[Boatanchors] First radio

[James Liles]

Hi Richard and all:

I don’t believe you will find NPO/COG ceramics in a small enough size for bypass or even coupling and you need to be careful where the high K ceramic disc caps are used.  They are not appropriate for circuits where you are concerned about stability or linearity.

You are quite right about the COG/NPO caps.  They are as stable as they get.  One thing folks overlook is, the Polypropylene orange dip or tubular caps are comparable in performance to the COG/NPO class 1 ceramics well into the VHF region and are as stable.  They do not suffer from varying capacity with voltage, hysteresis effects, and aging as the high K ceramics do.  They are also non-inductive. 

I would not use the high K, i.e. Z5U disc ceramic caps to do a mass replacement without thought.  They are good for bypass and low impedance coupling but that’s about all they can do well.  The Polypropylene do just as well.  Look at what the circuit is to accomplish before using them. 

Just a thought

Kindest regards Jim K9AXN