I've recently started working in microwave field, so I don't have big knowledge except for basic theory.
What I've noticed is that, in rf and microwaves printed circuits (for instance for microstrip lines, patch antennas etc) the substrate material is generally a non - magnetic material (relative permeability about equal to 1).
This tendency has become so standard that most of people (and even substrates datasheets) never mention the relative permeability, since it's known to be 1.
Here and Here you may find a list of typical substrates materials, which are all characterized by non-magnetic properties, and which are choosen mainly because of their loss tangent (dissipation) and dielectric constant.
So, my question is: why not using materials with relative permeability greater than 1?
High electric permittivity materials are used when we need to:
Increase the capacitance between two metal objects;
Decrease the wavelength (for a fixed source frequence). This allows to make the structure (patch antenna, microstrip line etc) seem bigger with respect to the wavelength, and so its size may be reduced. This allows to minimize devices sizes at the cost of higher parasitic effects.
Well, even choosing a material with high magnetic permeability can reduce the wavelength and provide the advantage 2), since:
The role of magnetic permeability on reducing the wavelength is exactly the same of electric permittivity. No difference. But why magnetic materials are rarely used?