Is it possible to add gain to a dipole antenna without changing its radiation pattern?
It depends what you mean by &quot;gain&quot; of an antenna.
Some people use this term as a synonym for the directivity of the antenna. You can't change the directivity of any antenna (dipole or any other kind) without changing its radiation pattern.
That's because the directivity is a description of its radiation pattern, that tells you how much power it sends in its &quot;brightest&quot; direction compared to the total power it emits. Improving the directivity gain of an antenna means changing the radiation pattern to emit more in the desired direction and less in other directions.
Other people use the term gain for a combination of the directivity and the efficiency of the antenna. You can improve the efficiency of the antenna by making out of higher-conductivity materials or larger-diameter wires, not surrounding it with lossy dielectrics, etc. But your stipulation that the antenna is &quot;not built out of ... lossy material&quot; seems to indicate you already considered this and can't improve in this area.
My understanding is that this definition of antenna gain does not include effects due to impedance matching between the feedline and antenna (source), so improving your matching network is not a way to improve the antenna gain.
Summary: You can not improve the gain of the antenna without either changing its radiation pattern or improving its construction to make it less lossy.