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Grounding a broadcast transmitter tower - engineering question




 
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Author Topic: Grounding a broadcast transmitter tower - engineering question  (Read 9033 times)
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WBear2GCR
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« on: June 07, 2010, 09:00:25 AM »


Anybody familiar with standard FM/TV broadcast practice??

Tower, ant, feedline, icebridge, transmitter building, transmitter.

How is that "grounded"??

Good idea to "bond" the feedline to the tower leg up in the air where it comes across the ice bridge?

Seems to me there are several disparate ground references lurking:
- the ANT at the end of the feedline (? they're talking yagis to make the requisite FCC pattern)
- the tower
- 20ft up the tower
- the "ground ring"around the building
- the AC mains if bonded to the ground ring
- the AC mains drop elsewhere on site


This also might apply to a similar ham situation...  Grin

Feel free to contact me via email or PM...



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« Reply #1 on: June 07, 2010, 11:19:50 AM »

It all depends on what service the tower is performing. 

If it is a series-fed AM stick, it will be insulated from the ground system of radials and pulled to ground potential by chokes that block the RF.

If it is merely a support structure, such as with FM broadcast, cell service, or holding up a beam, it gets grounded at the base via either a series of ground rods or a conductive web like copper screen or interconnected wire, laid out of the surface (this is done if it is a mountain top site and nothing but rock).

Ice bridges and anything else associated is usually tied together via Cadweld and heavy copper wire. 

All coax outer conductors on the tower are bonded to the tower by connections spaced out according to height.  Lighting power is in bonded rigid conduit.  If it is a "hot" tower (the radiator itself), cables on it are isolated at the base via ring transformers, isocouplers, or chokes in series.

The going thing today in addition to any grounding system is the addition of static dissipaters.  They will practically guarantee no lightning strikes.

www.nottltd.com/lightning.html

I hope this helps a bit.

DG
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WBear2GCR
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« Reply #2 on: June 07, 2010, 11:37:27 AM »


Ok, what about the differential between AC mains ground and the tower?

Not a problem??

                  _-_-bear

PS. not an insulated tower, used for FM and for VHF/UHF services afaik.
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k4kyv
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« Reply #3 on: June 07, 2010, 12:45:37 PM »

It is important to run the coax and other electrical lines down to the very bottom of the tower right at the ground point before they run to the transmitter building.  The common practice of having the coax exit the tower at some point 10-20 ft up from the base is asking for disaster, even if the tower is superbly grounded.  If lightning strikes a grounded tower, the voltage potential of the pulse may be thousands of volts above ground at a point 10 to 20 ft up the tower, and the coax shield is going to conduct that HV pulse right into the shack to destroy the equipment and/or start a fire.  I would run it from tower to building via a grounded metal conduit pipe lying right on the ground.

If, for any reason, a cable must exit the tower several feet up from the base, I would recommend using extra cable and winding several feet of it in the form of a large solenoid-shaped coil so that the external braid acts as an rf choke.

Lightning pulses are more akin to rf than to DC or 60~ AC.


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flintstone mop
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« Reply #4 on: June 07, 2010, 02:18:34 PM »

I'm pretty close to DG and his response.
From my former commercial world we had ice bridges for the waveguide to enter the building from point-to-point microwave 6GHZ. So this is not a hot AM tower.
Motorola standard was to ground the antenna coax at the top of the tower, at the ice bridge, and entrance to the building.
Inside the comm building was another system using #6 stranded copper and ALL pieces of electronics and racks were individually run back to the main ground or the #6 wire going around the inner perimeter of the building, which connected to the outside ground. Every piece of equipment/rack/battery chargers/electrical sytem  rises and falls the same amount during a strike.
Tower, of course, each leg connected to a massive mase of copper or aluminum burried to achieve some very low ohmage.......can't remember but it was down to tenths of an ohm.
We took direct hits almost every storm, on one 240 footer and sometimes the tower lights would pop, but nothing else was hurt, not even the high-tech tower light controller.

