Paul -
I can hear the influence of Wilf Carter, Roy Rogers and Elton Britt in Frank's yodelling, but he eventually developed his own style.

Roy Rogers had a great staccato attack with some wonderful syncopation to his yodels, as did Rex Allen whose work I admire too.

Slim on the other hand, really doesn't show his influences too much - possibly a little Elton Britt, kinda like Ranger Doug Green - really smooth.

They're all great!
BTW, didn't the legendary Slim Dusty yodel some too?

I distinctly remember a video of him from the late '50's singing
Muster On The Golden Plain with a tasty little yodel tag.

Seamus