Thanks Allison and Steve,
I'm new at this so having a go. Does this look right for the receiver input filter:
Assumptions made here:
Load impedance is 1500ohms (SA612A input impedance).
Source impedance is set by R21 at 51ohms.
Values of the variable caps were tuned in ELSIE to peak the output at about 14.1M.
Insertion loss was 1db at peak.
3db bandwidth was 2.3MHz.
Does this seem right?
I've gone on to tweak values to get for 40M by doubling the fixed cap and inductor values to the nearest standard values and then tweaking the variable caps to peak 7.1Mhz with similar looking results in terms of insertion and relative bandwidth (ie about 1.2Mhz at 3db down).
I'm trying to understand what is going on in this filter:
The LC network of L4 and CT1 does the bulk of the impedance matching between the source 50 ohms and the load 1500 ohms with L5 and CT2 doing most of the passband work. C34 and C41 act as coupling between the three main stages of source, resonant tank filter and load but I presume interact with reactive components on either side of them to cut out some lower frequencies. These added reactances are offset by impedances "left over" from the inductors by adjusting CT1 and CT2. Hardly scientific, I know. L4 and CT1 also acts as a low pass filter with rolloff at 14.8MHz (using the values above). Passband characteristics are enhanced by the low pass filter of L4 and CT1 and the high pass networks that C34 and C41 form with components of L4 and L5.
Any feedback on my dilettantish thinking would be greatly appreciated.