Fred
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« Reply #5 on: June 08, 2010, 10:46:34 AM »

Bear, why would there be a 'differential' from the mains to the tower itself?  You may very well measure a voltage between them, but that would either be a circulating current which will go away when bonded to the tower, or there is a fault in the wiring that needs to be corrected before going on.

If there were AC on the tower it is probably for lighting, perhaps to run a mounted radio up there like a spread spectrum link.  If it is not isolated for a 'hot' tower, it is treated just like any weatherproof outdoor circuit under the NEC.
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WBear2GCR
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« Reply #6 on: June 09, 2010, 09:05:16 PM »

The differential I am speculating is between grounds when you drop a few quadzillion amps down the tower or nearby via a lightning hit. Will the AC mains ground and the tower ground and the point 20 feet or so up the tower be at the same potential then?

At some point the AC mains ground and the feedline "ground" meet...

That's all I am thinking about... this setup just isn't  single point" ground, is it??

A Nautel publication shows a big arse ferrite choke between the xmitr and the outside world, but inside the xmitr building...
and the rest of this multiple connection ground system.

http://www.nautel.com/Resources/Docs/Site%20Recommendation%20Book%20-%20Issue%203%20%282004%29.pdf

I am a bit skeptical about this setup, but perhaps it works, but not because of  how or as it is shown? I'm thinking to myself, maybe the whole building and gear is "bootstrapped" in reality so that it is going to rise and fall with the nominally "grounded" tower ground radials, while the gear sits inside a Faraday shield formed by the building??

I'm missing something otherwise, because there are multiple "ground points" - all over the place actually... and while I won't go into too much side story, somebody is insisting that this is "single point grounding". Is it??

They (Nautel) caution that the setup won't work unless it is fairly carefully followed - the key to me seems to be that the building is strapped to the tower radials - assuming one can find and identify said radials, and that they exist in the first place???
Don't strap to the radials, and Pow!! Batman?

I tend to worry about real world issues like that more than the theoretical niceties that someone worked out, not that I ignore the theoretical, just that putting things into practice is usually not so 'clean' ??

Anyone??

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flintstone mop
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« Reply #7 on: June 10, 2010, 04:37:53 PM »

The Nautel paper isn't clear if these proceedures were protecting an AM or FM B'cast facility.
The spark gaps and the huge choke on the transmission line seems to hint that it's for an AM station. The choke will keep the lightning out of the transmitter. Lightning may have volts and amps, but it is a frequency that can be doused by a humungus choke as shown. Even  wrapping transmission line aas a huge coil will also work!
Most AM towers are hot with RF and cannot be grounded as others have posted above.
This link gives a nice illustration of the Motorola R56 standard

http://www.maplenet-inc.com/grounding

Enjoy
Fred
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« Reply #8 on: June 11, 2010, 03:39:18 AM »

Hey Bear,

Last month I set Phoenix' first Nautel NV20 FM transmitter atop South Mountain, and I did put two big ferrite rings (5" ID) on the 3-1/8" rigid coax immediately to the transmitter per the factory. 

Each leg of the 3-phase input has a smaller one as well. 

The NV20 works quietly, uses a lot less juice than anything else I've ever seen, and colloquially known as a big computer hooked to a lightning rod. Wink

The entire site is a bonded Faraday cage, and there is a custom designed surge suppressor (I designed it some 15 years ago) on the main 480-Volt disconnect section.  It can handle some 30 kJ, with less than 2nS rise time via Joslyn OEM parts.

The coax lines going up the 240' tower are bonded in three places, and the 12-bay ERI antenna is bonded via each bays' support frame.

The top is protected by a set of Ron Knotts' Gila-Stats installed a few years back.

We had lots of strikes accompanied by various levels of damage before them, none since...

My gist: If you have dissapators, lightning is not near the problem as without, even with all that we had done.  Lightning rods may be better than nothing, so the analogy is of two choices: a rod is like armor to keep the bully from possibly hurting you, whereas the 'stat keeps him going somewhere else for his fun...

73dg
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WBear2GCR
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« Reply #9 on: June 11, 2010, 08:25:08 AM »


Yeah ok - seems like my "reconin'" was about right - bonded faraday cage... now my only question is where do you want to ground that faraday cage??

