Topband: High-Power Unun Components?

[Jeff Maass]

What characteristics of a unun determine its power-
handling limits? 

I am interested in the case of matching a 160 meter
inv-L with good ground (1.5:1 unun), for 1.5 KW with
a safety margin: perhaps a 2+ KW continuous rating.
This is a single-band application: only 160 meters.

Do any / all of these affect power handling capability?

1) Core diameter? 
2) Multiple (stacked) cores?
3) Wire size (#14? #12?)
4) Wire type (Thermaleze - HAPT?).
5) Wire supplemental Teflon insulation / spaghetti?
6) Core ferrite material (Type K, u=290? )
7) Construction techniques?
8) Other?

This question grows in part from an incident during
ARRL 160, when we were using an Amidon Sevick-designed 
"1.78:1-HMM50" multi-match unun, rated "conservatively"
at 1KW continuous and 2KW peak, to match 50-ohms to
75-ohm hardline near the shack end. Output was 1.5 KW,
SWR was good, but the unun was overheating and the SWR
at the amplifier climbing with each transmission after
a time.

I have an inv-L antenna with good ground which I'll
be using at 1.5 KW for the first time in a few weeks.
The feedline is fairly short, and so we will be putting
much of the 1.5 KW into the unun. I'd like to head off
any problems by making a truly "conservative" unun to
use at the feedpoint in case problems arise with the
existing unun.

73,

Jeff  K8ND