Owen Duffy wrote in
:

....
The construction at
http://www.mydarc.de/dk7zb/DK7ZB-Mat..._Isolation.jpg is quite
complex, containing what might be seen as as an electrical quarter
wave transformer and the balun formed of a U shaped stub of physical
length 0.5*0.25*0.66=0.08 wavelengths shunting one side of the DE, but
this stub lies with its hot side against the cold boom forming another
o/c TL element. If the balun shunts the DE with a high impedance, it
will be due to not just the loop length, nor just the s/c stub length,
but parallel resonance with the boom / half loop o/c stub.

I should have explained that looking at the reference picture, and
considering the insulation between cable and boom, I made the assumption
that the braid nearest the boom was connected to the DE, and other braid
end was effectively grounded.

It may actually be the other way round.

In any event, the physical construction is not IMHO well approximated by
the electrical equivalent shown, nor the description "This line is a
simplified coaxial sleeve balun to avoid sleeve-waves on the braid of the
cable running to the station". Making this balun adequately effective may
involve more than just cutting the line sections to the dimensions
indicated.

Owen.