5.2. Ball Gap

A nice ball gap can be constructed from round "dummy" doorknobs, i.e. those that don't include the latch. These are generally cheaper than "real" doorknobs. Just cut the knob off of its base with a Dremel tool or hacksaw and you have, in effect, a hollow metal sphere with an opening to the interior. Glue a copper endcap to the inside of the knob with conductive epoxy. Slip one end of a copper tube into the cap, the other end into a tee. Using two such knobs connected to opposite collectors, you have a nice gap that can be adjusted to any gap size.