how to reduce the s21 in the exponent taper balun

i design a taper balun ,but the result of s21 seems not good ,could you give me some advice to reduce the parameter.plz

Microstrip losses come-generally-from substrate losses.There may also be some radiation losses and conductor losses but substrate losses are more dominant.
That's why since we don't know anything about your circuit and the substrate which you've being used, there won't also be a satisfactory answer.

hi bigboss
The relative dielectric constant is 2.2 ,h=1.5mm,the thickness of microstrip lines is 0.035mm.the radiation is perfect .

What about TanD ? Loss is proportional with it.

hi bigboss:
the tanD = 0.0009; and any other cause the problem?

what is that a plot of "Y1"? The ripple suggest to me that it is NOT well matched to a 50 ohm load, hence the ripple as frequency moves (think of a smith chart, and its matched to, say 60 ohms. now draw a circle around that 60 ohm point on a smith chart of s11. As freq changes, the circle sweeps from 75 ohms to 50 ohms, and part resitive/reactive in between, hence causing the small ripple)

You might also consider : R. W. Klopfenstein in a paper titled A Transmission Line Taper of Improved Design as an alternate.

Microwaves101 (dot) com is a useful place for additional information.

hi Azulykit
i have read the paper ,and do a simuation like its siad ,the result is good but i cannot make it continuous .could you give some example?



hi biff44
i cannot understand clearly .but you said seems professional .i not good at this part ,could you give me more detail.plz