logarithmic variable capacitor for tuning?

Thread Starter

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?

1621786998709.png
This log-cap seems to suggest by it's construction that
it capacitance increases significantly as it is rotated
clockwise.
 

Thread Starter

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,
it appears to be anti-log in function.)
 

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,
it appears to be anti-log in function.)
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.
 

Thread Starter

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.
 

Thread Starter

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
 
Last edited:

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”.
 

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.
 
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