[ARC5] R-25 dial nonlinearities

[Bruce Long]

I gave this issue a lot of thought many years ago and even wrote a draft magazine article.

For any given tuning ratio Fmax/Fmin there is a unique tuning capacitor plate shape.  As the tuning ratio decreases to a value of one, the tuning capacitor plate shape converges to semi-circular.  (Assuming there is not a fixed capacitor in series with your variable cap) your problem comes about because the modified variable capacitor has the "wrong " plate shape for the current tuning ratio.


You could grind the plates to a more circular shape--- not easy, not fun
You could add an an inductor having the "right" value  in series with the tuning capacitor that would have the effect of increasing the tuning rate at the top of the band.  This would improve the tuning linearity but also change both the tuning ratio and the center tune frequency.  I would use a computer to determine component values first as this would be very difficult to get to work using cut and try methods.



It is important that you determine if the person who butchered the tuning capacitor variable cap also added fixed capacitors in series with the variable cap in order to further reduce the tuning range.  If he did remove these capacitors.  Removing these series caps will improve the linearity, shift the tuning range and increase the tuning ratio.  You will need to retune to the desired tuning range by changing the values of the other LC tank circuit components.
Assuming no tuning ratio limiting series capacitors I think your only option would be to  remove even more tuning capacitor plates to narrow the tuning ratio to the minimum needed then adjust the other LC tank component values for the desired frequency coverage.   With narrow tuning range, RF-LO tracking problems decrease in proportion.   Tuning linearity will never be great but it will improve.






      From: J Mcvey via ARC5 <arc5 at mailman.qth.net>
 To: "ARC5 at mailman.qth.net" <ARC5 at mailman.qth.net> 
 Sent: Saturday, January 7, 2017 7:57 PM
 Subject: [ARC5] R-25 dial nonlinearities
   
I got around to marking the dial on the "bandspread" R-25 .(This is the one that someone had pulled the plates out of the variable cap.)It tunes from just below 1700 to a bit over 2000.The 100khz spans on the dial increase in size as it progresses to 2000 to the point where the distance between 1900 and 2000 is about twice the 1700 to 1800 span.
Is there an easy way to correct it or it that just the way it's going to be?
The interaction of L and C vs dial calibration has always been my nemesis!


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