# logarithmic variable capacitor for tuning?

#### Hamlet

Joined Jun 10, 2015
448
I'm contemplating experimenting with radio receivers. To my understanding, when tuning, the variable capacitor often squashes the upper bands, tuning gets fiddly, as smaller and smaller increments of rotation of the variable-cap have large changes in freq. selection (or maybe I have it the wrong way around)?

Anyway, my question(s) is, would it be advantageous to construct a tuner with the variable-capacitor having logarithmic characteristics? And if so, where should the capacitance have the greatest change when moving up in freq., at the bottom or top of the dial?

This log-cap seems to suggest by it's construction that
it capacitance increases significantly as it is rotated
clockwise.

#### DickCappels

Joined Aug 21, 2008
7,434
Bottom of the dial as illustrated in the photograph you posted.

#### Hamlet

Joined Jun 10, 2015
448
Bottom of the dial as illustrated in the photograph you posted.
Thanks Dick, I think you answered my question, but I'm still confused.

Generally, do we want lots of capacitance at lower
freq., with wide swings of cap. at small rotations, & then, with the high end of the dial
experiencing tiny amounts of capacitance, with small changes for larger
rotations of the dial?

(Presuming we want an intuitive tuning, a clockwise rotation would have decreasing
capacitance. It seems the picture I provided would need gearing to correct, as natively,

#### Yaakov

Joined Jan 27, 2019
3,155
Thanks Dick, I think you answered my question, but I'm still confused.

Generally, do we want lots of capacitance at lower
freq., with wide swings of cap. at small rotations, & then, with the high end of the dial
experiencing tiny amounts of capacitance, with small changes for larger
rotations of the dial?

(Presuming we want an intuitive tuning, a clockwise rotation would have decreasing
capacitance. It seems the picture I provided would need gearing to correct, as natively,
The intuitive quality of the direction will depend on whether you tune by wavelength or frequency.

#### MisterBill2

Joined Jan 23, 2018
8,761
The reason is explained by the formula for the resonant frequency, and by the fact that as the capacitance is reduced, a change of any fixed increment becomes a larger part of the whole capacitance.
AND, for any given frequency tuned to, a fixed increment change in frequency becomes a larger portion of the actual frequency.

#### Hamlet

Joined Jun 10, 2015
448
I don't have many large variable caps on hand, so I had a go at making
a multi-leaf style variable-cap that uses a screw-press arrangement with threaded
rod to increase capacitance. The ratio came out good (35:1), and logarithmic, but I anticipate
fussy operation because of the short range of motion, and right-hand threaded
rod will make for a counter-intuitive anti-log tuning.

#### MisterBill2

Joined Jan 23, 2018
8,761
I don't have many large variable caps on hand, so I had a go at making
a multi-leaf style variable-cap that uses a screw-press arrangement with threaded
rod to increase capacitance. The ratio came out good (35:1), and logarithmic, but I anticipate
fussy operation because of the short range of motion, and right-hand threaded
rod will make for a counter-intuitive anti-log tuning.
Use a string and pulley arrangement like tuning some older radios and reverse the rotation that way. Cheap but complicated. OR use a gear drive. A couple of gears from a failed mechanical clock.

#### anniel747

Joined Oct 18, 2020
1,063
Use a string and pulley arrangement like tuning some older radios and reverse the rotation that way. Cheap but complicated. OR use a gear drive. A couple of gears from a failed mechanical clock.
Meccano gears are cheap and work well.

#### schmitt trigger

Joined Jul 12, 2010
440
I remember the days when those things were ubiquitous.
They have gone the way of the dinosaurs.

#### Hamlet

Joined Jun 10, 2015
448
I remember the days when those things were ubiquitous.
They have gone the way of the dinosaurs.
Yes, meccano gears are expensive on ebay, a collector's item it seems,
and so are variable capacitors.

I'm boiling up some pcbs to make guillotine style variable cap. right now.
If that isn't suitable, a trombone variable is in the cards.

#### sparky 1

Joined Nov 3, 2018
543
basic radio builder open 15pF in the low and medium bands like the picture or 350pF when the log shaped fins are
closed. Below 15pF the hand capacitance gets touchy. Example I want Xl=XC with a reactance of 1000 ohms 50pF 50uH 3.18 MHz
If I adjust to 15pF gives 5.8 MHz the reactance now 1825 ohms at 75% 260 pF 1.4 MHz reactance 438 ohms
The reactance is up to you. A combination of weak signal and accurate tuning giving up bandwidth for better selectivity.
I like to start design at 50pF but it might end up at 25pF and that is your high range while the more accurate tuning at 350pF a station of interest might be at 75% its cheaper to make round plate capacitors but logs are nice

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#### MisterBill2

Joined Jan 23, 2018
8,761
Adequate isolation / shielding will avoid the hannd capacitance effect, but iit does require extra effort. But that is why some oof those reallyold radios had the tuning capacitor inside a steel box.

#### Yaakov

Joined Jan 27, 2019
3,155
Adequate isolation / shielding will avoid the hannd capacitance effect, but iit does require extra effort. But that is why some oof those reallyold radios had the tuning capacitor inside a steel box.
And,

#### MisterBill2

Joined Jan 23, 2018
8,761
Isolation couplings are certainly helpful, and very much required when both sides of the capacitor are at some high voltage level. So thanks for the reminder. That is quite the collection!

#### Yaakov

Joined Jan 27, 2019
3,155
Isolation couplings are certainly helpful, and very much required when both sides of the capacitor are at some high voltage level. So thanks for the reminder. That is quite the collection!
Not mine, unfortunately. I did have quite a box of tuning caps and couplers but they… are gone. Years of apartment living and many moves took a toll on the boxes of stuff I couldn’t ”justify”.

#### MisterBill2

Joined Jan 23, 2018
8,761
I am aware oof how cruel life can be.

#### schmitt trigger

Joined Jul 12, 2010
440
Those General Radio test equipment pieces were one step above everything else.

#### Yaakov

Joined Jan 27, 2019
3,155
Those General Radio test equipment pieces were one step above everything else.
I am going to start a separate thread about that capacitor. I just tested it and while the maximum value doesn't match the calibration tag, the dial reads precisely .001nF increments on my meter, as it is supposed to do. Amazing.

#### anniel747

Joined Oct 18, 2020
1,063
Yes, meccano gears are expensive on ebay, a collector's item it seems,
and so are variable capacitors.

I'm boiling up some pcbs to make guillotine style variable cap. right now.
If that isn't suitable, a trombone variable is in the cards.
Try the Meccano website, the guy that answers is from Borgfeldt and the gears cost a buck or two depending on size. Also RC gears come useful at times. Otherwise SDP/SI also has gears. There is another good place but their name evades me for the moment, maybe someone can chip in.