Mark K5LXP
Albuquerque, NM
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RE: Grounding system design questions
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by WB6BYU on November 27, 2005
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Is conduit best for mast?
Probably not, depending on what forces will be acting on
it. The problem is that electrical conduit is DESIGNED
to be bent. It is relatively heavy for its size compared
to aluminum tubing and some other options. A TV push-up
mast might be a better choice. But, if it is already in
place, go ahead and use it.
There is no requirement that an antenna support be metal,
though a metal mast does reduce the amount of lightening
current that flows through the coax in case of a direct
hit. In fact, if the rest of the antenna is supported
by trees, there is no reason that the feedpoint can't
hang from one also. Your coax may evaporate in a lightening
strike, but you'll probably end up having to replace it
anyway.
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RE: Grounding system design questions
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by K4SAV on November 27, 2005
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As I see it you have two choices: 1) ignore the grounding issue, or 2)add a grounding system to take care of a lightning strike.
For a dipole, I don't know of anyone who has ever done number 2. That said, I do know of at some who wish they had. Most people just put up a dipole and don't ever use any grounding unless the grounding is already in place for a tower system. Probability of a direct lightning hit on your antenna is a function of where you live, the height of your antenna, and the height of nearby objects. For most folks, this is a low probability. However, having spent 38 years in the lightning capital of the US, I have seen and been a part of a lot of lightning events, and have seen some really weird things happen. The probability of a hit in that location was not low.
I had a strike go right between 4 big 55 ft pine trees (which also held a 20M loop) leaving these untouched and hit a puny 10 ft high tree only 15 ft away.
I also had a low dipole (35 ft) surrounded by 55 ft trees. It had a single ground rod. I had a direct hit on the wire. The lightning traveled down the wire to where the ropes were attached, blasted the ropes loose, turned the copperweld into a wet noodle, vaporized the feedline (no conductor left at all, only the plastic left), ignored the ground rod, traveled down the ionized trail where the feedline was until it found the corner of my house, and blew a big hole. Most of the charge went into my concrete pad. There were also small fingers of lightning feeders which hit other things in the immediate area, such as outside spigots and chimney. Needless to say, everything in the house that was electrical was toast.
So what do you do about this situation? Add a good ground system? Move to a better location? Actually I did both, but that doesn't work for everyone. Protecting a dipole from a lightning strike is very difficult because there is no way to guarantee where the current is going to go when the feedline disappears. Also a good ground system is a lot of trouble. It's about the same as for a tower, lots of ground rods, and for most folks just doesn't seem to be worthwhile. I may agree with them if the dipole is near your house. (A dipole supported by a tower is a different situation.)
You pays your money or you takes your chances.
Oh, and your insurance (depending on where you live) may only be good for two lightning claims before you find yourself without insurance.
Jerry, K4SAV
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RE: Grounding system design questions
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by AA4PB on November 28, 2005
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The most important thing is to ground the coax shield near ground level before it enters the house.
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