I had the urge to test to build a QRO low pass filter yesterday, which will probably be needed when we get the 40m yagis mounted in the 60m tower, since they will be rather close to the 20m beams. There are the expensive BPF filters from 4O3A but I figured that for TX filtering a simple LP filter should be just as good and by combining that with the regular BPF between the radios and amp that should be sufficient.
I did some simulations in a software called Elsie where I after I got pleased with a design I simulated the currents through and voltage over the capacitors and noticed that regular doorknob caps should be sufficient for QRO use. The simulations were done in OrCad with PSpice.
The filter is a 7-pole Chebyshev filter which has got 0.2dB passband ripple. However I did place the -3dB filter breaking point to close to 40m so I will need to readjust the values of everything and move it up a bit in frequency, to improve the insertion loss on 40m. If I get down to 0.2dB I will be satisfied. However when adjusting the inductors I accidentally broke a doorknob of a value which I of course did not have more of. So I have ebayed after some extra caps for more testing.
Here are some pictures