Topband: Inverted L SWR Jumps ???

[Tom W8JI]

The sloper loads fine all the way up to 1500 watts. The inverted L loads 
just fine to about 700 watts and then causes the Alpha amp to fault out. I 
think I am getting a sudden change in antenna impedance. The antenna is fed 
through a 5 KW rated choke balun. The feed line exits the base between 
radials. I've tried various feed line lengths, I've replaced every component 
in the system except for the antenna wire. The antenna does climb along the 
branches of a tall pine before L-ing outward at about 55 feet. I think the 
problem is worse at night time when things are cold (and perhaps more 
humid).>>>>

There could be many things at work. One problem common with amplifiers are 
defective or misplaced lightning arrestors, or bad coax connectors.

I high-pot all of my connectors. Ohmmeters will not test for the stray 
strands or bad insulation that cause problems like yours. A good PL259/SO239 
combination will hold off 5,000 volts peak, as will any reasonable size 
cable (RG58 or larger).

You also (as Timmy pointed out) have two antennas, and on 160 it is pretty 
tough to get enough spacing to really be safe. An UNUSED antenna can 
sometimes arc at the open feedpoint.

Lightning arrestors have historically been problematic for damaging 
amplifiers, including blowing bandswitches or causing SWR trips. A lightning 
arrestor has a rating of power/SWR. An arrestor that triggers at 3000 watts 
peak into 50 ohms will trigger as low as 1500 watts into a 2:1 SWR at the 
arrestor insertion point, or as low as 1000 watts peak into a 3:1 SWR. Most 
manufacturers of protection devices fail to tell consumers this important 
information. They also can go bad.

It is unlikely anything except the area out toward the open end of the 
Inverted L can arc, so I would primarily suspect antenna coupling, a 
defective component or connector, or a lightning protection device.

73 Tom