Suggestions for automated antenna disconnect?

[WA2SQQ]

After sustaining some lightning damage earlier this year, I decided to try and automate some additional protection. I want to build an antenna disconnect system using two relays, in series. The first relay, nearest to the transmitter, would disconnect the transmitter and connect it to a dummy load. Why a dummy load? Just in case I get a brown out and momentarily loose power, I minimize risk of damaging the transmitter. The second relay will ground the antenna to my service ground which is located about 5 feet from the shack. Both relays will be controlled by the station power, so when the shack has no power it’s activated.

What are your thoughts on using two DPDT relays with ~20A rating? I can add a few PF of capacitance to cancel the inductance the relay contacts will likely introduce.

The Internet connection is already protected by using an Ethernet to fiber to Ethernet switch I picked up for $119 shipped. Total DC isolation!

[W1ITT]

Before we get into the discussion of transmitter isolation and antenna grounding relays. it would be good engineering to start with your ground system.  You may have something better that you didn't mention, but the service entrance ground rod alone is not good enough.
I have recently done work worldwide on our government installations (and yes, they have lots our our tax money at their disposal to really do it up well) upgrading grounds to protect both transmit and receive sites.  We usually run a buried ring of 4/0 around a building, grounded with rods at various points away from the disturbed soil around the foundation.  At towers, we have a couple rods at the base, connected to a ring about 20 feet out with rods at the corners, and 4/0 under each guy line to rods at the guy anchors. And it's all tied together with the building grounds via the 4/0 in the cable trench.
Again, this is all fine if you have the budget and lots of land unencumbered by the patio, grill, garage, etc.  At home my own installation is a bit more modest, with a ring of #8 around the house grabbing the power entrance ground, the ground rods near the tower, and various buried radials going out across the lawn to rods driven in 20 or 30 feet away from the house.  When I dug in a yard drain tile, copper wire went into the bottom of that too.  I have often thought that if I were to build a new septic system, I'd run fat copper out through the leach bed as well,  Flush twice for a couple extra decibels!
The point is that our ground systems probably won't approach mil standards, but we need to do better than a ground rod pounded into the backfill around the foundation.  Get as much copper as you can afford out around there, get it into undisturbed damp soil, and try to get your connecting wires at least a foot below the grass roots. (This is different from radials for a vertical antenna where you want them above all the lossy soil.)
Terminating radios into dummy loads, and grounding antennas is a good idea, although our government HF sites stay connected all the time.  Just be sure that you have a ground that's worth connecting to. 

[WA2SQQ]

That's already a work in progress that started when I upgraded the electrical service last summer. I took an indirect hit that destroyed my rotor control box, router and two switches. Fortunately, it did not take out my Flex which was disconnected while I was relocating the shack. This same relay switching idea will also disconnect the rotor each time the station is powered down.

[KJ4OLL]

Another point of view, from the Polyphaser white paper "Ham Radio Station Protection":

"Just a word to those who tell us that they are safe from lightning because they always disconnect the
coax from their equipment. When asked what they do with the disconnected line(s), they usually respond
that it is placed on the floor. Now if you stop and think about the last few thousand feet that the lightning
has jumped, you can see the fallacy of their thinking. In fact, they made it worse since arcing involves
ignition temperature plasmas inside your house. True, the radio may still work, if it survives the house fire. "

http://www.polyphaser.com/SiteMedia/SiteResources/WhitePapersandTechnicalNotes/1485-013.pdf?ext=.pdf


Where I work, we have a trunked Motorola public safety radio system. 20 sites, spread out over 1500 sq. miles.
Lots of lightning activity in this part of N. America.
We don't disconnect anything, instead, we do a good job of grounding and bonding, and perform routine maintenance on the bonding.

The sites follow this Motorola "R-56" method of bonding and grounding. Each site takes many hits every Summer, rarely a problem.

http://www.repeater-builder.com/antenna/site-stuff/are-fifty-six-man-2005.pdf

So, anyway, the point is that:
- A disconnect lets the energy pick where it wants to go.
- A good grounding & bonding system improves the chances that the energy will go where you want it to.

Frank
KJ4OLL

[KA2DZT]

I disconnect my antenna lines and then ground them.  I agree just letting the lines lay is not a good idea.

