How to make a small microstrip balun ？

how to reduce the size of microstrip balun,when it works at 2-6G ,but i just have 25mm ?i have try with the klopfstein taper but its result of s11 seems not good enough. do you have any good ideals?help

If you use high di-electric coefficient substrate ( for instance Alumina), the dimensions will be smaller.Marchand Balun can also be used so that it has many variants.

i use the fr4 as substrate，the result is not well at the2-3G，other higher freq is ok

FR4 is not very much recommended substrate for beyond 1-2GHz even it looks like working.A high quality and high dielectric substrates should be choosen for precise and application specific structures.Balun is one of them because here Gain and Phase balance is important for a proper performance.FR4 has also poor fabrication tolerances and di-electric coefficient shows some variations between manufacturers.
For 3GHz, lambda/4 will be 11.9mm for FR4 ( Er=4.4) substrate, 7.9mm for Alumina ( Er=9.8 ) and losses will be much less than FR4.

thanks，but i do not understand why you use lambda/4.may books say balun size should more lambda/2 .

Yes that's true but because of symmety, a section is considered as lambda/4.
Look at this paper..
http://www.sonnetsoftware.com/suppor...s2012-1129.pdf

thaks ，but i should design the balun like the microstrip lines for my antenna ，but i just have 25mm to use .use high dielectric is a good choose.