[HBR] Under construction HBR-??

[hbr@mailman.qth.net]

In a message dated 3/15/04 9:56:11 PM US Eastern Standard Time, 
Bill@LizCurtisHiggs.com writes:

> 
> The HBR-?? ( who knows how many tubes this thing will end up with?) 
> continues under construction both on paper and on metal. Having bowed to the sage 
> advice of Walt and others, I have mounted the variable capacitor from my poor 
> cannibalized BC-453 on the front panel. I was able to obtain a 454 cap from a 
> fellow mad scientist (!), I compared the two side-by-side. Much to my surprise 
> the plate shapes are identical, the caps only differing in the number of 
> plates, and therefore the total capacity. With this in mind, I'm either going to 
> go with a high c oscillator circuit, or put a series mica in line, and then 
> live with whatever nonlinearity I encounter. After looking through one of the 
> early "Single Sideband for the Radio Amateur" books, it seems that several 
> of the fellas used the 454 cap for roughly a 5.2-6.2 Mc tuning range, and the 
> pix showed a dial with reasonable linearity (no worse than about 2:1). I 
> played around a bit with my 453 cap, a toroid, and an FET (shudder!), and was 
> able to get about a 3:1 linearity (tightest 100kc was about 1/3 the space of the 
> widest 100kc) while tuning 5050-5750. Output was a bit lower than expected, 
> so the high C circuit might not work (either in tube or FET configuration). 
> The final receiver, of course, will use a tube.
> 
> 

Bill,

The following is something I posted back in February.  I couldn't find an 
archives on the web for this list, so I had to type it again.

I got involved in designing a linear tuning capacitor VFO about 30 years ago 
while in college.  Here is what I came up with for 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 the 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.

If you use a BASIC program or MATHCAD, etc., to plot the shape of the plates 
it is very enlightening.
The smaller the frequency range, the more nearly circular the plates need to 
be.  For a small range like a ham band the plates look almost like a circle 
with the shaft offset from the center.

Dan K9WEK

Please don't be offended with all of this.  This will give you the shape of 
the plates, the real problem is finding or modifying a capacitor to have this 
shape.  Once you see what it should look like though, it gives you a starting 
point.  You can also play games like changing the frequency ratio until the 
shape matches a capacitor you have.

If there is something you don't understand about the above info maybe I can 
help clear it up some.

Thanks,
Dan K9WEK


 


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