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 Post subject: Vacuum variable cap in HB regen?
PostPosted: Oct Fri 20, 2017 2:31 am 
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Joined: Jan Mon 28, 2013 9:35 pm
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Location: Santa Rosa, CA
Thinking about the next regen project. Part of the trick to building a
top performer is using high quality parts and of course
mechanical layout. It seems like a vac variable should work very
well.

Has anyone tried this? Most of these type of caps are quite large
and meant for use in transmitters. Do anyone know if physically
smaller caps were made?
Steve


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 Post subject: Re: Vacuum variable cap in HB regen?
PostPosted: Oct Fri 20, 2017 4:56 am 
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Why would you expect a vacuum cap to work better than an air variable? If you're concerned about oxidation of the plates causing a super tiny loss in the Q of the tuned circuit, you could use a silver-plated air variable. They're fairly commonly available -- they were used in higher quality test equipment and military gear.

Air in itself should not introduce any loss in a variable capacitor.

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 Post subject: Re: Vacuum variable cap in HB regen?
PostPosted: Oct Fri 20, 2017 5:03 am 
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Tom Albrecht wrote:
Air in itself should not introduce any loss in a variable capacitor.
True, but humidity could.

I've not personally seen any vacuum variables small enough for use in a receiver. There are some "relatively" small ones, but "small" only relative to the 8"-diameter ones.

- Leigh

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 Post subject: Re: Vacuum variable cap in HB regen?
PostPosted: Oct Fri 20, 2017 5:17 am 
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Interesting! Hadn't thought about water vapor. Polar molecule, so it sloshes just a little in response to RF, which will pull a little energy out and heat the air. Or were you thinking about a little bit of lossy surface conduction due to water film on the surface of the plate? I suppose either could be responsible for a little tiny loss.

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 Post subject: Re: Vacuum variable cap in HB regen?
PostPosted: Oct Fri 20, 2017 5:39 am 
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Hi Tom,

I was thinking more abstractly, simply about the desire to eliminate humidity and other contaminants from the device by encapsulating it in a vacuum chamber.

Air contains lots and lots of "stuff", water vapor being just one example.

- Leigh

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 Post subject: Re: Vacuum variable cap in HB regen?
PostPosted: Oct Fri 20, 2017 3:02 pm 
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Chances are you will have more real-world losses in the inductor than in the cap. There are decent variable caps from old military gear available.

Peteh

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 Post subject: Re: Vacuum variable cap in HB regen?
PostPosted: Oct Fri 20, 2017 3:05 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 820
Location: Santa Rosa, CA
Tom Albrecht wrote:
Why would you expect a vacuum cap to work better than an air variable? If you're concerned about oxidation of the plates causing a super tiny loss in the Q of the tuned circuit, you could use a silver-plated air variable. They're fairly commonly available -- they were used in higher quality test equipment and military gear.

Air in itself should not introduce any loss in a variable capacitor.


VVC's offer a number of advantages. They are multi-turn (15) which allows
for less touchy tuning as compared to air variables which only offer
180 degrees from min to max capacitance. VVC's have the highest Q of
any variable cap I know of. They are also less prone to environmental
effects (due to vacuum and large glass or ceramic insulation). What I
don't know is how sensitive they are to temp changes and if they
are straight line capacitance, SL frequency or SL wavelength. Probably
the first which will bunch up tuning at the max capacitance end. I have
a number of VVC's here and the smallest are 2" dia, 3.5" long (ceramic).
Hoping to find something smaller. One last advantage is many VVC's
offer a very wide range of capacitance due to their multi-concentric
meshing plates.

I've been doing a lot of experimenting with regens and the biggest
weakness of most are due to the tank circuit. Standard 180 degree
air variable are the worst IMO and the ones most seem to use. The best
VC assembly I've found so far is the one used in the BC-221/LM
freq meters. These are SLF caps to boot which makes them nearly
ideal. The one problem they have is with the wiping contacts.
They create noise, much like a noisy pot due to the high gain found
in a regen detector. There is a way around this but haven't tried it yet.

So to answer your question Tom, I don't know for certain VVC's will
be better but the only way to know for sure is to try them. As far
as I know no one else has.
Steve


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