Miniaturization of Patch Antenna

Hi every one.
I want to design a dierectional antenna at 400 MHz. Which type of antena I should choose?

In my case, I choose a Patch antenna. I successfully simulate a Patch antenna, size 9x10 cm, resonate at 500MHz, S11=-13 dB. However gain = -4 dB.
Normal partch antenna without miniaturization is 32x35cm at 400MHz, gain =1.5dB.

My question is that when the size of antenna is smaller, the gain also smaller. Is that right? Because antenna gain is proportional to antenna volume?

Many thanks

1. Maybe your miniature antenna maximum gain at another frequency, for example 600 MHz.
2. You can try a little more expensive substrate with higher dielectric constant. This gives smaller patch size.
3. You can try use microstrip without ground plane - monopole antenna. At ground side u can put machined cavity (i guess the same as with horn feeding), cavity can be filled with dielectric for smaller size

In general, if an antenna is smaller then the gain is also smaller.