Posted by: m1paf | May 30, 2009

Vacuum Variable Cap Arrived

The cap for the tuner has arrived.

It is a Comet CV05C-500XIH/5, 16 turns 5 – 500pF rated at 3KV. Now I need to couple  it to the stepper motor and arrange the end of travel limit switches. This should be fairly straightforward as the cap has a position marker that travels along a slot in the drive mechanism. It looks like the cap originally had a fibre optic end travel sensor and I may use a similar technique as it would keep the switching devices away from the RF in the tuner.

73 Paul.


  1. Good progress Paul! The process of sourcing parts unfortunately takes more time than we would like.

    Have you a schematic of your tuner at all? I am interested in the fact that you are constructing a balanced tuner, yet appear to have purchased only one HV vacuum variable capacitor.


    • Hi Kevin,

      The tuner is a balanced L match using 2 inductors and 1 capacitor with a 1:1 balun on the input side (i.e. 50 ohm side) as shown here:

      ———– L ————
      I |
      N |
      P C
      U |
      T |
      ———– L ————

      The capacitor can be switched between the input and output of the tuner depending on whether step up or step down impedance transformation is required.

      Sorry for the rubbish formatting of the diagram but a quick Google will find you a better one.

      73 Paul.

  2. good schematic of L tuner is available on my site, in page automatic antenna tuner, as well a DC motorisation for vacuum variable capacitors and roller inductors.
    73’s de dominique

  3. I have taken the plunge and ordered a couple of roller inductors and a couple of two gang variable capacitors rated at 1.5 KV. I will get my balanced tuner working first in “Armstrong” mode and then think about putting in micros to control stepper motors, measure SWR and feeder currents.


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