Sparks

John Dunn - March 11, 2013

An antenna for the 40-meter ham radio band is a pretty large thing. A half-wave dipole antenna is around 66 feet long from end to end. Mine was center fed with RG-11/U coaxial cable terminated with a PL-259 type of UHF connector at the transmitter end of the coaxial cable feed line.

One day, I had disconnected that cable from my transmitter for some reason or other and just left the open PL-259 sitting there. Thunderstorms started rolling through.

Suddenly I noticed a sound. It was "snap..........snap...........snap........................snap....snap.." and so on at a varying rate. I couldn't imagine what it was until I saw the open PL-259. There were quite energetic sparks jumping across the gap between the connector's center pin and its outer shell. It wasn't even raining at that moment, but there was enough charge buildup via that dipole antenna for electrostatic discharge (ESD) activity to be going on and to make me very nervous.

I just left the cable totally alone and let it spark to its heart's content until the thunderstorms had gone.

Lessons learned:

ESD events don't require direct hits from lightning

Never design anything that's going to be connected to an antenna without providing an intentional discharge path as protection from electrostatic charge build-ups.

4 Comment(s)

Flagstaff Rich

This is one of those things that every generation rediscovers. Lightning protectors were used back in the early days of radio, too, and they evolved into quite a clever design. The ones I refer to were cast from clear glass and had a pair of neon bulbs within - one from each side of the ladder line to ground. The casting was in the shape of a battleship and when the bulbs lit she appeared to be firing a broadside! I hook mine up for grins when conditions are right for some naval action.

3.12.2013 4:05 PM EDT

EIMEWORRY

I first saw the antenna build-up effect while fishing as a kid off a lake Michigan pier. The static in the air accumulated on my fishing pole and discharged with static hisses.

3.12.2013 3:25 PM EDT

G Man

Neat story! Nothing like a lesson learned eh! Kinda bummed it didn't involve the connector being in the "lap" vicinity and causing mayhem that can only be laughed at later. After all, what is life without some good hair raising stories? But I am not one to laugh at others pain, glad it all turned out well. :-)
Living in the radar world was a never ending source of "surprises". Interesting that those old radios tolerated so much.

3.12.2013 1:33 PM EDT

AC0QG

When I was a boy, I liked to do shortwave listening and installed a dipole antenna on the roof. I had read that to protect my receiver, I should add a Neon lamp from the antenna to ground and a knife switch to ground the antenna. After doing so, I noticed that if the knife switch was not closed, the lamp would be continuously lit. The antenna was picking up enough induced voltage from the nearby overhead power lines to energize the 50V lamp.

3.12.2013 1:14 PM EDT