Straight-Line Frequency Air Variable Capacitors

[aa5wg]

Hi Everyone:
Can anyone recommend or do you know of a simple to use Windows software program to design straight-line frequency air variable capacitors?
It would be nice to design straight-line wavelength and straight-line capacitance air variable capacitors too.

Thank you for your kind consideration.
73,
Chuck Pool - AA5WG
Cedar, Michigan

[KA4DPO]

I have never found a straight line air variable. Cardwell made some that worked OK at frequencies below 7 MHZ but even they weren't perfect. I wish you luck in your efforts and let us know how well you do. Theoretically it should be possible but in practise that hasn't been my experience.

[ka5s]

What DPO says.

In practice you will also need to slot the outer rotor plates and adjust the sections to deal with nonlinearities and parasitic capacitance.


Cortland
KA5S

[AG3Y]

On the other hand, I have seen some FM broadcast tuners ( the REL Precedent comes to mind ) that used straight line inductors consisting of copper foil wound on a glass tube, with a ferrite slug running through the center of the tube. The straight line frequency characteristic was achieved by winding the coil of foil in a non-linear fashion, with the turns closer together at one end, and spreading apart at the other. Somewhat like some roller inductor antenna tuners I have seen. It would seem that would be easier to design than a cap with straight line characteristics.

I have seen some caps that have almost an exaggerated airfoil cross-section as opposed to the semi-circle form. I wonder what characteristic these caps have. Unfortunately, I have never seen one in a radio and compared the dial with the shape of the caps.

I remember when Heathkit sold high-power antenna tuners with "build-it-yourself" capacitors. The plates, spacers, etc. were all stacked together in a certain order to achieve a multi-plated tuning cap !

It sounds like it could be a fascinating project!

My thought, plug the formula into a spreadsheet, and then make a graph of the results, and see if it looks like a capacitor plate ! It just might !

Worth a try ! 73, Jim

[ka0gkt]

Besides stability, the reason Collins gear used a PTO (Permiability Tuned Oscillator) was to provide a linear sliderule dial.

73 DE KAØGKT/7

--Steve

[VK2TIL]

Chuck's question is also over in Q & A;

http://www.qrz.com/ib-bin....=163496 (http://www.qrz.com/ib-bin/ikonboard.cgi?act=ST;f=5;t=163496)

Any mathematical formula will always be an approximation; Mother Nature doesn't always follow our "human" rules.

I thought the question interesting so I did a bit of Googling. I found this;

http://www.quadibloc.com/science/freqint.htm

A good overview of the subject.

Like Jim, I've often been intrigued by the "airfoil"-shaped plates on older variable condensers (to give them their "vintage" name http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif ).

Later ones are not so extremely-shaped; many are just a "flattened-D" shape. They often have the slots mentioned by Cortland.

The shaft is usually offset and this must help with obtaining a near straight-line characteristic.

[AG3Y]

I just picked up an old WWII Philco SW-BC console. #The lower half of one of the SW dials covers 6 to 9 Mc ( old terms ) and the upper half of the dial from 9 to 18. # The other SW dial is 1.5 to 2.1 and the upper half 2.1 to 3.5. Not exactly linear, I would say! #Wouldn't you know it would miss the whole 80/75 meter bands, including the #AM phone frequency ! # The high SW band will be great for 40 meter SWBC, though ! #BTW, the plates are semi-circles.

Oh, BTW, I once read how to use those slots in the plates to "trim" the bandspread. A bit involved, consited of bending the plate sections closer together or farther apart, and you had better not do it too many times, for fear of fatiguing the metal !

73, Jim

[aa5wg]

kA5S, Jim and Steve:
If I know the required inductance and capacitance for all the HF bands than how would I use these numbers to come up with a straight-line frequency capacitor? That is, how would I find the shape of the rotor and stator plates for above straight-line frequency capacitor?
Thank you.
Chuck AA5WG

[aa5wg]

kA5S, Jim and Steve:
If I know the required inductance and capacitance for all the HF bands then how would I use these numbers to come up with a straight-line frequency capacitor? That is, how would I find the shape of the rotor and stator plates for above straight-line frequency capacitor?
Thank you.
Chuck AA5WG

[N0WVA]

Quote[/b] (AG3Y @ July 29 2007,18:19)]On the other hand, I have seen some FM broadcast tuners ( the REL Precedent comes to mind ) that used straight line inductors consisting of copper foil wound on a glass tube, with a ferrite slug running through the center of the tube. # The straight line frequency characteristic was achieved by winding the coil of foil in a non-linear fashion, with the turns closer together at one end, and spreading apart at the other. #Somewhat like some roller inductor antenna tuners I have seen. # It would seem that would be easier to design than a cap with straight line characteristics. #
Ive seen this on AM radios for cars,too. Gobs of turns on one end of the slug tuned coil, and tapering off to a few on the other.

[VK2TIL]

Yes, the slot sections were bent for adjustment; a tricky business but, in the "analog" age, people became adept at this kind of thing.

Let's say that you wanted a bit more C at the low-C end of the capacitor's range; you would bend the section at that end in a bit.

Considering that there were at least two capacitor sections and perhaps three slots, that would have been an exercise in patience.

Hand-calibrated dials were a natural adjunct to this process.

[N0WVA]

Quote[/b] (VK2TIL @ July 30 2007,19:20)]Yes, the slot sections were bent for adjustment; a tricky business but, in the "analog" age, people became adept at this kind of thing.

Let's say that you wanted a bit more C at the low-C end of the capacitor's range; you would bend the section at that end in a bit.

Considering that there were at least two capacitor sections and perhaps three slots, that would have been an exercise in patience.

Hand-calibrated dials were a natural adjunct to this process.
Im pretty good at that sort of thing. One of the first things I do when I get a boatanchor is calibrate the dial.

There is also a trick in getiing the l/c combination just right. You can "spread out" or "squeeze" the dial by that combination. I once had a National receiver that could not get enough inductance to cover the frequency range I needed. It resulted in giving too much trim capacitance, so the minimum capacitance fell and I couldnt get the whole "band" on the dial.

The problem was the core material lost its inductance over age , and the solution was re-winding the coils with a few more turns of wire.....After getting it close that way, then I would "tweak" those little vanes on the variable capacitor, even sometimes twisting them a little.