Topband: Doorknobs

[Rob Atkinson]

There are doorknobs and there are doorknobs.  Most hams know this.
But you can't always use just any class 2 RF doorknob, especially in a
critical application like a series connection between the transmitter
final and pi output tank.  Why does this matter to the 160 m.
reflector.  Because paradoxically, as you go down in frequency, the
dielectric stress increases.  A capacitor in series with RF on 160 m.
during a long continuous duty transmission (i.e. AM or RTTY) is
probably one of the toughest jobs for a doorknob.

It isn't enough to meet the voltage and capacitance value needs.   You
have to use a capacitor that has the manufacturer current carrying
data for 1000 kc available, and that current rating has to be over
what you will be needing.  In a transmitter for bypass and blocking, I
favor overkill and source doorknobs purchased new from suppliers who
provide the 1 MHz current specifications.  You want to minimize the
need to pull a chassis and get into a rig to make repairs.  Pay the
bucks, get new HECs from a place like RF Parts and call it done.  If
you have to, parallel a few to get the current capability.  Save the
hamfest doorknobs for experimenting, use in feedline matching networks
and doghouses where you can easily get to the circuit and see what's
going on and there's no high voltage around.

You don't have to use a ceramic capacitor of course.  But they are
popular because of their small size and ease of mounting.  If you have
room and the value and voltage requirement is met by an air or vacuum
capacitor, or a large cast broadcast mica, then go ahead and use it.

73
Rob
K5UJ