By G8MNY
     Here is a simple filter that will remove most unwanted signals often found
as unwanted images in 23cms Rx. eg UHF TV signals x2. It will also give some
static protection for preamps as the double tuned line is grounded, but the
loss can be as high as 2dB depending on materials! Bandwidth is quite narrow &
the filter will need tuning top to bottom of 23cm band. It is not suitable for
Tx over 1-2W due to the trimmer losses.

The I/P & O/P can be coax leads soldered straight to the box & line or BNCs/F
connectors etc.

         <-1.5cm->                          <-1.5cm->
            Coax or BNC               Coax or BNC
  ^      ======= ! ======================== ! ========
1.5cm    !!_________________________________________!!
  v      !!                  [^]   Brass strip      !!\...End Wall
         ====================! !======================
                           0.5-3pF                    \..Side Wall
         < - - - - - - - - 8cm - - - - - - - - - - - >


It is made from double sided PCB 2cm tall all seam soldered up with the bottom
& end pieces slightly larger to allow inside & outside soldering of the side
walls. Leave the lid off until after the initial tune up.

The brass strip is 4mm wide (can be thick copper wire) & is soldered to both
end walls. The trimmer is a bolt in type, but soldered in ceramic ones from old
mechanical TV tuners will do.

Once it is tuning well, tack solder the lid on & re-tune, you should have
slightly better Q, make sure the tuning is OK before seam soldering up.

73 De John, G8MNY @ GB7CIP