I can confirm the insertion loss stated above for the 80m BPF, the one intended to work with the U3S Rx (not the one in the QCX). As you can see in the picture attached , my BPF had even slightly more insertion loss than that the 5dB of DK3UW above.
I can not confirm the behavior of the trimmers: mine worked as intended, did what they are supposed to do. But one has to know where to look after the effect of turning the trimmers: the S21 transmission curve (red in the picture attached) does hardly changes it's absolute position. To see what the trimmers do you have to watch the S11 reflection curve (blue in the picture). The effect of the individual trimmers is clearer to be seen watching the two distinct "dips" of the S11. This is the nature of the beast.
The blue curve showing the reflection factor S11 at the input of the BPF also tells us something else. The reflection factor in the middle of the curve is about 30% on the linear scale chosen (corresponding to a 10dB Return Loss or, if you prefer, to a VSWR of abt. 1,8). With these figures in mind one can say that the mismatch is the cause of some of the insertion loss. This assumption is right, but the loss due to a VSWR so low is only about 0,4dB, not more.
Probably the rest of the uncommon high insertion loss comes from the red toroids used. I have experimented changing the fixed capacitors with no success and the trimmers will be the same (not checked).
The -3dB bandwidth of the BPF being almost a half of MHz (well suited for the purpose), increasing the coupling to decrease the insertion loss is not an option. There must be something in there eating up the Q Factor: the design is straight forward and can not be blamed.
But the issue is of no concern, at least not for me. As Hans states in the manual, the filters are a huge field for experiments. Myself, intending to use the Rx together with an U3S WITH a LPF multi band switching board, I think of replacing the BPF with a HPF. What do I mean by that? Let me explain:
Using a multi-band U3S with a monoband Rx would be a "no-brainer". Unless there will be somewhen a switching board for the BPF's too, I think of following: Replacing the BPF with a 5 or 7 pole High Pass Filter (HPF) with a cut-off frequency of about 3,2..3,3MHz would get me rid of that nasty QRM coming from below, all those switching power supplies, computers and what's left from the poor good old AM Radio.
Together with the LPF that is in front of the U3S, the "combo" would give me some sort of Band Pass Filtering, at least for the lower bands (80..60..40..30m) near the cut-off frequency of the HPF. Yes, I know, there is lots of QRM in-between those HPF-LPF corners, but ... I think I can live with this and I prefer this solution to having to manually change the BPF each time I change the band of the U3S. Used for band observation, his Rx is not supposed to become my ultimate DXCC chaser. Any better ideas welcomed however.
Thank you for the bandwidth!
73, Razvan DL2ARL