DC Grounded Antennas?? Good Idea?

[WBear2GCR]

The question is how about DC Grounded Antennas?

I'm pretty sure I read that this is a good idea on a website about lightning protection.
Thought that I made a post about it here on the amfone forum, but I can't find it, and it seems worthy of it's own thread.

The basic idea is the same as an RF choke - let the RF pass by due to a high impedance, but the DC goes to ground, out at the antenna...

The obvious question is, what to use for the choke?

- does it have to be a purpose wound RF choke? (easy enough to wind up...)
  -- if so, wind it w/heavy wire, or use a spark gap, or both?

- or how about a standard choke, iron core,HV,  maybe a HV tranny's secondary?
  -- will this work at RF, or will it arc through??

And, of course, is this a good idea in the first place?
Is anyone doing it, or know of someone who does??

It seems like a naturally good way to go... certainly a way to keep static charges/discharges/noise off the receive side?

         _-_-WBear2GCR

[Steve - WB3HUZ]

Any antenna can be DC grounded, when you are not using it! Just ground the coax or feeder wires. Much easier than messing with special antennas or chokes. Not sure why you would need DC grounding protection when you are operating, unless you plan on doing so during T-storms or have a high static environment (dry and dusty, or dry snow, etc).

An open wire fed dipole can also be DC grounded, if you are using a link coupled tuner. Just ground the center of the link coil on the antenna side.

[WBear2GCR]

Let's discuss the relative merits of the "static drain choke"... now that we know the technical name for it?

...seems to me that it would be applicable to both a coax fed beam and a coax fed dipole, where a "box" is used at the base of the tower/bottom of the feed (as appropriate) to contain the pass thru including the "static drain choke"?

Also, would not it be best to wind with something heftier than #16 if possible? And, include a Spark Gap to handle the "over voltage/current" should there be any??

As far as disconnecting the ant, good idea, and I do that all the time, but it seems to me that another layer of "protection" as free and simple as this is would be a smashing good idea!   

So, does anyone here actually do this??
Any other thoughts?

   _-_-WBear2GCR

[W3SLK]

I had used a 160M loop with one end "HOT" and the other end grounded. It worked FB and didn't subject itself to winter static build up. I was concerned whether a current could be induced into it from a nearby lightning strike. I disconnected it regardless. But it was a "DC short" antenna.

[The Slab Bacon]

Bear
      My flat top is DC grounded through my home brew balun. I have a short length of coax coming from the operating position to the balun and the rest of the feeders are ladder line. The balun center tap is grounded to tbe ground rod for my tower.
This keeps the static build up off of it but I still would be leary of a direct hit. So I still keep ALL antenna lines disconnected when not operating. This way I dont have to worry about it if I'm not home. I definately feel better that way! You know the old "ounce of prevention" thing.
                                                The Slab Bacon 
     
   

[flintstone mop]

Hi Bear,
I definitely do what the Slab Bacon does during the Summer time. I disconnect the radios from the antennas when not in use. When I know that a bad wave of storms are heading our way I unplug all shack power from the radios.
So far no problems.
Fred

[k4kyv]

Over the years I have received a few very nasty shocks from touching an ungrounded antenna under certain weather conditions.  Sometimes when the wind builds up before an approaching thunderstorm, or during rain or snow even when there is no apparent thunderstorm activity.  This is exactly the same phenomenon that occurred with Ben Franklin's kite experiment, and the purpose of the static drain choke is to prevent static charge buildup on the insulated antenna.

This problem can be particularly severe during winter snowstorms under windy conditions and low humidity.

[W2VW]

Saafety choke in my final is across antenna terminal. Antenna is dc grounded all the time. No protection for a strike but might help snow static a little.

[Glenn NY4NC]

Isn't it only across the antenna when you're transmitting?

Saafety choke in my final is across antenna terminal. Antenna is dc grounded all the time. No protection for a strike but might help snow static a little.

[Steve - WB3HUZ]

Dave only transmits.

[Glenn NY4NC]

 

Dave only transmits.

[W2VW]

Nope the choke is right at the SO-239. It's across the antenna all the time. Some others are similar.

[w3jn]

Such an arrangement is excellent for protection against failure of the plate blocking capacitor as well.