Fred

[WA2SQQ]

Somehow, the main reason I started this post got lost. Has anyone had experience using relays we'd normally use for switching B+, as an antenna relay? Would a 20A contact rating handle 1500W? The relay would be constantly closed, so I'm not anticipating and hot switching scenarios. I'm thinking a few PF of capacitance could be used to cancel the inductance that will be introduced - or so I was told.

[W1ITT]

For antenna switching, I have a commercial DX Engineering remote coax switch.  It grounds unused antennas, and only opens up a connection when 12 volts DC is supplied so I suppose it's "fail safe".  The relays are maybe 1 1/2 inch cubes, with 20 amp contacts and I believe they rate it at 5 kw RF.  Running at legal limit, occasionally at SWRs higher than unity, I have had no problems with it. 
So the short answer is yes, AC relays should do OK as long as there is good spacing between the contacts and ground.  It might be useful to bypass the coil voltage connections for RF with .001 disc ceramics of proper voltage rating, to keep RF out of your control voltages.
As far as using "a few pf of capacitance" to cancel inductance, I'm not sure what you're getting at.  The inductance of these relays would be in series and not much microhenry.  In the 160-10 meter range, it would not be significant enough to worry about unless you have clip leads hanging all over the place.  Use short direct connections.
But, as pointed out above, make sure your outdoor grounding work is done and done well before you spend time and treasure with the relay games.  As much of a pain as outdoor relays can be, you might consider doing this switching outdoors if you are concerned that lightning energy will be an unwelcome guest indoors.

[W4DNR]

To be fully protected, you need to have more than a small gap ( relay ) in your expensive transceiver input.    I don't know of anything better than pulling the coax and keeping the disconnected coaxes several feet away from the gear.  Your outside ground needs to be much better than your inside ground.

W4DNR

[N0WEK]

To be fully protected, you need to have more than a small gap ( relay ) in your expensive transceiver input.    I don't know of anything better than pulling the coax and keeping the disconnected coaxes several feet away from the gear.  Your outside ground needs to be much better than your inside ground.

W4DNR

If you're just going to pull the coax connection it would be much safer to connect the loose end to a coax connector with both sides connected to your ground buss or at least some ground connection.

[WBear2GCR]

I'm not anything like expert in this, but it seems to me that rather than opening the ANT line with a relay, it might be better to short it to ground, preferably outside the shack. Opening the line after the short to ground, before the rig, at the same time would be a plus.

I'd consider something like a HV vacuum relay for the series line - the Ruskies sell 'em online for QRO operation...

In one ham installation that is a club station with a 100' tower on top of a very high hill, they used a 2' x 3' aluminum plate, grounded to whatever their outside ground is (I don't actually know) with coax/hardline feedthroughs at the entry point to the shack. Seems to have been effective for decades. I presume the idea is that the energy traveling down the outside/shield is shunted to ground.

the idea of the loop around the QTH out of copper, is daunting. The cost of running about 250' of heavy copper around this QTH seems impractical. Since the ANT comes in the opposite side of the house from the AC mains, the differential in distance of the two unrelated grounds (the AC and the ANT ground) bothers me too.

Fwiw, a potential solution that I have considered regarding the unrelated AC vs. ANT ground problem is to use a (big 240 to 240/120-0-120)high quality isolation transformer to provide shack power at the shack location, and bond that to the ANT ground system outside the shack. Any grounding experts care to comment on this idea??

I could bury a considerable amount of copper pipe outside the ANT entry, maybe 20-30' linear feet, plus ground rods, if needed.

               _-_-

[WBear2GCR]

And, any thoughts on the efficacy of fab'ing a spark gap + RFC type box at the base of the ANT line (for the center of coax, or both legs for 'OWL'), before it heads to the shack??

[W1RKW]

protecting from a direct hit is futile. 

think of the gap lightning spans between cloud and ground and the instantaneous voltage to do that.  Another 1/4 inch isn't going to matter against 50000 ft.

Best to disconnect.

[Opcom]

A very large DPDT knife switch grounds both sides of the feed line through a #6 and long ground rod. Also disconnects the antenna feed from the section going into the building. The top of the tower is 7 FT higher than the center of the dipole, which hangs from a spar about 5 FT out. I don't expect protection. I expect a lessening of damage. It's worth the inconvenience of throwing the switch.

