reactive/radiative near field inside material

Hi all,
I have a question of how to measure the radius of the radiative/reactive near fields.
according to Balanis (Antenna theory and design) the outer radius of the radiative near field for example, is 2D^2/lambda.
where lambda is the wavelength, and D is the largest dimension of the antenna.

So my question is this - if my antenna is implanted inside materal, how then will I calculate it ?
do I need to use lambda of the material? and is D need then to be the electrical length, instead of the physical length ?

Let's say real example, where D=2cm (physcial length) and lambda = 400MHz (inside material 100MHZ). what is the correct math for this ?

please help.

Well, since the calculations happen in medium in which the wave produced by your antenna propagates (I mean now the desired medium), you have to use lambda in your medium (most probably air :)), and D is the maximum physical dimension of your antenna (not electrical length in your substrate).

thanks. sounds convincing.