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:

Conclusion:

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

Back

Copyright 2015  ©  Cwmbran and District Amateur Radio Society

CADARS Logo
Home About Us GB3RT Repeater News and Events Education Local Information  Local Conditions Useful Info

Cwmbrân & District Amateur Radio Society

For access to this area email webmaster


Members Only