Thanks to GW0OAJ for this article
Next morning it was on with the project and I bread boarded a cascade of two filters in Ugly style:
Just to get the show in the road, the stages were coupled with a straight wire connection. This was always going to give a problems, and as expected, it did :)
Getting rid of the direct connection between the two stages and instead inserting a variable capacitor produced interesting results. This method gave an indication that more capacitance was needed and of the kind of values. I then got rid of the variable and inserted increasing amounts of standard preferred values:
Below a certain threshold level the result was insertion loss and too sharp a response peak:
Above the threshold there was increasingly too much bandwidth coupled with increasing insertion loss:
I settled on 470 pF and tweaked for centre and flat topping. It is based on the CW end of the 80m band:
A very workable system for making a pretty cheap, decent single band pre-selector coil pack. The advice I would give, if cascading for better performance, is to couple the stages with a 470 or 680 pF capacitor. A caveat, of course, is that this work so far only applies to the 80 metre band version.
So after an encouraging session, next attempt will be to make a more durable version!
80m bandpass filter2
Copyright 2015 © Cwmbran and District Amateur Radio Society
Cwmbrân & District Amateur Radio Society
For access to this area email webmaster