[Fwd: LF: Surge arrestors and lightning]
Mon, 29 May 2000 23:26:45 -0400
Doc Gruis wrote:
> Thank you for these interesting posts.
> I have had minimal lightning problems over the years I have been doing
> radio type things. I have high hopes of a new antenna this summer for
> the next season. On the last 45 foot mast, I had a small gas engine
> spark plug between the bottom of the antenna and ground / earth. It
> showed some pits on its points so it must have done something! The
> little neons across the inputs on WW-II type gear were used for several
> years. Perhaps they helped with some slight noise reduction or were
> talismans of sorts, but the fact that they were used for several years
> indicates they must have had some marginal utility. A direct strike is,
> of course, devastating, but the field around it is more easily
> dissipated. Confession: I still use them from time to time. And, then
> again, maybe they would just dissipate local static. I have noticed a
> slight noise reduction in putting a NE-2 in parallel with a high
> resistance static drain resistor.
> Regarding the gas tubes sparked (pun unintentional - but not deleted) a
> memory about the "keep alive voltage" in radar T-R switches...
> Doc, K0HTF and Long Wave "D"
> Iowa, USA, EN31
> Alan Melia wrote:
> > I am no expert on lightening protection but I did work about 30 years ago on
> > high voltage protection of (the pre-optical) submerged repeater amplifiers.
> > These are power-fed in series and when your friendly Trawlerman gets snagged
> > and puts his axe through the cable, it is in effect shorting 7 miles of coax
> > (100pf per foot or so) charged up to anything up to 15kV. We used gas
> > discharge tubes to protect the power supply together with BIG Zeners. The
> > gas tubes have to be specially made (I have never quite understood what the
> > little neons on the old wartime BC348 and command sets were supposed to do)
> > The problen with discharge tubes in the dark is that eventually all the odd
> > ions recombine, then when you want the tube to strike it has to reach very
> > high voltages before it will take current. Purpose built (gas) surge
> > arrestors have a small amount of radioactive material in them to ensure that
> > there are always a few ions floating around to start the breakdown avalanche
> > off. I think that gas tubes took somewhere in the order of a few hundred
> > microseconds to fire, but they took the brunt of the power.
> > The amplifier input was a 3:1 step up transformer feeding the base of a
> > bipolar transistor. Having found some diodes that would absorb 10kV for
> > 5usecs we put a pair across the the transformer input. We found that whilst
> > they protected the transformer from fusing, in this position they did not
> > protect the transistor. We found the transformer 'rang' and produced a surge
> > in the secondary that destroyed the transistor. So a second pair of back to
> > back diodes was connected directly across the base and ground. As these
> > where wideband amps, intermod was inportant so we actually used a pair of
> > diodes in series in parallel with another pair with reversed polarity. The
> > reason for this is that the capacity of a junction diode varies with the
> > square of the voltage applied, so if you half the voltage swing you reduce
> > the capacity variation by a factor of 4, and a resultant improvement in the
> > intercept points.The diodes used were a standard in the 1N3595 range from
> > Fairchild. The secret being a large 'S' shaped spring with a reasonable
> > thermal inertia that made contact to the top of the diode chip, and absorbed
> > the rapid rise in temperature at the junction. The output stage was
> > protected by a zener and small diode in series, the zener providing a
> > 'hold-off' voltage.
> > I noted that my old FT101ZD had a small 'lamp-fuse' in series with the
> > aerial feed to the receiver section. Induced voltages took that out but it
> > obviously protected the rx front-end.
> > I would suggest a spark gap might be the best safety device ....2 sharp
> > points about half a cm apart (it probably depends on the rf voltages around
> > but air breaks down at about 10kV per cm gap , I think) What you must watch
> > is that if the static causes a flashover whilst you are transmitting the
> > spark will be maintained by the RF even though the voltages are not enough
> > to initiate a discharge. The safest way is Mal's....ground it all and go and
> > make a cup of tea until the storm passes.
> > Cheers de Alan G3NYK
> > Alan.Melia@btinternet.com