1.25M LPF suggestion #filter #lpf

Steve KA6PRW

I have built several (about 18) LPFs at this point, so decided to take on the 1.25M (222MHz) LPF. 

I must admit that I do like to tinker and so I spent about 2 days attempting to align the filter using the inductor winding suggestions given at the QRP-Labs site.
Bottom line : I was not happy with the results and so started down a side road, trying to figure out how to make the inductors adjustable.

So - I have attached a couple of photos and associated VNWA screen shot for my final design:

The adjustable cores are #6 BRASS screws.  I intentionally used brass so that the inductance would be reduced the further I inserted the core.  I 'overwound' the coil length (intentionally created an inductor that was higher value then the final value) and this additional length allowed me to stabilize the core by inserting the core further into the coil.   

Before Hot Glue!

After Hot Glue 'stabilization':

Finally the results:

I hope this gives others ideas about how they can improve their design.

Steve Jones KA6PRW