if you want to replace the original coax, why not use a halfwave multiple like sirio do, ever wonder why they give you exactly 3.6m on an mag145pl?
if it didn't do anything why would a company like sirio go to the bother of exact cutting and not just stick random lengths on?
Not arguing against your statement on the 18 foot myth, however, I seem to remember someone on this forum emailing Sirio and getting a response that they use the length that they think it takes to get form the antenna to the radio... Although it may have been some other company...
I'll have to look for the post it I guess...
The DB
Here is at least one of the posts in question...
link