Thanks again... we are playing with Q degrading and antenna matching independantly from the transmitter at the moment. almost ready to integrate the antenna. I find that i need to match with two capacitors at the antenna input to get near a resonating 256 Ohm impedance, since our loop is so inductive.
I would like to clear up one other issue. Well two little issues... called Cm2 and Lm2, on the opposite end of the antenna. Am I correct in thinking that all the matching occurs at the input to the antenna? That is Cm1, Lm1 (Cm3 in our case)? And that the components on the other end are just to provide a bias (through Lm2) and an RF ground (through Cm2). I suppose to leave Lm2 at the same value as in the EVB, and just insure Cm2 is 2 or 3 times bigger that Cm1?
Well, Colm, I am going to be a bit coy here and play with words a bit, but with a point to it. Yes, in principle, you affect an impedance match by adjusting the components that are attached to the input of an antenna. However, this loop antenna is a balanced structure, with two conductors arranged in parallel to feed it. So, to be precise, we can say that the input to the antenna includes both of those conductors. So it would be logical to say that CM1, LM1, CM2 and LM2 are all attached to the input of the antenna.
It would be a lot less confusing if Melexis had designed a balanced output from their transmitter because then you would have two pins, labelled Out1 and Out2, each attached to alternate sides of the loop through an L pad of an inductor and capacitor. The schematic would probably make more sense to you then. In fact, their output is unbalanced, having only one output pin, and so they have used a matching configuration that compromises and allows an unbalanced transmitter to feed a balanced antenna structure with reasonable efficiency.
I note that the Melexis EVB, which has a loop antenna about double the size that you are working with, is matched with very different values for CM1 vs CM2 and for LM1 vs LM2. I am a bit confused by these differences and not entirely clear on what their matching procedure is. I also note that Mr. GrosJean advises to do fine tuning using CM2 and LM2. The instruction sheet for the EVB also describes all four of these parts as "impedance matching" components. So it is pretty clear that we should also consider CM2 and LM2 as useful in achieving the desired input impedance. I think that answers your original question fairly clearly.
Since your loop is so much smaller than theirs, I wonder if you can get away without adjusting the values of CM2 and LM2 as you suggest. Are these components merely to provide bias and blocking? Well, no, as we discovered above. To get more insight though, consider the impedance of each part. The capacitor, CM2 is 3.3 pF which is roughly -j111 ohms at 433 MHz. The inductor LM2 is about +j185 ohms. These values are marginally "high", especially when compared to the loop inductive reactance (I think you said it was about +j50) or CM1's impedance (about -j20 ohms) or LM1, +j 73 ohms. If LM2 were meant to be simply an RF choke for bias, its impedance would be better put at around 400 ohms or so while I would expect the CM2 impedance to be an awful lot lower if it was only an RF ground component, like close to 1 ohm. So it is safe to say that these are not simply bias and ground coupling parts as you suggest, they are relatively involved in establishing the impedance seen by pin 8 of the IC.
What to do with your smaller loop then? Well, I am not an expert at loop matching, nor am I willing to spend a few hours to analyze your configuration on a simulator, so it might be wise to consult further with someone who has been through this already with this transmitter. Short of that, I might guess that it might be necessary to lower the value of LM2 by perhaps one half, and raise the value of CM2 by about 2 as a new starting point. But then, I am only guessing.
edit: on further reflection, it seems that my final advice above is not sound. If I were truly matching a balanced very low impedance loop to a balanced fairly high impedance source, I would expect the shunt C on both sides to be somewhat large in value, and therefore low in reactance, while the series L would want to be relatively high in reactance. As you decrease the size of your loop, you are lowering the reactance, so it would logical to maintain the higher reactance of LM2 and decrease the reactance of CM2 (by raising the capacitance). Perhaps begin with a significant adjustment of CM2 as suggested followed by some pretty meaty changes to CM1 and LM1. Safe to say that a large change in the loop will require something other than "fine tuning" and therefore you must give major attention to changing LM1 and CM1. That might be the better path to take if you are going to tune by trial and error. I prefer to do much of my trial and error on a simulator as it is much more controllable.