> I'd like to thank all of those who offered suggestions regarding
> "relative power idicators" for my link-coupled antenna tuner project.
>
> Now I have another question. The schematic I'm following for the tuner
> calls for a 1700 pf capacitor, in series to ground, on the input link.
>
> I have a 4000 ph vacuum cap that I could use. However, it's big and
> heavy. I also have a 500 pf vacuum cap that's light and small. I
> realize I could add fixed capacitance in parallel to achieve the correct
> value.
>
::My feeling is that this is a poor choice. Vacuum caps are too
slow to adjust, requiring lots of rotations for a relatively small
capacitance change. If the tuner includes a bridge and control circuitry,
and a motor system to drive the capacitor, then I'd go for it; but for a
manually adjustable tuner, vacuum caps are silly. Also, if the actual
circuit design requires 1700pF max (seems like a lot for an input cap) and
you parallel fixed cap(s) across a 500pF variable, you don't have nearly the
tuning range that a 1700pF variable would have, unless you use lots of small
shunt caps and switch them in and out -- sounds like a lot of bother.
-WB2WIK/6
> My question is: Would "doorknob" caps be a good choice for this added
> capacitance? If so, any particular type?
>
> TNX, Barrie, W7ALW
>
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