Static discharge and drain

[rv1pop]

I have had some telephone, forum and email discussions with some members of the forum about lightning and static build up on or dishes. I have measured over 1000 volts between the ungrounded center conductor and earth (single point) ground. I used a 470K ohm resistor tapped in with a tee across the co-ax to ground, but I ran across this solution and it is so much better calculated than my solution, I felt I should post it. For our purposes the gap should be about as close as you can make it - definitely a business card or less. With the 470,000 ohm resistor the voltage never went over a couple of volts, here.

The graphic shows a method of draining charge from an antenna while also providing some protection against high voltages from static electricity. The basic idea is presented on the K2AV Folded Counterpoise antenna website. The value of the 5-Mohm resistor is not critical and just needs to be high enough to minimize its effect on antenna impedance while surviving normal operation. Power ratings of 2 watts or better are generally recommended in this application. The spark plug can be mounted directly to a grounded plate, as well.

An ordinary spark plug and a high-value non-inductive resistor (carbon composition or metal oxide) provide some inexpensive protection for static charge building up on your antenna. (Thanks, K2AV) From an ARRL newsletter, today! That is a 5 meg-ohm resistor. My 470K was a 2 watt as that is what I had.

 

[turbosat]

Wow pop you are swift...I never thought of such. Can you collect enough electricity that way to run any electronic devices? I'd put one of those big outdoor lights up if I could power it with the dishes, lol.

 

[rv1pop]

Well, thanks, but this is a refinement or adaptation of the old spark gaps we used on ladder feed lines years ago. Even though the adaptation is not mine. My dad drilled and tapped a piece of copper plate for two spark plugs as arresters before a "Twin Lead" came into the house. I recently put a co-ax tee and resistor on my rg6 because I got a nasty zap off the center conductor of a cable I was putting a connector on in the yard. The coax was not grounded at the dish.... Grounding at the dish is against the code that demands a single point ground!

This post was in an ARRL newsletter I got today, and was very well done, so I used it. If this power could be harnessed safely, Ben Franklin probably would get the credit. Wind turbines and batteries can do a better job, and LEDs are getting brighter and cheaper. Saw a 90 LED fixture that runs off 12 volts -- puts out the brightness of a 100 watt incandescent bulb for $6. seems they said 250ma draw.... Four of thoses would be awesome. But we were in a hurry and could not take time to check it out.

BUT this thread is really about safety. Equipment is getting destroyed and maybe people hurt by the strange weather. A wildfire started in the last 24 hours from arcing from a disconnected phone line. A police officer or forest ranger saw it start. Phone line was confirmed to be a two mile abandoned, unconnected line.

 

[Cham]

Great idea for antennae where ground coupling is an issue. Usually here in heavy clay soil there is no problem but this year it has been very dry so I might have to re-think that issue.
The spark plug idea has been around for a while. I set the gap really close until I start seeing increase in SWR or peaks cutting off (1kw or more). The plug is best close to the antenna feed point where the voltage seems to be lowest on the peak of the sine wave. It has saved many arc plugs in my lightning protection system by keeping a lid on that induced voltage. Snow is the worst offender as it can build up really high charges over a long period of time. I never thought of using a resistor though. What about resistor spark plugs?? Might have to try one...

Ive never noticed high voltages on any dish here, all of them are grounded at or close to the dish (contrary to code?) rather than piping the lightning surge into the house system. I see the logic in not grounding the dish differently than the mains supply, in doing so can cause a difference in potential between ground sources that can result interference/electrical noise... A 10' dish is quite a lightning rod so I elected to do otherwise. So far no issues, maybe because the dishes are very close to the transformer pole that supplies the power to the house. There is a #2 ground conductor running in the cable between the transformer, ground rod at the pole, ground rod at the house, and subsequent electrical panel.

Always an interesting subject.
-C.

 

[rv1pop]

A resistor plug has the resistor in series with the gap, and therefore will not drain the slow buildups.

I had a heated discussion with the instructor for our NEC code class. He even took over some of my points! The single point ground and common ground plane potential rules disagree, and I believe the common plane potential is safer. {{[[ A Plane is in the air and a plain is a piece of ground -- to make it clear is plain - so which is right referring to an electrical ground? ]]}}

If your equipment is never disconnected and reconnected, and you do have A good ground, you may never see a problem. (Just a ??? thought: Could blowing switches happen when there is a ground potential difference, from static buildup OR high resistance - ground loop voltage - on the cables?)

Another thought, again from ARRL, use stranded wire for static drains. Static is mostly RF according to these experts and stranded wire drains it off quicker! It is not the current so surface area counts. The statement was a number 12 stranded drains more static in the time frame than a number 6 solid. "Braided shield is quieter than continuous metallic (aluminum foil) shield."

 

[turbosat]

Agree, it is a very interesting subject.

 

[Panavision]

I find this topic very interesting. I've wondered about grounding with my dish. Right now there is none. I wish I understood what you guys are talking about better! I've read some ARRL stuff in the past. This thread has rekindled my interest in ARRL antenna matters!

 

[primestar31]

My dishes are on metal poles, with cement, 4 feet in the GROUND. That automatically grounds them! You won't get any build-up of static up here in states that get rain on a regular basis.

 

[Larry1]

Even in areas of rain, static buildup can be a concern. I remember some time ago out fishing before with the weather getting cloudy as a storm was approaching. The weather was not bad at that time, as the storm was still some ways off. Well, every time we raised our fishing rods to cast, we could hear a clicking sound. It was static from the graphite fishing rod sparking off the tip. After putting the boat on the trailer and talking to the people at the marina, they told us that it was quite common to see a few outboards with blown ignition control modules after an electrical storm.
Currently I'm repairing an antenna pre-amplifier for my TV antenna (not satellite) that has gone bad and it has two diodes across the input coax connection (in parallel, one in each direction) to limit the input signal to the amplifier. Reading this thread, I thought this would also do a great job of bleeding off the static once the voltage built up to the diode junction voltage.

 

[Cham]

A resistor plug has the resistor in series with the gap, and therefore will not drain the slow buildups.

Yes that's absolutely right! It will do nothing... the parallel resistor will do the job.

Blowing switches: well normally the shield would equalize any potential between the two connected items, however when disconnecting (and especially re-connecting) the center conductor might be the first contact point so certainly any potential could damage sensitive equipment like switches. Maybe a weakness in the F-connectors we use for TVRO. N series (N, TNC, BNC, etc) connectors will have the shield as the first contact point so they would be better I suppose but more expensive. Back to the "plane" reference... when you fuel up your Cessna, you always use a grounding strap to drain off static charges before attempting to fuel up to avoid sparks that would ignite the fuel; using the shield or outside of the connector to drain off charges before connecting might help avoid damaging electronics...

Most Ku LNBFs use a plastic body that isolates the unit from the dish (including the LNBF holder on the dish) so unless there is a grounding block at the dish any ground potential current would be eliminated (except for stray RF) but static charges could still exist. Wonder if it would be better to have the LNB/LNBFs isolated like this in all circumstances to minimize exposure to the air and/or ground potential issues even on large C-band dishes?

Grounding cable is almost always braided, most electrical current runs on the surface of a conductor so the more small conductors in the same area the more surface area to drain the charges. At least that's the theory I learned...