I used one of his older coil antennas years ago, before he started adding capacitance hats to them, it worked fine.
It is another coil antenna, lots of guys have used flat stock for the coils, use the Search Forums link above and search for boomhauer antennas or monkey made.
Some claim that the flat bar provides more surface area than a tube.
Measure the thickness, width and length of the flat stock used in the coil and use one of the online calculators like this one:
http://www.onlineconversion.com/object_surfacearea_box.htm.
Then get the OD of the tubing and the length of the coil and do the same thing.
Compare the surface of the flat stock with the round tube.
There are a lot of factors that affect how well a coil can work, but it in the end it is still a loaded antenna.
You are not going to see a monster increase in field strength or gain between say a 10K antenna vers his antenna or another loaded big coil antenna of similar length.
Dan was calling the crossed circles on top of one of his antennas, "Audio Balls" at first,
Lol, He got a good ribbing over that if I remember right.
He is a good machinist and is well know up in the north west corner.
73
Jeff