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The pcb substrate does not have a frequency limit. Some parameters of it (Er), besides the physical dimension of your traces will affect your transmission lines characteristic impedance (Z), and others will affect propagation (tg delta - loss factor). Some PCB substrates have loss coeficient higher than others. Usually, you can find these in substrate manufacturer's datasheet. Some examples of substrate are at www.isola.de . Most used is FR4, with copper . I use it in high speed design. Some parts are working at 2,2GHz, but a special atention is required for traces impedance, and they should be as shorter as possible (1-2 inches max.) If you need to design at high frequency or you need larger boards, then you should use substrates specially designed for RF. The important features are substrate uniformity, lower loss coeficient. Also, traces must be prepared for high freq. (gold metalisation ot other).
fr4 with copper is most ordinary pcb I know. It has a dielectric constant of 4,3 to 4,9, depending on manufacturer. But to be more specific: at what frequency do you want to use it? Maybe if you give me some details I can help you.
I'm member in NanoSat group in ASU Egypt, my task is to design PA with acceptable power budget and satisfy the Link budget, the main problem is the operating freq....my guess is to make prototype @ 50MHz then increase the freq. gradually until specified freq, to be able of tracing any error ( all discrete )....thats why I need to know the limit of PCB operating freq.
Here is my opinion: the most clasical pcb substrate used for low and medium frequencies is FR4, in thickness of 0,4 and 0,8mm, used in multilayer boards and 1,6 for single or double layer. It is widespread and relatively cheap, and almost all pcb manufacturers have it and use it. In RF, FR4 can be used up to 2-3GHz, if the PCB designer pay special attention to some details. First, it is preferably to use only smd components, as small as possible (considering power requirements and skills of assembly), not through hole. This will minimise the contact area between your component and pcb substrate. The main problem with FR4 at high freq is that the substrate is not homogeneous from an electrically point of view. That means the electrical properties may vary from one small area of the pcb to another (or from one production lot to another), thus making hard to perform exact calculations of traces impedance. Also, the dielectric constant of the material is not really constant, it may vary a little with the frequency and temperature. If you can make some testings with a VNA over a pcb board (put some microstrip traces with different widths on board and then measure their impedance), you will be able to calculate your board parameters more accurately . Further, I think it is not a good idea to desigh whole PA at one frequency and then increase it. This is because theoretically impedance of a track do not vary, regardless of the freq., but in practice, due to Er variation with freq., you board parameters may vary also. Design the PA at the final freq.