[Elecraft] K2 and SWR protection?

[Rolf Moberg, OH6KXL]

Hello!

I installed "standard" MFJ's G5RV antenna. Middle point is about 10 
meters from ground level. One end is a bit lower, another is at 10m. In 
the bottom of 450 ohm ladder line there is a choke:

http://ugly.blerp.org/~oh6kxl/choke_balun.jpg

After the choke I installed about 20 meters of RG213. It'll be replaced 
by shorter and better coaxial later. I don't have KAT2 or another 
antenna tuner in line with G5RV.

K2 is giving full output _according_ to requested power and RF LEDs in 
all hf bands with exception in 18 MHz where K2 is giving only 8 
according to power meter when pressing TUNE.

I understand K2 will decrease output due to improper load in 18 MHz. But 
why this doesn't happen in other bands? 20 meters of RG213 between choke 
and K2 surely transfer some of the RF power to heat. According to my 
understandig from past ham radio years there should be more SWR-related 
power limitations in this situation.

Comments, please?

Rolf Moberg
oh6kxl, K2 #4759