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Author Topic: EMT Conduit EMI Protection for Rotor Cable on Tower vs. Additional Shunt  (Read 1547 times)
KI4RAX
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Posts: 3




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« on: December 25, 2012, 08:48:00 AM »

I just put up a 100' tower and am working on the grounding system.  My shack is 220' from the tower on the far side of my house where I'm establishing a single point ground along with Polyphaser coax and rotor cable surge shunt type protectors, etc.  According to my reading of Polyphaser, being over 80 feet away from the tower results in significant impedance / reduction of currents aimed at my station.   I've read that surge protectors should be placed at the single point ground; but, I've also read that they should be as close as possible to the base of the tower.  In addition, Polyphaser states that having an additional shunt protector for my rotor cable at the rotor close to the top of the tower is better. In my case which should it be? at the shack, at the bottom of the tower and / or at the top of the tower or some combination?  Note: I'm also putting a lightning rod on my mast as well as a heavy conductor from the mast to the tower to reduce the impulse at the rotor.

The coaxial cables are to be grounded at the top and bottom of the tower as well as at my single point ground entry panel.  Which got me thinking about the rotor cables...and saving money by not having to buy an additional shunt for the rotor at the top of the tower and doing a better job protecting them.  NOTE: It turns out that the cost of the EMT, connectors, etc. is about the same as the additional shunt at the top of the tower. But the EMT conduit may significantly reduce EMI for the rotor cable and keep them from being degraded.  My idea was to Faraday protect the rotor cable wires as they exit the rotor by covering the 8-pin connector panel with appropriate metal  & metal tape (perhaps AL shell and tape, not sure what would be best yet) and then place the cable in flexible conduit and then standard EMT conduit.  I would then ground the outside of the EMT at the top and bottom of the tower just like a coaxial cable.  At the base of the tower, I planned to run my coaxial cables and the rotor cable EMT into a larger EMT (grounded at the same point at the base of the tower).  About 90' from the tower, I planned to transition to plastic (grounding my coaxs and Controller EMT to the end of the large EMT carrying all) conduit.   From there, I would bring coaxs and Controller EMT to the shack grounding all to the single point ground plate, and pass them through the aforementioned shunt protectors.

Any guidance from someone who knows would be greatly appreciated.
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N4CR
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Posts: 1650




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« Reply #1 on: December 26, 2012, 07:35:17 AM »

Sounds to me like it would work, but a few turns through a ferrite ring at the top and bottom would work great and require less shielding of connections. And it's a proven solution.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
KC4MOP
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Posts: 729




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« Reply #2 on: December 27, 2012, 04:39:21 AM »

metal conduit would be another maintenance headache.
You are on target for the protection at top and bottom of the tower and entry into the shack. The way the pros do it.
I'm assuming you are focusing on protecting the rotor motor or the controller in the shack?
I have a rotor cable protector at the base of my tower and no problems from this past Summer's close lightning strike that wiped out a lot of internet stuff and the TV and two Ethernet ports on two computers.
Me thinks the strike came in through the ungrounded cable TV (internet) and Sat dishes.
Fred
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KI4RAX
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« Reply #3 on: December 27, 2012, 08:14:59 PM »

Fred,

Sorry to hear about your lightning strike!

I'm protecting the controller in my shack and was hoping to protect the rotor at the top of the tower enclosing all in aluminum--and counting on most of the inductive pulse to skirt the rotor by traveling on the outside of the tower and the remaining high voltages to stay on the skin of my rotor conductors and metal conduit.  If this is unlikely, I will need to buy another 8 pin shunt to protect the rotor at the top of the tower.

I've got a Polyphaser 8 pin shunt at the shack single point ground.  I'm wondering if another shunt at the foot of the tower is overkill.  Polyphaser 8 pin rotor shunts cost about $128 / unit which has me a little breathless.  I've thought about other brands...any suggestions?

