ask about the dielectric filter design

dear sirs,
I am now tring to design dielectric filters of about 2 G.with the bandwidth of 15MHz.
the dielectric resonator's dimension is 40mm*40mm*33mm.and a
dielectirc disc with height of 11 and diameter of 28mm is placed in the center. A hard foam is taken as a substrate with the same heght
as the disc. but as a result, four DR cavity filter dont has better feature than 5 coaxial cavity filter. It seems that, the Q factor of the DR
is no higher than the traditional coaxial cavity. so is the cavity diemension
40mm*40mm*33mm too small?
The second question. a dieclectric resonator must have four times size
of a coaxial TEM resontor, Is it too bulky? For the fixed desired feature,
is DR filter smaller than coaxial cavity filter?
third question. Is there any miniaturization techniques for DR Filter?
Thanks!

Liu Yun
lycloud1978@163.com

1.when work frenquency in 2GHz,DR filter is smaller than coaxial cavity filter.
2.you can use small scaled DR ,but the Q smaller.