[Amps] minimum length of 1:1 transmission line balun ?

[Carel, pc5m]

Hi, busy constructing  a QRO 2 mtrs power amp with balanced FET's.  It will
a.o. be utilizing a rudimentary (coax) 1:1 balun at in- and output  to
transform 50Ohm unbalanced to 2 times 25 Ohm balanced (single run of coax,
outer braid connected only to ground at 50Ohm end).  I tried to figure out
what the length of this piece of coax should be ? Doing some RF simulations
with Agilent ADS seems to indicate that this length can be very short, one
tenth of wavelength, although I got into the issue of determine the even-
and odd-mode impedance of a coax cable ..would odd-mode impedance of coax
equal to his characteristic impedance and even-mode impedance just be the
reactance of a the coax outer braid ?

 

Anyone can point me in the right direction for the design criteria regarding
the length of  a (coax) 1:1 balun ?  I have found some app notes / designs
but a bit reluctant to blindly copy something I do not fully understand ;-).
Of course the shorter those baluns can be, the happier I would be.

 

73's Carel, pc5m