[TowerTalk] how best to protect RC ham gear from lightning

[Dick Green WC1M]

Hi Dave,

What's the distance between the tower and bunker? Does the tower have a
separate ground system? If so, is it bonded to the bunker's ground system?

There are a number of good texts on lightning protection. I recommend the
Polyphaser papers on grounding and lightning protection. It sounds like your
installation already follows the important guidelines, which is good.

In theory, if you follow all the recommended practices, you should be able
to get away with not disconnecting cables during lightning storms.

In theory.

Unfortunately, I found out the hard way that theory isn't always reliable. I
followed the commercial recommendations  to the T, but experienced extensive
damage from a nearby lightning surge. Total cost of the damaged equipment
was $30K, cost to repair/replace was $10K (luckily, insurance covered all of
it.)

In my case, the problem was caused by sensitive semiconductors connected to
the tower. I believe the best practices commonly used in the commercial
broadcast industry contemplate a combination of RF transmission lines and
possibly control cables for switches and rotators. Gas discharge tubes and
MOVs at both the tower and shack end work pretty well to protect cables and
devices like that, provided you have a good ground system installed with
proper materials and procedures, etc. (like bonding cable shields at the top
and bottom of the tower, as you say.)

But when you directly connect semiconductors to the tower, and/or they lead
back to a computer and hence to other devices, you can get some serious
damage. In my case, the trigger voltage of the MOVs and fuses I had on the
lines connected to semiconductors that led back to the computer were too
high. I've since replaced them with lower-voltage units, but it's tricky
finding a voltage that's higher than the semiconductor operating voltage but
low enough to shunt potentially higher voltages to ground. I'm uncertain
enough about the MOVs that I always disconnect all cables when the station
is not in use. Built a large patch panel for that.

If you can't be there to disconnect cables, consider using remotely-operated
relays to disconnected all vulnerable devices from the tower when the
station is not in use. This isn't a perfect solution because lightning can
still jump the small gap inside a relay, but it's better than nothing. Also
make sure that when the station is not in use the antenna selector switch at
the tower is not connected to any of the antennas. That should confine any
surge on the cables to the tower end of things. 

It's very important to determine if there are any paths to other equipment
through devices that will remain connected to the tower. Focus on those
paths.

Hope this helps.

73, Dick WC1M

-----Original Message-----
From: David J "Dave" Windisch <davidjw1 at cinci.rr.com> 
Sent: Friday, July 27, 2018 11:44 AM
To: towertalk at contesting.com
Subject: [TowerTalk] how best to protect RC ham gear from lightning

Subject: how best to protect RC ham gear from lightning

 

Hi, Steve, and all concerned:

I'm simply nervous and jerky about how best to protect RC ham gear from
lightning, since I won't be around to disconnect cables "the old way".

 

My remote site is a former AT&T microwave relay site, on 920ft ASL property,
near Cincinnati OH.

There's a pic on my QRZ page.

The tower is 120ft tall to the approach beacon plate.

The bunker is about 57ftX57ftX16ft. 

 

A retired AT&T engineer familiar with these sites, told me there is a ground
mat under the entire site, to which _everything_ is connected, probably even
the fence around the site.

20-mile el profiles to major world DX regions taken from GOOGLE EARTH, as
well as HFTA, show the site as "a nice bump" of good ole Ohio farm dirt,
iow, a good potential lightning target.

Yet, minute inspection of the entire tower top yields nothing I can
interpret as evidence of lightning strikes during the 50-odd years the site
has existed: there are no telltale marks in the paint or galvanizing.

 

Electricity, antenna feeds, and internet enter the bunker, from under
ground, at a single point.

 

A home 1.9mi away on 750ft ground(!) took a _major_ hit last summer. Lotsa
damage, no injuries.

 

My thoughts, so far, are to start from the highest point, with:

1.   A mast with rotator, bonded to the tower, with the topmost antenna
down, say, 5ft from the mast top so the mast forms a sorta lightning rod

2.   Cable shields tower-bonded top and bottom

3.   All cables in a trench to the single point bunker entry

4.   Control cables protected by, eg, ICE or similar voltage clamps

 

I'm stuck here: 

How can I best protect the sensitive data and rx/tx ports on the gear, which
will be set up on a grounded metal operating table, inside the bunker?

 

Brgds,

Dave, N3HE