[TowerTalk] Element Mounting Plates

[Mike Harris]

G'day all,

I'm wondering how the "effective diameter" of element to boom mounting 
plates is calculated.

In the ARRL Antenna Book the mounting plate for the 20 metre Yagi elements 
is 0.375 inch thick, 6 inches wide by 8 inches long.  This is quoted to be 
electrically equivalent to an effective diameter of 3.7063 inches for the 
heavy duty element and 3.4194 inches for the medium duty element.  So 
element diameter is also a factor.

The calculations in the spread sheet "Element Strength and Equivalent 
Length" Leeson 1991, released by the ARRL seems to come up with a 
different number i.e. for the heavy duty element 2.08 inches, if I'm 
reading things correctly.

Now this isn't a big deal, initially for my 20 metre dipole element to 
stack above my 18-30MHz log, it can simply be brought to resonance, 
however, it becomes a deal if trying to use available materials in lieu of 
the ARRL plate sizes when constructing a multi element Yagi.

My 20 metre dipole element will eventually transform into a 30 metre 
dipole element by simply extending it appropriately.  I have achieved a 
calculated wind speed survival figure (no ice) of 120mph with two 
additional taper sections to 1.5 and 1.75 inches using the above spread 
sheet.  This is better than the ARRL element which uses 2.25 inch 
diameter.

Regards,

Mike VP8NO