Bonding Tower to Service Common Point Ground
I have a question I thought I knew the answer too, but the more I researched it the more unsure. I am about to place my antenna on a new tower, this is what I am going to do, please correct me if you think its wrong. The tower is only 30 ft, with 8 ft. of mast beyond the top. It's on a cliff line so effectively its much taller. Anyway, I have 3 ground rods for the tower 1 per leg. I am grounding the coax hardline shield at the top and bottom of the tower with the bracket and the lower one has a lightning arrestor attached as well. Then my feed line coax runs 250 ft. back to my shack. This is the new one on me, as my antennas have always been close to the shack ( 15 ft. ). My shack already has , I think, a good ground system I have been using for years. I have a service entrance bus with lightning arrestors grounded to the copper plate bonded together with common point ground. This is bonded with the shack buss bar etc,with all the shack equipment. I have the outside buss bar also bonded to the AC house ground as recommended, and I have an array of ground rods for this system all bonded. Now for my question, my grounding knowledge says I need to bond the tower back to my common ground buss bar. It's about a 300 ft run. It seams illogical to do this even though I believe that is the book way. I have spoke with a few hams with lightning hits and they say with a good tower ground and coax ground, the rf from the lightning has always bled off before it got back to the shack and didn't damage any equipment. They stated this in reference to a long coax run from the tower to the shack as in my similar situation. It seams if I provide a nice low resistance ground wire from my shack to the tower it would defeat the bleeding of RF and may facilitate more energy to enter my shack.. Confused ? Thanks for any help.
Last edited by WH2HAO; 09-10-2012 at 06:45 AM.
I recommend that you bond the tower. Good idea..
Not bonding the tower is called a OPEN LOOP.
Lightning looks for the shortest path to ground.
Most lightning strikes do not take place at the tower, but through the power lines.
The electric company GRID is massive, your antenna is minscule in the grand scheme of things.
But your antenna system is a electrostatically charged device.
It basically acts like a lightning rod.
I don't know who advised you to put your antenna so far away from your shack , or how you figured there would be some type of gain by placing it closer to the cliff then the shack.
Electric - radio waves can travel faster through air then through wire and the difference of a couple of hundred feet in length of wire vs the difference between the antenna being at the edge of the cliff vs being next to your house - negates any gains associated by the increased take off angle.
Move the tower next to the house and add 30 more feet of tower and your new tower will out talk and listen your old tower...
Thanks for the reply. Sorry for some confusion , maybe I was not clear about the tower. I put the new tower up the only place it could go, I didn't purposely distance it from my shack. I did not have a tower prior to this. I live on a cliff line, so the tower location was picked where it least looked like it would fall down the cliff. Also, in my area, there are no power lines as everything is buried. The tower is clearly the tallest metal structure in the area, So do you think I still need to bond the tower. Thanks.
Wow. Thats a long coax run! I hope you don't plan to do any VHF/UHF with that tower!
As pointed out, They should be bonded together. For such a long distance, A good conductor can be some lower cost soft (Roll type) copper tubing of say 1/4 inch or so. The normal rule of thumb is to install ground rods along the ground about twice the distance apart as the depth. (8 foot deep rods spaced about 16 or so feet apart.) But in such a long run, I would just add a few along the path, spaced further apart.......
Note that the normal location for a lightning arrestor is at the entrance point to your building, Not at the tower base. At your "Single point ground panel" where the coax enters.
For some tips on how to do some of this on a low budget:
(Give that site plenty of time to load)
The amount of current flowing towards the house depends on the length of the wire from the tower to the house and the amount of grounding at the tower. Adding an additional ground wire will help reduce that current by draining off some of it to the soil by direct contact between the wire and the soil. The additional ground wire running next to the coax lines won't significantly change the inductance of the path between the tower and the house, and that is what determines the impedance between the tower and the house. (Impedance of this path is determined by its length, which determines the inductance. DC resistance is insignificantly small by comparison.).
Note: Increasing the inductance of this path by coiling the wires, as sometimes recommended, isn't a good solution. The voltage developed across the coils can be enough to cause arcing across the coils.
