[TowerTalk] 2 element quad on a 17.5' roof tower?

[Jon Knodel]

I am looking at putting up a decent hf antenna on my 17.5' Glen Martin roof tower.  Apart from yagis, I'm thinking about a Gem Quad or standard 2 element quad.  My question is, how much interaction with the roof will I have and how much will it degrade performance with the bottom of the loops being so close to the roof?  I figure the bottom of the 20m loop will be roughly 8' above the roof.  The house is typical wood construction with asphalt shingle roofing.  I've been modeling with EZNEC and can accept the compromise in overall height (take off angle, etc.) but am just not sure what effect the close proximity with the roof will have.  Any help is appreciated.   Thanks.

 

N7XW