Greg wrote:
I am trying to build a broadband UNUN 9:1 from 1.8 mhz to 30mhz. I
have had no luck.

Question is: Where are you going to find an antenna that
exhibits a feedpoint impedance of 450+j0 ohms from 1.8 MHz
to 30 MHz? Here is a Z0=450 ohm ladder-line-fed all-HF-band
antenna that exhibits a feedpoint impedance at my QTH
between 40+j0 and 87+j0 ohms on all HF bands and needs
a 1:1 balun with no conventional tuner.

http://www.w5dxp.com/notuner.htm

Would you mind describing the antenna system that requires
the specified balun?

The question is simply how do I build a broadband UNUN or
Autotransformer from 1.8mhz to 30mhz and what core do I use?

It's extremely difficult to build a balun that functions
properly over a 16:1 frequency range. It is relatively
easy to build one that functions properly over a 4:1
frequency range. I would suggest building two baluns and
switching between them, one for 160m-40m and the other
for 30m-10m.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com