[Hallicrafters] air variable capacitors

[Julian Bunn]

But capacitance is inversely proportional to the distance between the 
plates.
So if the distance between the stators is d, then the capacitance
with the rotor plate in the centre is

C = e.A/(d/2) + e.A/(d/2)

where e is permittivity, A is area of the plates, and we have
two capacitors, one on each side of the rotor.

Thus C = 4eA/d

Then if you move the rotor closer to one stator, say to d/3, you have a d/3
spacing on one side, and a 2d/3 spacing on the other, which gives

C = e.A/(d/3) + e.A/(2d/3) = 4.5 eA/d

i.e. the capacitance is increased. Not a net difference of zero.

Is that right? My A-level physics may be a bit rusty :-)

Julian

jeremy-ca wrote:
> Capacitance would be increased to the closer plate and decreased to 
> the other. Net difference is zero.
>
> One place where it does make a difference are the end plates. Tweaking 
> those is often used for tracking adjustments and later caps often had 
> segmented end plates to make it easier.
>
> Carl
> KM1H
>
>
>
> Carl
> KM1H
> ----- Original Message ----- From: "Ginger Harris" 
> <elvispharris at rtmc.net>
> To: <Hallicrafters at mailman.qth.net>
> Sent: Thursday, December 06, 2007 5:43 PM
> Subject: [Hallicrafters] air variable capacitors
>
>
>> Since everything is so quiet, I might as well ask a question.  Is it  
>> safe to assume that  in a receiver, the rotors are always centered  
>> between the stator blades at the factory?  Could there be an  
>> exception among  those big spaced capacitors of the 1920s?  It seems  
>> that off center spacing would increase the capacitance a little.  I  
>> am puzzled.  Thanks and an early Merry Christmas,   JP    KI4ZVX 
>> ______________________________________________________________
>> Hallicrafters mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/hallicrafters
>> Help: http://mailman.qth.net/mmfaq.html
>> Post: mailto:Hallicrafters at mailman.qth.net
>> ----
>> List Administrator: Duane Fischer, W8DBF **for assistance**
>> dfischer at usol.com
>> ----
>> HHI Halligan's Hallicrafters International and www.w9wze.net
>>
>
> ______________________________________________________________
> Hallicrafters mailing list
> Home: http://mailman.qth.net/mailman/listinfo/hallicrafters
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:Hallicrafters at mailman.qth.net
> ----
> List Administrator: Duane Fischer, W8DBF **for assistance**
> dfischer at usol.com
> ----
> HHI Halligan's Hallicrafters International and www.w9wze.net
>