That's important!
ICE says that they will work with impedances of 300-600
ohms. Although the characteristic impedance of a line may
be within this range, the impedance *seen* at a particular
point could be very high or low.
Multiply line SWR by line impedance. Most ladder line is
around 400 ohms, so with a 20:1 SWR you could have 8000 ohms
down to 20 ohms on the line depending in where you are at
along the line and what the antenna impedance is.
sqrt of (8000*100) = 894 volts RMS or 1265 V peak.
on the other hand 1500 watts with a 600 ohm impedance is
sqrt of (600*1500) = 949 v RMS or 1341v peak.
So you can easily have higher peak voltage with 100W into a
high SWR (20:1) than you can with 1500W into a low SWR.
This is why lightning gaps or lightning arrestors on open
wire line systems are a waste of time unless SWR is low.
The other thing to keep in mind is that the effectiveness
of any lightning arrestor depends on the quality of the
ground system and the layout -- that is, you need a single
entry point for all antennas, power lines, phone lines,
etc., everything has to be bonded together, etc. There's a
lot to keep in mind and it all needs to be done correctly.
99% of it is in the bonding of entrance cables.
Despite dozens of direct hits a year I get away without any
lightning suppression in my system and I always keep things
connected. As a matter of fact WW4LL operated right through
a lighting storm this weekend in the NAQP contest. He never
even knew it was storming except for the QRN. But the common
points are all good and everything enters through a single
entrance common ground point. **I'm not trying to encourage
others to not use lightning suppression devices, I'm just
pointing out I don't have problems without them.**
I would never bring ladder line inside that way, however. My
ladder line tuners are all outside! If I had to bring ladder
line inside, I would disconnect it outside when not in use.
My barn entrance is at:
http://www.w8ji.com/contest_station_w8ji.htm
My workshop and house entrances at:
http://www.w8ji.com/station_ground.htm
73, Tom
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