I have a board that has capacitor in series with FM antenna socket. It is SMD, so no markings, I measured it and it is around 1nF, it matches to the known 1nF capacitor I have. I could not find exact specification for the board but different sources suggest that it should be around 18pF. Quite a bit of difference. What should I expect with 18pF. The 1nF seems to work fine, could be better maybe. Is lower cap going to make a lot of difference?
using SI4735 ?
no KT1025A
OK.
I remembered reading those values for the SI4735 as they describe it in the AN383 note
may be that can be a source of inspiration on what to look for
Interesting and confusing, why would they put 1nF on the board I have? so what would 1nF do in comparison?
EDIT, ah ok, 18pF is for short wave, 1nF will do
Did you measure it in place (on the board) or did you remove it from the board first?
I don't know
it probably goes hand in hand with what's around it. Isn't there an inductor somewhere? (An antenna behaves efficiently when operated in a condition of resonance and those components are usually chosen to favour a specific band)
At 100MHz 18pf has roughly 50 times the impedance of 1nf. So if it's just a coupling capacitor, 1nf could be better.
Thanks
@J-M-L Just going to get inductor and ESD protection as well, why not, gonna do it properly.
Often, the antenna matching components are selected by testing and refinement (we don't want to call it trial and error, do we?), not design. Usually the design values are just a best guess because the antenna impedance is so hard to guess in situ.
That is why you will often see multiple pads for SMD components, but no components placed in some locations. The pads are provided for worst case matching where for example a complete Pi network is needed (3 components).
We don't know what the FM antenna looks like either. If it's a piece of hookup wire at some random length, tuning the circuit will be different than using a known parameter antenna.
So it looks like they've solved it for you.
A floppy antenna like that can never achieve a good match that is permanent. Usually designers worry more about a TX match because transmit power is more critical and reflected power has a big influence on the PA.
If it's just an RX antenna, don't "sweat it".
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