[HBR] Re: Linear Capacitor Tuning

[hbr@mailman.qth.net]

I thought the listing I posted the other day may be of interest to some of 
the HBR group, but I haven't heard anything out of anyone.

This was one of those projects which I put quite a lot of effort into back 
when my calculus skills were quite a bit sharper than they are now.  I'm pretty 
sure the equation is valid, though.  For example, I did the shape of a linear 
tuning capacitor for the AM broadcast band and got a pretty neat looking 
shape.  For just the 80 meter band, simply offsetting the plates about 1/16th to 
1/8th inch gives nearly linear tuning with circular plates.

Let me know if you try it and/or find it interesting.  TNX, Dan K9WEK

> I got involved in this about 30 years ago.  I must admit that I never put 
> the 
> following into a practical project, but here is the theoretical shape of the 
> 
> plates of a linear-with-frequency tuning capacitor:
> 
> R=SQR((-4*K)/(H*(K*A+F2)^3)+C^2)
> 
> Where:
> R == Radius of plates as a function of rotation (inches)
> A == Angle of rotation (radians) -- the variable in the equation
> K == Rate of frequency change with rotation -- (Hz/rad)
> F2 == Higher frequency (freq with plates unmeshed) (Hz)
> C == Radius of cutout in stator to clear shaft (inches)
> H == Ck/(F1^2*C1)
>     Where: Ck == Farads per square inch of capacitor used -- this accounts 
> for the plate spacing and number of plates
>         F1 == Lower frequency (Hz)
>         C1 == Capacitance at F1 (Farads)
> 
> The equations look much simpler when you put them in standard math notation.
> 
> Since this is only theoretical, fringing of the electric field is neglected, 
> 
> but this should have only a minor effect.
> 
> Now, does someone know how to build a capacitor once you design it?
> 
> Anyway, it is fun to see what the capacitor you want looks like.  Remember I 
> 
> did this back before home computers.  I had a TI SR-10 calculator which was 
> great because you could do square roots on it!
> 
> Dan K9WEK



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