Lightning protection for a dipole fed with ladderline (also called
window line, or window line sold under the name Ladderline) is somewhat
problematic if it takes a direct hit. The problem is that if you are
unfortunate enough to sustain a direct hit, the first thing that happens
is that the ladderline immediately evaporates. It then becomes very
difficult to control where the current is going to go when you have no
controlled path for it.
What usually happens after the ladderline disappears is that there is an
ionized path that has been established and most of the energy goes down
that path, but because that path now drops a huge amount of voltage,
other fingers break off and attach themselves to anything close by, that
looks like a decent ground. If your feedline was close to a house, the
house is at risk. Well, actually more than "at risk", it's almost
certain that some damage will be sustained by the house. Also note that
if the termination point for the ladderline is at the house, that will
be the point at which the major portion of the strike hits.
If you are really worried about a direct hit on a dipole, it needs to be
located a significant distance from your house and the feedline needs to
run very close to an object (like a tower or mast) that has a good
ground system and can take a lightning hit safely. That's where the
spark gaps need to be located. This is all a big pain, and I will leave
the details of how to configure something like this to those who think
the effort is necessary (there are a few, and I know of one place where
this is a necessity).
Spark gaps and transient suppressors by themselves with a ground rod are
good for handling induced voltage from near strikes, but they are
worthless for direct hits (without the tower or mast).
Jerry, K4SAV
Bill Turner wrote:
>I need some advice on using ladder line if you would be so kind.
>
>I have a Palstar AT5K 5kW tuner but sp far I've only used it with coax
>feed and a W9INN trap dipole. I'm considering using my 160 meter
>inverted vee with the tuner on all HF bands other than 20/15/10, where
>I have a yagi. The main reason is to avoid having a remote coax switch
>outside. I can just switch the Palstar from coax for the yagi to
>balanced for everything else.
>
>Any pitfalls here? I have lightning suppressors on the coax, but what
>about the ladder line? The voltages would be pretty high depending on
>the SWR, so I'm wondering if maybe a DPDP knife switch to ground the
>ladder line when not in use might be a better idea. Or possible some
>homebrew spark gaps on the latter line? Or both? Or something else?
>
>Also, how well would the vee work on the higher bands? According to
>EZNEC the pattern is pretty wild. Woould I be sorry?
>
>All suggestions appreciated.
>
>73, Bill W6WRT
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