This discussion has been very informative, but has failed (as I see it) to respond to the original question, which is, what are the potential sources of insertion loss for the QRP-Labs standard bandpass filter kit.
What are the potential sources of loss, and how would one identify them?
One potential source mentioned is the T37-2 toroid. How would you (anyone) go about testing this?
Are there other potenital sources of loss? (Other than poor construction technique.) And how would one go about testing them.
I have built a number of these bandpass filters, and, with the exception of the 80 meter filters, the insertion losses have all been below 2dB. I have been unable to achieve a loss of less than 5dB on either of the two 80 meter filters I have built.