Hi,
Can you explain to me your Yagi-Uda antenna, pls? I didn't see the driven elements, the reflector, and your dipole antenna.
Did you check the phase diff between the 2 poles of your antenna? The gain of your antenna is positive? What is the polarization of your antenna?
There are several technologies to increase the bandwidth of your dipole: use the parasitic elements, use a bowtie dipole, fractal antenna, use a high loss substrate, use a very thin substrate, etc. I'm not sure that a balun can help you broaden your bandwidth, it is used normally to match the impedance discretely.