The only better suggestion to really disconnect stuff would be to use a set of old 3KW 'lighting control switch board' plugs and jacks and just walk the feed away from the building to a purpose built ground point and plug two conductors in there. Those with coax could also walk it away from the building and screw it onto a jack on a weather-protected metal post.

The modern 'safe' version of these single pole high power connectors are called cam-loks. The rotate-to-lock function is a small annoyance but the key is small and easy to remove with a strong tight contact maintained, for antenna purposes (never doing that for mains use).
http://www.cooperindustries.com/content/public/en/crouse-hinds/products/industrialconnectivity_interconnect/single_pole_connectivity.html

About those old style straight pin plugs, a digression to recall the high school days, when I was involved in the school stage lighting team, operating and maintaining a 12 FT long console holding 6 or 8 giant rheostats controlled by big levers and rack-and-pinion mechanisms, rows and rows of single pin jacks with 120VAC from those rheostats just waiting there for you to do a scene change and replug the board with sweaty hands getting a tingle.

It was already ancient when I was there, and made just like a telephone switchboard with plugs that drew the wire up with them to be inserted into the jacks, and these plugs were simple single conductor slotted pins about 1/4" diameter and 1" long, with a recessed but not insulated set-screw in the side of a phenolic handle to hold the thick stranded conductor. The jacks were like as not to be a bit hot, with only a 3KW rheostat's few Ohms preventing a big load like a lamp from operating. Two operators would sit behind this on chairs and operate it for school productions.

Afterward the carbon tetrachloride was used to clean the plugs and the rheostats, the latter of which required one to lie on one's back and slide into the cabinet underneath them. These rheostats were about 18" diameter and all the contacts were to be cleaned, which were on the circumference, so you had to watch where you put the back of your hand and hold the rag so that you did not tough the contacts. I miss carbon tet. It worked better and would not catch fire as would the alcohol. Anyway not being buzzed by the mains was a developed skill.

The main breaker really a disconnect switch was some 300A monster located in a crawl space under the floor next to a distribution transformer the size of a chest of drawers. It was badly planned, 30 FT from the crawl-way door and inconvenient as well as time consuming to get to, and if you demanded the power be cut to the whole board just so you could clean the rheostats, you were considered a sissy and likely to be booted off the lighting team.

Being a parochial school, such a failure would undoubtedly result in being forced to participate in some other and much less desirable part of the production, generally one of those being left unspoken for and involving prancing about in tights.

Working on that stuff and operating it was a rite of passage, now gone. I guess the modern technology of plastic consoles and DMX has just sucked all the potential fun right out of it but no amount of technology can remove the awkwardness from a pair of lime green tights.

[WB4AIO]

I would think your relay gaps of a quarter inch or so would provide some protection from induced currents, if they aren't too large. But a gap of twenty feet is better.

Outside, just before my open wire line comes through the wall of my shack, the open line is connected with a pair of 30 amp power pole connectors to the inside run. When storms threaten, or I am away from home, I just pull the line free and throw it back into the woods patch behind the house, where it lays on the ground.

Someday, I'd like my shack to be in a separate building away from the house. Then I will have a similar manual disconnect of appropriate dimensions for the AC line and power company ground, which I will also disconnect and move away from the building when storms threaten. Then my gear and my shack will be isolated from the entire ground/cloud complex where the current wants to flow. They will be isolated, I believe, to a significant degree (not perfectly, though -- that's impossible) not only from a direct strike to the antenna but also from induced currents from nearby strikes.

I know that there will still be wires in the shack connecting my computer and transceiver and tuner, et cetera, together. And I know that currents will be induced on them. But those few wires, just a few feet long, present a much smaller profile for receiving induced currents than a shack connected in any way to an outside antenna or long power mains or ground wires.

Years ago, I had a deafening lightning strike right outside my home. It didn't hit my house or antenna but it made a black mark on the ground less than ten feet away from my bedroom window. Several electronic devices connected to the AC line were damaged by induced currents. But a Sony 2010 sitting on my bedside table, running on batteries and not connected to the AC line, and which was turned on at the time, was not damaged at all, even though the 2010's RF amplifier FETs are famous for dying from normal Winter fingertip static discharges.


73,

Kevin.