The next problem is adding AC power to my single point ground (SPG)...Polyphaser has a unit for $205...Once again, the product looks good, but, the price!!?  I'm looking for comparable quality at a better price or a circuit diagram that I can use to build an AC lightning pulse shunt and/or surge protector on my SPG. 

My satellite internet and television services were connected by technicians to my house partial ground loop; however, I'm thinking of running their grounds to my service entry which is directly connected to my single point ground by a four foot 2" wide copper strap.

Thanks,

Bob
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K4SAV
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Posts: 1807




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« Reply #4 on: December 28, 2012, 09:11:31 AM »

Note: I'm also putting a lightning rod on my mast as well as a heavy conductor from the mast to the tower to reduce the impulse at the rotor.

The lightning rod on top is a good thing.

Some people use a strap between the mast and the tower, thinking that would shunt the current around the rotator to protect the bearings.  I didn't think that would work, so I did some simulation to prove it.  I also found something surprising.  The top section of the tower where the mast and rotator are located acted like a large leaky transmission line.  When current is conducted by the tower legs, there is an opposite current induced into the mast inside the tower, even if there was a zero impedance connection between the tower and mast at the thrust bearing.  The shunt most people add is usually a braid strap.  Its impedance is orders of magnitude greater than the path thru the rotator, so it does nothing.  The only way to keep current from flowing in the rotator is to completely shield the tower section where the rotator and mast are located so that fields cannot penetrate the inside of the tower.  I don't think I have ever heard of anyone implementing that.  You would have to keep the currents flowing on the outer surface of the tower legs.  If it flows on the leg surface facing the inside of the tower, a current will be induced into the mast.

My tower doesn't have a rotator strap and it takes a direct hit two to three times a year average, and no rotator problems so far.

Here is a TowerTalk discussion on this subject.  Read K0RC's post.
http://lists.contesting.com/archives//html/Towertalk/2006-12/msg00472.html

Jerry, K4SAV
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KC4MOP
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Posts: 729




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« Reply #5 on: December 28, 2012, 09:17:28 AM »

The WX has interrupted my progress to finish my grounding. Doing the same as you to get the electrical and the Sat and Cable TV shields to be at same ground potential.
The lightning pulse must have come in through the cable TV ground and did its damage.

I may have to rebuild what I had from a previous QTH.
I still have these:
http://www.chatsworth.com/products/grounding-and-bonding/insulators/

The red ones.

I secured a piece of copper pipe to those standoffs and silver soldered #6 stranded to all pieces of electronics in the shack. Separate runs. And #6 out to the house ground / electrical service entrance / Sat and Cable.
If you want to get anal, you can use an exothermic or CADWELD process for these wires. The same for the tower. One #6 stranded to each tower leg ( get Polyphaser connector or CADWELD directly to the tower) and then to the 2-3 ground rods around the tower. All bonded together and then bonded to the house grounding system. If something happens, your entire installation will rise and fall with the same potential from the nearby strike and nothing damaged.
Motorola Grounding R56 standards. We never had anymore lightning problems with our trunked radio sites after we had those standards installed.
I copied the best I could from what I saw and the Chatsworth insulators were the only things I bought. Of course, the #6 stranded and silver solder and those nice crimp connectors.
That SPG panel from Poly is very pricey. That's why I went with a length of copper pipe, etc etc.
I only use one of those Polyphaser rotor wire protector at the pole. All cabling for my rotor and RF are in a plastic pipe 5 feet down and come into the basement shack.
Fred
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KI4RAX
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Posts: 3




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« Reply #6 on: December 28, 2012, 06:30:51 PM »

Jerry & Fred,

Looks like I'll need an 8-pin surge protector at the top of the tower to protect the rotor circuits from the rotor control line surge possibility.  Apparently, a strap at the tower top would not make much difference and neither would running the rotor cable in conduit from the top of the tower.  I can return all the conduit and couplers and then have about enough for another 8-pin surge protector at the top of the tower.

Thanks,

Bob
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