planar filter design using High dielectric material

I am trying to design a coupled line band pass filter on a very high dielectric material (epsilon_r = 70 ). my question, Beside reducing the size of the filter and reducing the radiation loss(due to the high dielectric material), is there is any other advantages of using the high dielectric material?, for example, is the high dielectric materials help in increasing the coupling or allow for other different modes to propagate and increase the filtering?.

Hi,

according to my experience, you will only have troubles with such high dielectric constant material as manufacturing tolerances will play important role in making your measurement result close to simulations. Not quite sure, but I think the coupling will be hard to increase for high dielectric sublstrate. It will require extremely narrow slots and will again be prone to tolerances. What is your reson to use such high dielectric constant in the first place?

flyhigh

I built an elliptical lowpass filter once on high Er material. If I remember, the cuttoff was around 1.5 GHz. It worked fine. I needed to shoehorn it into an existing design that had no space. I kept the thickness high so the line widths were not too small. I think you might have trouble due to aspect ratio if you went to higher frequencies.

Dear flyhigh,
The main reason of using high dielectric material is to minimize the size of the filter when designed at lower frequencies. My question that I am looking for an answer for it, can the high dielectric material act as a small resonator and concentrate the fields around the filter structure and thus enhance the filter performance.

Hi gladiator,

you gave more explanation but stil it is not clear enough what is the frequency range you have in mind. If you have to use such high dielectric constant, I assume the operating frequency is relatively low. In this case, I am not quite sure that coupled line filters are the best choice for you (not saying that they are impossible, just might not be the optimal solution). You might be better off using lumped components instead.

High dielectric constant substrate will certanly decrease the size of the filter as electrical length is inversly proportional to square root of effective dielectric constant, and it is again proportional to relative dielectric constant of substrate.

You can think on Er effect like it is making the field lines more dense. For the same reason in order to couple resonators to achieve desired filter response, space between resonators must be small. Depending on substrate thicknes, precise fabrication of small gaps might be a problem, thus filter will be different from (any) simulations and will have wide spread of characteristics in production.

I suggest as thick substrate as you can get to make line and gaps reasonably wide. Length of resonators should not change much compared to thin substrate, so you will have your small dimensions and resonable tolerance requirements.

Good luck

flyhigh