I had a Hallicrafters SR-150 once years ago, bought at a local ham store used.  Worked fine on their big beam, but when I got it home it wouldn't transmit.  Took it back, they put it on a dummy load, it had full power.  Took it back home, wouldn't transmit.  Repeat this 3 times until the store owner got a bit impatient.

Turned out there's a 700 uH choke across the antenner jack that's also the DC return for some biassing somewhere.  If it opens up one of the stages goes into cutoff and no transmit.  My antenner was a dipole w/no balun and hence no dc continuity from the antenner jack to ground.  Of course the balun on the store's beam and their dummy load provided the DC continuity.  I replaced the choke and all was good, but it took a LONG time to finger out what the deal was.  Anyway, that's a double safe design.  If the plate blocking cap shorts and some dumb ham replaces the PS fuse with a 20 amp (as I've seen), the choke will blow but put the xmitter into cutoff, so hopefully the ham will turn it off and figure out what's wrong.

[K1JJ]

Turned out there's a 700 uH choke across the antenner jack that's also the DC return for some biasing somewhere. 

Wow, that's an unusual lashup. I can just imagine how frustrating it was going back to the store and it worked there every time.... HA!

I wound up six big RF chokes (50 ohm/antenna bypass) for each of the new rigs here.  Each choke is a 3/4" diameter pvc tube about 8" long. I wound #18 insulated wire on each. I finger the HV will smoke and open up any small 2.5mh type tiny choke, so I wound these tough to kick the breaker for sure.

Being at a 50 ohm antenna point, the inducatance doesn't have to be that big. I ran some tests and found absolutely no loading effect when across the receiver 50 ohm line. On TX, they don't warm up, even on 160M at 1KW.

As for receiver static discharge, the antenna field here is so noisy during snow static and rain storms that I don't see much difference with them on or off. Guy wires, towers, reflectors and directors NOT connected to any DC bypassed feedlines arc like heck anyway and the other antennas pick this up via magnetic coupling.  The antenna doesn't know the difference when receiving real signal RF or arc noise...

T

[Mike/W8BAC]

To protect the transmitter and tuning network, the feeder going from the tuning network to the feedpoint should be coiled into a one or two-turn loop. This provides a high impedance to the steep-fronted lightning discharge, but it does not interfere with the performance of the antenna at MW frequencies.

Hi Phil

I have seen this arrangement several times and have been told it stops lightning but how is the charge dissipated at the loop? The ones I have seen had solid shield coaxial feed lines without jackets. I assumed the charge would arc at the loop but than what? A ground rod nearby?

Mike

[W3SLK]

John said:

Such an arrangement is excellent for protection against failure of the plate blocking capacitor as well.

I have seen this arrangement before, including the HN-500. The principle idea is right on the money but the question I have is does that kind of RF choke withstand the event or 'glitch' as they say on the AMPS reflector enough to dissipate all the energy to ground? The wire is very small.

Mike said:

I have seen this arrangement several times and have been told it stops lightning but how is the charge dissipated at the loop? The ones I have seen had solid shield coaxial feed lines without jackets. I assumed the charge would arc at the loop but than what? A ground rod nearby?

The only thing I can think of is that the coax will overheat and 'explode'. I have seen this where things as simple as knots tied into power cords to prevent lightning hits on the power line from getting into the device. Most of the time, the lightning stops at the first knot and blows the cord apart. So instead of paying for a new "insert electrical device here", you pay for a cheaper power cord. I know this sounds like a myth but I have it on very good technical authority that it works and have seen the results.

[w3jn]

John said:

Such an arrangement is excellent for protection against failure of the plate blocking capacitor as well.

I have seen this arrangement before, including the HN-500. The principle idea is right on the money but the question I have is does that kind of RF choke withstand the event or 'glitch' as they say on the AMPS reflector enough to dissipate all the energy to ground? The wire is very small.

Mikey, depends on how scroteful the choke is and what the PS fuse is rated at.

[flintstone mop]

Hello
I would have to agree with Steve, The HUZMAN. During the Summer I always disconnect the radio from the aerials. I do not ground them when not in use. Why attract the lightning to the "grounded" antenna? Your property will not survive a direct hit any way. B'cast stations go throught the grounding routine 'cause they have to stay on the air and make money and they even hope that it's not a direct hit. What can survive a kajillion volts and a million amps?

G'day
Fred