Blocking stray RF with diamagnetic materials

For 1.5GHz the exact type of capacitor is important as well the the capacitance value. Many types of capacitor will self resonate well below 1.5GHz. The capacitor does not act like a perfect capacitor, instead it acts like a capacitor with an inductor in series.

If your capacitor was perfect and your antenna impedance at 1.5GHz is entirely resistive then you would only need to consider the impedance of the capacitor and make it low enough that it has no effect on the rf. A perfect 100uF capacitor would have an impedance of a tiny fraction of an ohm at 1.5GHz. In reality a 100uF electrolytic capacitor dosn't work at 1.5GHz due to inductance of the leads and the coiled plates.

You have to use a tiny surface mount capacitor for 1.5GHZ.

If you have a typically antenna system characteristic impedance of 50-75ohms then making the capacitor impedance under 1ohm means it has no practical effect. Maybe you want to try for 0.1ohm if you want to get the last 0.1dB of performance if you are designing expensive satellite equipment.

TO figure out the minimum capacitance value to get below 1 ohm at 1.5GHz you can use the 1/2.pi.F.C formula from any basic electronics textbook. It works out that you need a capacitor of 100pF or larger. The practical problem then becomes picking a capacitor that has low enough parasitic inductance and dielectric loss to actually work.

The behavior of common components at microwave frequencies is often not specified by the manufacturer. This is when you realise why RF is seen as a black art. You sometimes need to spend some time reading, looking at stuff other people have deigned and testing things in your lab full of RF gear to find out what works.

If you antenna impedance is significantly inductive or capacitive then you can make it more complicated by using the block cap as part of a matching circuit.

Whenever two pieces of metal are near each other, there is capacitance between them. If there is a voltage difference between them, there will be some equal and opposite charges accumulated on the two "electrodes". Conversely, in order to establish a voltage difference between them you must supply enough charge produce that voltage.

So if your signal in your rf circuit travels along a metallic conductor (like a wire or PCB trace) and that conductor is near some other conductor (like pins of a chip, or the lid of your enclosure or mounting screws or whatever), there will be capacitance between those two objects. If that isn't capacitance that the designer intended to exist, then we call it "stray capacitance".

To be honest, usually in rf we design the signal paths to be reasonably well shielded from sources of stray capacitance, and so this isn't usually an issue (but I've never tried to do rf on a breadboard). It's more in precision analog design that I've seen problems with stray capacitance.