[Antennas] W4HDX Zepp antenna query

[JOJ006 Mail]

Hi - my first posting here I believe. I wonder if anyone on this forum has simulated or built the W4HDX Zepp antenna? The original publication can be found here and other places...
http://www.southslope.net/~kb0sk/img/zepp.pdf

He claims low noise pickup with good performance and easy ATU matching by designing the top to be flat  or non-resonant on any of the intended bands and he uses a double coax feed with the balun at the sending end of the twin feeder arrangement rather than at the antenna.

I'm puzzled as I would have intuitively thought that for an antenna to radiate efficiently, it would need to be resonant on the intended bands. I assume that the twin coax feed is a matching section but he says it can be any long length over a certain minimum rather than a quarter wave etc. and can even be buried so it's not expected to radiate as, for example, a Windom feeder would.  In fact a quarter wave has to be avoided. 

Is it that the ATU is part of the antenna resonance, as it might be for example with an end fed long wire, rather than it being a simple impedance matching device?

I'm trying to get my head around this one.  If it was really that good I'd set about making a Carolina Windom version to suit the geometry of my site, but I'm sceptical.

What do others think and has anyone simulated it?

Thanks, John, ei1em/g3vlo