Receiver preselect filter design

[brandon7861]

I built an excel calculator for a simple preselect filter and ran into something curious.
The calculator solves for C and L of both the tank and match in this circuit:

based on the following inputs:

Antenna resistance​

LNA input resistance​

series resistance of the coils (which effectively puts parallel equivalent resistance to ground and is not shown in schematic)​

edit: and desired bandwidth and center frequency​

​

It assumes the capacitors are ideal and that any reactance in the antenna or LNA input is already cancelled (resonant source and loads). It outputs both X and C values and the realized BW and dB loss.

The excel calc and simsmith are in good agreement, but something unexpected is happening. I figured I could choose an LNA input impedance and design the match for that, but it turns out that, for this high pass match, I need to first vary the LNA input resistance to find realistic values for the match components followed by raising the desired BW until the tank capacitor is small enough to be practical. So, for practical values, in reality, I have little control over the LNA input or BW.

So my question is how do I regain control over LNA input impedance and BW? Do I need to add the option to use low pass filtering and/or cascading networks? Do I need to first use a transformer to change the antenna impedance seen by the preselect filter to reduce loading? What do the professional radio designers do when they want to achieve a design goal here?

 

[brandon7861]

I know there are a billion ways to implement a preselect filter, I am just diving into the one example used in the Radio Design 101 tutorial by MegawattKS on youtube to better understand it. This calc I made is pretty much the process he follows in his video (with added consideration for the inductor resistance).