With poor grounding at the tower, there could be enough current flowing thru the coax lines to damage those lines, and in that case an additional ground wire could save the coax and control lines. A way of reducing that current is to add more grounding at the tower. The amount of grounding necessary at the tower (ground rods) is difficult to estimate. The soil conductivity is a big factor. Many people recommend 3 rods, as you have. I usually recommend 9. That was based on some ballpark calculations I made. Commercial sites nearly always have more than 3.
Although the maximum current from a strike happens in the microsecond range, some strikes have large amounts of low frequency energy. There have been strikes measured that produce waveforms with tails that have 200-800 amps for up to 1 sec. Also strikes have been measured that produce 10,000 amps for 10 ms. These large low frequency waveforms aren't common, and I don't have numbers for what a typical strike produces. If you get a strike that produces one of these waveforms, the impedance of the path between the tower and the house is much less. The impedance produced by that long wire isn't enough to reduce the current to a safe level. That low frequency energy can melt wires that don't have a lot of current capacity. Certainly a coax line couldn't handle this, so an additional ground wire could help. A better way is probably to add more grounding at the tower. Most of that current needs to get dumped to ground at the tower.
So I gave you enough information to make your own decision. There is always a certain degree of risk and uncertainty with grounding systems, and especially with making recommendations for a system you can't see. If you want to know what I would do, I would add more ground rods at the tower and forget the additional ground wire to the house.
You said that your coax lines are 250 ft from the tower to your shack and that the ground wire from the tower to your ground buss is 300 ft (if you add it). That doesn't seem right, and there may be another problem here that could be uncovered with more explanation or a diagram of your system.
Thank you Jerry,
Very nicely explained, I will take your advise. The 250 ft coax vs. 300 ft ground wire was an error. They would obviously both be 250 ft. I originally measured 300 ft, and did not change it to 250, the final coax length run. I will ground the tower to the max !!
Well do not be confused, there is no question about it all electrical codes and radio tower installation/engineering practices all require you to bond the tower electrode system to the shack and AC service ground electrode system together to form a common Ground Electrode System (GES). It is NOT an option, it is mandatory. Refer to NEC 810.21 (J). Anyone who tells you differently does not know what they are talking about and could get you killed.
If you were to fail to do that and if the tower is struck all the equalizing current between the tower and your shack will flow on the shield of your coax. You do not want that to happen. By bonding the Tower GES to your Shack/House GES will shunt most of the equalizing current around your coax shield and bleed it off to earth.
Last edited by KF5LJW; 09-12-2012 at 02:55 PM.
-It was important enough to run coax 250ft from the antenna to the shack
-Then it''s important enough to run a bonding wire from the tower ground to the shack ground.
Electrons don't care about convenience or "priorities".
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Yes that is true. I should have cautioned you about the potential problems. If you ever disconnect the coax cables between the tower and the shack entrance panel there could be a voltage present at this disconnect point, either from AC leakage or a lot of voltage present if the tower or AC lines get struck. This would not be safe. It is also not to code. The code requires a separate ground wire other than the coax shields. If its going to be inspected, you will need that wire. Even if it is not to be inspected, you have to recognize this potential problem and avoid it by never disconnecting the coax lines between the tower and the entrance panel. You should never eliminate this ground wire if the run to the shack is short, even if you don't care about complying with the code. Violating the code is always risky because of the legality, even when you are sure that what you are doing poses no safety risk. It's also risky when someone else is messing with your system and doesn't know what you did.
Originally Posted by KF5LJW
That is also true, and is the reason I suggested more grounding at the tower if you omit this separate ground wire. (Actually more grounding at the tower is a good idea whether you have this ground wire or not.) Current will split between the ground rods and the impedance of the wires going to the shack. The impedance of that long wire to the shack at 1 MHz should be a couple of orders of magnitude greater than the impedance of a good ground system, so most of the current will go into the ground. A poorly grounded tower will likely blow out the coax lines. Dereck's statement about the ground wire reducing the amount of current flowing toward the house is also true if that ground wire is buried in the dirt.
Originally Posted by KF5LJW