View Single Post
  #38   Report Post  
Old June 23rd 09, 02:08 AM posted to rec.radio.amateur.antenna
Dave Oldridge Dave Oldridge is offline
external usenet poster
 
First recorded activity by RadioBanter: Jul 2006
Posts: 234
Default lightning protection Coax + Ladder Line

"Rick" wrote in
:

MUST ground the shields that's OK and you can use lightning
arrestors on both coaxes.


This is probably not going to work in most cases. Remember an open
wire feedline can have
rather high voltages on it compared to a matched coax liine. This is
because they are often
mismatched at the antenna.
If a voltage peak should happen to occur at the place where you have
the arrestor, it can fire
the gas tube because of the rf voltage.
For example, a gas tube arrestor made for legal limit typically has a
firing voltage of about 800 volts. In a matched
50 ohm system, even with a full 1500 watts into 50 ohms the peak rf
voltage is in the neighborhood of
600 volts. But even a moderate power into a high impedance open line
could be enough to fire
the gas tubes. Neither the transmitter nor the gas tube would be happy
with this situation.


That's true. You would probably need air-gap arrestors that can be
adjusted to not arc on normal transmitted voltages.

Still, the only really effective way of feeding a non-trap wire system on
several bands that I've encountered is to use open wire.

--
Dave Oldridge+
ICQ 454777283