Any good references for variable capacitor design?

[Ady2017]

I have recently decided to put together a 1930's style crystal radio as a father-son project to get him started in electronics. The lack of availability of now-obsolete variable capacitors is causing a big headache. From what I have seen on the internet, a number of people have resorted to making their own variable capacitors but have not given any details about how they designed them. Does anybody have any old references for the design of variable capacitors which includes details such as plate spacing, plate size, number of plates, best materials to use, etc.

[stj]

Re: Any good references for variable capacitor design?

your question will relate to the range of capacitance your after

did you try ebay/aliexpress for a crystal radio kit??
the chinese are really stepping up in a big way with eductational kits since radioshack died.

[petehall347]

Re: Any good references for variable capacitor design?

try searching tuning capacitor on ebay and google .. i see an an abundance of them

[eccerr0r]

Re: Any good references for variable capacitor design?

capacitance has always been number of "active plate surfaces" times dielectric constant of air times area divided by distance between plates. Not much to it to make an air capacitor. Note that the plate material is immaterial as long as it conducts electricity. However the capacitance you need is dependent on the rest of the design, most specifically the corresponding inductor coil or antenna coil and what frequency you're trying to tune.

The reason why the old tuning capacitor looks funny is the effort the designers made to make the dial look linear. Making a makeshift capacitor for a homemade radio, this doesn't really matter.

[petehall347]

Re: Any good references for variable capacitor design?

when i was building crystal sets i knew nothing about pf values i just threw them together and they worked fine . i did however know the differences of am and fm tuners though .

[redwire]

Re: Any good references for variable capacitor design?

Old crystal sets are always tuning using the (slide) inductor. I've never seen a homemade variable capacitor. They are treated as a component you buy.
The common value of 365pF is a lot of capacitance to make, you need a multi-ganged rotor and too much mechanical ballyhoo.

Internet Archive - you can look through old radio magazines going back https://archive.org/search.php?query...=TXT&sort=date but I couldn't find a way to filter just old i.e. 1910-1950's. Many old radio magazines showing how to make one, but none have a DIY variable capacitor.

edit: surprised to see some countries taxed people with crystal radios, I think to help cover broadcast costs.
1922 crystal radio kit $4.75 or $84 in today's dollars. Not sure about the difference in wages though.

[CapLeaker]

Re: Any good references for variable capacitor design?

Is it possible for the op to post the project? Maybe scan it and upload it here?

[petehall347]

Re: Any good references for variable capacitor design?

wish i still had the book i used all them years ago .. all simple stuff really . i used to try different windings around a ferrite core and capacitors connected to an external antenna for selectivity and sensitivity .. its been a long time since i played around with simple radios. 50 years and more . or was it cap to copper water pipe ? i must have driven my dad and brother mad with all the wires etc .

[eccerr0r]

Re: Any good references for variable capacitor design?

indeed the main problem is the plates need to be quite rigid and any slop that bends the plates will change the capacitance and cause your frequency to go off.

I think for the most part crystal radios are so nonsensitive and nonselective you're only going to get local stations and you don't even need to be tuned just right, and yes tweaking the inductor is probably easier/more stable. been a while since I played with a crystal set like with the radio shack spring terminal radio kits, but indeed they weren't that great... at least they got the inductor and capacitor just right to tune in the AM 300m band.

[petehall347]

Re: Any good references for variable capacitor design?

i remember adding different capacitors to the earth and the antenna for selectivity and sensitivity .

[eccerr0r]

Re: Any good references for variable capacitor design?

are we talking actual capacitor design, or tuner/antenna design, makes a difference here...
if you don't have any variable capacitors I'd think you're looking at the former. If you already have a pile of variable capacitors, then the latter is the issue?

I recall this one rat shak radio kit that actually seemed to be a bit better in terms of at least sensitivity, it was a regenerative receiver instead of the usual crystal + amplifier. Selectivity wasn't as good as standard off the shelf receivers, but it was able to pick up stations without the usual ground wire and really long antenna.

Either way if you don't have a variable capacitor at all, it'd be mighty hard to tune, at least the capacitance side. You can still tune the inductor side...

[desert-rat]

Re: Any good references for variable capacitor design?

I got a var cap off Amazon. Took a couple of months to get it. If you look at electronics suppliers they are available. As I remember the cap was around 365 pf . Don't remember the micro Henries of the coil . But it's easy to find. You can also use a fixed cap and a mid taped coil. I have seen a few videos on YouTube on how to..
PS ,all you really need are 2 metal plates . One fixed the other is movable.

[eccerr0r]

Re: Any good references for variable capacitor design?

Or you can just move them closer/farther apart (which unfortunately is very nonlinear and capacitance drops very fast as you move farther apart, making frequency even harder to tune.)

The "only" two metal plates have to be very big to get appreciable capacitance and very stiff to make sure they don't touch each other...which becomes a very good antenna for picking up 50/60Hz...

BTW the tiny variable capacitors for full band tuning purposes use non-air dielectric to increase its capacitance in such a small volume...

[stj]

Re: Any good references for variable capacitor design?

the caps in am radio's dont move plates, they move mylar film between the plates