Antenna Coupling Circuit

[csheldon]

Hello,
I was experimenting with 433 MHz saw based transmitters and found quite a lot of materials about topic, but all they teach you is how to calculate the values of LC tank circuit , and that's fine. But what troubles me is how to calculate the antenna coupling circuit (L2 , C5) in the schematic below. Antenna is a quarter wave monopole , and if I got it right, it's impendance should be around 35 ohms.... L2 and C5 values in this schematic are 470 pF and 120 nH. Can somebody with experience please help me about this? Is it important to know the output impendance of the generator (transistor and surroundig circuit) to calculate this right. This is the link : http://wireless.murata.com/media/products/apnotes/sawbasedtransmitter.pdf

[T3sl4co1l]

Well first of all, what are the reactances?

I think you will find C5 is a coupling cap (much lower reactance than the parts around it), and then L2 will be somewhat of a lowpass filter, but also accounting for load capacitance (if the antenna is electrically short, not a properly resonant monopole), or matching to the oscillator (which itself has an oddball impedance, it's hardly a resistive amplifier).

Tim

[IkariShinji]

They are for antenna tunning. Here is another app note from Murata. In the last page, it notes L and C "can be used to tune to an unknown antenna".

http://wireless.murata.com/media/products/apnotes/highdataratesaw.pdf

They pick a large inductance value in the reference to avoid loading on the resonator circuit.

With a proper monopole antenna, I bet you are better off without L2 at all.