Seems like they are doing some sort of sequential/distributed charge dispersal scheme, and hoping a bit, rather than a true single point ground (which presumably would have to be at the base of the tower??

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« Reply #10 on: June 11, 2010, 12:55:53 PM »

Doesn't seem to be any single 'best way' used by everyone.  Most sites default to the 'more grounds are better grounds' in terms of earth contact, and most sites have most everything inside bonded to those grounds, i.e. via ring or single point topology. 

Coax lines on a tower are bonded in several places, top & bottom inclusive.  Maybe Polyphasers or feedthrus on the entrance panel.  Sometimes just grommets, with a outer conductor grounding kit installed there.  That panel will usually have a heavy connection to earth ground as well

'Course, all this is based on the premise that you will take a strike every so often and you have to do something with all those Coulombs.

Me, I use the gizmos that prevent the strikes, whilst leaving the existing grounding/bonding in place.

73DG
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WBear2GCR
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« Reply #11 on: June 11, 2010, 02:03:38 PM »



Yeah, ok.

I get it.

At this point I am merely gathering info for my own edification as the wonks who are actually going to have to do this project are set on following people who are mostly blowing smoke out their rearmost exhaust pipes... big cool and not all that much actual experience.

In the meantime, the wonks are the ones who got the LPF license, not me. So who's the fool??

For example they are "afraid" to ask the tower company about what sort of "air terminals' they have on top, and if they can put what they want there or not... but they still want to do what they want their way... etc.,etc...  don't ask any questions...it goes on.

They'll work it out as they go apparently.

Should be interesting IF their IP with a transmitter from the local transmitter mfr company gets fried - there is no back up, and no plan to acquire one either. Heh.

It's not my money, and not my project, I only volunteered to help on the technical side, so whatever happens happens. A lot of the time things come out ok anyhow, and then they think that they were on top of things and cool...

Otoh, there was this college kid at a local 10kw college station - the "chief engineer" - who had zero background but liked the title I guess (for his resume no doubt) who stuck his arm in the transmitter cabinet. Dunno if it was still on or just had a charge left. He was in the hospital for quite a while... but that made no difference and had no effect on the operations at that place either... you might think that they would have maybe perhaps tried to call upon some local station's engineers to help? to volunteer? to train? Nah. What for??  Shocked Shocked Shocked Shocked Shocked

At least this is a solid state transmitter, less chance to go to heaven fast...

Ignorance is bliss.

Sometimes i don't know why I bother to ever to try to do things properly, ever...

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« Reply #12 on: June 14, 2010, 04:00:55 PM »

An LPFM license? 100 watts power or lower?
This group is co-locating on an existing tower and they are asking questions about placement of their antenna??
That should have been figured out before the application was submitted to the Feces. They could cause serious interference to existing services with a continous RF carrier.
Ohhh welll
Fred
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« Reply #13 on: June 16, 2010, 11:03:11 AM »

No 3kw ERP...

And they have their ducks lined up with the FCC and the space on the tower and the tower company.

The issues are only with me, I question things that other people just wave their hands at and try to think ahead...which is why I asked the questions...

As I said the guy who is doing the "consulting" on the install and fire-up is not terribly experienced afaik and claimed this grounding method is "single point". It just ain't "single point" anything... it's distributed charge dissipation as far as I can see...

I figured I ask here and see what experienced people have to say, since talking to them right now is like pissing upstream in a strong wind... and best be sure I know what i am talking about before I open my big fat mouth??  Roll Eyes

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« Reply #14 on: June 16, 2010, 11:52:16 AM »

Ah, just another class 'A' FM.  Hang the bay(s) at the height needed on the CP, use a grounding kit on the Heliax up there, one in the middle, and one when it is at the lowest point.  Maybe one to the bridge if there is one.  One at the entrance panel.  Since it is a rented tower, hope for the best at that point, it will probably be OK for a long time.

I couldn't make it any simpler.

73DG
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« Reply #15 on: June 16, 2010, 08:13:59 PM »


Ain't up to me what they do...

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