Tower & Grounding

[tibadoex]

OK. Have a question for the tower people out there in the forum.

I have a 30' tower I will be installing shortly for my ham radio. Will be adding a 5' to 7' mast on top and finally antenna. Ground level to top antenna is approximately 50'.

Here are some of my grounding questions:

1) Will be driving a 8' ground rod for each of the three tower legs. Is it best for them to be in the hole and pour concrete around them or do you get a better gnd by driving rods directly in the earth as close to the concrete as possible?
2) What gauge wire for connecting gnd rod to each leg? 4 or 6 gauge maybe?
3) Should I run a gnd wire up tower to the bottom of mast pole also?
4) When I run gnd wire to main house ground rod (60 ft) does it matter if wire is buried or ok to leave top of the ground dirt?

Hope this makes sense.

 

[prcguy]

Its not a good idea to tell you what to do but I can tell you what has been done in the past. You should follow NEC at the minimum and seek professional advice going forward.

In the systems I've been associated with which were designed by professionals, a 10ft ground rod was used at each tower leg separate from the foundation and the routing of the cables from the tower legs to the ground rods is critical with no sharp bends allowed. The rebar in the tower foundation is sometimes bonded to the ground system. There is also a ring of wire buried around the base of the tower as the main attachment point for grounding and all buried ground wires were 4ft below grade. The wire size in all the installations I've dealt with is bare stranded 500MCM cable that was cadwelded at all junctions. In some cases where the tower is a distance from the building the buried ground cable extends outward from each tower leg for upwards of 60ft underground with a 10ft ground rod about every 20ft.

The mast is generally not grounded but on every satellite dish there are air terminals or lightning rods with their own cables held away from the satellite dish on insulators going directly to the ground ring. This is to help shunt massive currents around movable parts like motors and bearings and to minimize currents induced into the wiring back to the building.

There is a lot to consider and every installation is different, so some things I've mentioned might be good for you and some may not and you might need more of something else. That's why you should seek professional advice from someone who can see and comment on your specific install.

 

[tibadoex]

Thanks for the reply. I probably should have reworded my question a little when I first posted.

I have already checked over the NEC & local inspector for my area. In process of getting info from the other professional hams in my area that have done this and their experience. So I'd figure I would throw it out the the forum for personal experience.

So for those who put up towers, did the go by the minimum recommended by the codes or take it a step further. Just curious.

Definitely would not go blindly into this and follow anyone's advice without research and codes as a basis.

Thanks again.

 

[prcguy]

NEC is for human safety from electrocution and not necessarily for lightning protection. You can read the Motorola R56 manual and Polyphaser used to have a good white paper on the subject, but every situation is different and you might need more of something that your not aware of.

Thanks for the reply. I probably should have reworded my question a little when I first posted.

I have already checked over the NEC & local inspector for my area. In process of getting info from the other professional hams in my area that have done this and their experience. So I'd figure I would throw it out the the forum for personal experience.

So for those who put up towers, did the go by the minimum recommended by the codes or take it a step further. Just curious.

Definitely would not go blindly into this and follow anyone's advice without research and codes as a basis.

Thanks again.

 

[JerryX]

Flat copper strip 3-4" wide is better than round wire as it has lower inductance.

 

[prcguy]

Its common to use wide copper sheet strips to bond lightning arrestors and Heilax cables to the ground rod system and it is low inductance, but you would generally not use it for grounding a mast or tower to a ground rod or for direct burial. The only thing I have seen used in my career for bonding towers, steel building frames, satellite antennas, etc, to ground rods and for establishing a ground ring in a commercial environment is 500MCM bare stranded copper wire. Its 13/16 diameter and about $7/ft and you would want to cadweld all connections.

Flat copper strip 3-4" wide is better than round wire as it has lower inductance.

 

[JerryX]

W8JI uses copper strips to bond his towers to ground rods, as do I.

Antenna Grounds

 

[scanmanmi]

Don't forget about putting a lightning arrestor in the coax line.

 

[kc5uta]

I've heard anecdotally that depending on the earth composition it's better to not have the rods encased in cement. But when you look up "Ufer ground", it has a different perspective. It seems that buried cement likes to maintain a certain moisture content. It stays a "bit wet" so to speak, even after it's well cured, makes for a good grounding system. That said, a good hefty grounding cable is preferable over a skinny one IMHO. As for the rest, I'm not sure. BUT, when I put up my replacement tower, I am going to ground the HE** out of it, including buried copper wire fanning out away from the tower, the guys will be insulated from the tower. I am fond of shunt feeding a 65 foot tower for 160, and 80, it needs a real good ground system.

 

[prcguy]

An Ufer ground using the tower foundation is ok as a supplement to a larger ground system, but you don't want to use it as your only tower ground unless its substantially large. There are many cases of lightning blowing a tower foundation apart due to the massive current flowing through the rebar super heating things and making high pressure steam that can't escape when its the only ground available.

Grounding for RF is different than grounding for lightning. A bunch of short radials is better than a few long ones for an RF counterpoise but you can use very large size conductors, possibly 6ga for three or four of the ground radials and drive in 10ft ground rods every 20ft or so along the length of those ground radials to help dissipate a lightning strike, so the radials can do double duty.

I've heard anecdotally that depending on the earth composition it's better to not have the rods encased in cement. But when you look up "Ufer ground", it has a different perspective. It seems that buried cement likes to maintain a certain moisture content. It stays a "bit wet" so to speak, even after it's well cured, makes for a good grounding system. That said, a good hefty grounding cable is preferable over a skinny one IMHO. As for the rest, I'm not sure. BUT, when I put up my replacement tower, I am going to ground the HE** out of it, including buried copper wire fanning out away from the tower, the guys will be insulated from the tower. I am fond of shunt feeding a 65 foot tower for 160, and 80, it needs a real good ground system.

 

[kc5uta]

yup, heard the same about the exploding cement thing during a strike...yikes