Well, you want the series capacitor to pass RF at 1.6 GHz with as little
attenuation as possible, while you want the choke to have the highest
impedance possible at 1.6 GHz. Obviously just about any capacitor would
block DC, and any choke would pass DC, so it's the AC characteristics that
matter. However, no components are ideal, so the capacitor has inductance
and the inductor has capacitance, and these can be important at 1.6 GHz.
First, the capacitor. It needs to have an impedance at 1.6 GHz that is
significantly lower than the 50 ohm characteristic impedance of the
transmission line. Considering capacitance alone, anything from about 47 pF
or larger should do. But as the capacitance increases, the parasitic
inductance usually increases too, and you want to minimize that. So the
smaller the capacitor the better - physically smaller. That's why I suggest
47-100 pF surface-mount ceramic, for minimal inductance.
Now, the absolutely *ideal* pass capacitor would have just the right
capacitance *and* inductance to be series resonant at 1.575 GHz. This
perfect capacitor would have near-zero impedance at that frequency because
of the resonance. It would have *lower* impedance than a larger capacitor.
And that's why I suggested 47 pF, because some guesses about inductance
suggested that it would be closer to being series resonant than a 100 pF cap
would be. On the other hand, I have no test equipment to measure this. If
a larger cap passes more signal for you, then it's clearly a better choice.
You do want to make sure that the self-resonant frequency of the cap is
*above* the operating frequency, if you can't put it *at* the operating
frequency. A capacitor behaves like a capacitor below resonance (i.e.
impedance decreases with increasing frequency), but it behaves like an
inductor above resonance (increasing impedance). This shouldn't be hard if
you stick to ceramic chip caps in the 100 pF range.
The inductor should have enough inductance not to load the RF signal,
ideally 500 ohms or more at 1.575 GHz. The inductor I described should meet
that. Again, the *ideal* choke would have just enough parasitic capacitance
to be parallel resonant at 1.575 GHz, giving nearly infinite impedance at
that frequency, and that's what I aimed for when choosing diameter and turns
and spacing (as well as easy construction, of course). But again I just
have some mathematical models of chokes to work with, no test equipment.
Again, you want the choke's self-resonant frequency to be above the
operating frequency, since it behaves like a choke below resonance and like
a capacitor above resonance. You probably want to avoid commercial chokes
because they all have inductances much higher than needed (i.e. 1 mh).
These probably have resonant frequencies in the MHz, and are capacitive at
1.6 GHz.
All the active antennas I've taken apart don't use a discrete coil for the
RF choke at all. They just use a narrower trace than the signal (so the
characteristic impedance is 100 or 150 ohms just from that, and most signal
is reflected at the junction with the 50 ohm path). Then the designer adds
a number of zig-zags in the trace to increase its inductance. If you have a
reasonable way of designing and etching PC boards, that's a simpler way to go.
Dave