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> Variable Capaictor., how many watts?
Skeith
Posted: March 15, 2012 05:50 am
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I was wondering, How many watts of RF would a typical variable cap from an old AA5 radio or similar be able to handle?

I am working on experimenting with a magnetic loop antenna and am trying to select a capacitor to tune it.


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Geek
Posted: March 15, 2012 06:11 am
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Caps are rated in volts, not watts.

At resonance, E=I*Z. Off resonance, reactances come into play and things get complex real quick.

If you're talking magnetic loop, those hare nasty hard on caps, as the smaller the loop, the higher the voltage. A 160M loop that can fit on a city lot can easily see a good 10KV across the cap before you come near legal power.

With larger loops, they start to carry lotsa current and less voltage.

Seems all my bookmarks for magnetic loop calculators are now bad sad.gif There's a program for this, give a Google smile.gif

Cheers!


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Colt45
Posted: March 15, 2012 07:31 am
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The voltage is about the air spacing on them - which is why bigazz RF amp stuff has 3/8" airgap between the plates, instead of 1/16"...

Anyway, on AA5's they're usually in areas with almost no bias and not much RF either, hell, the whole sets run on 100V... although I'd still think they're probably good for 500V? I don't know.

At least in the desert where we live, maybe not in sub-tropical cascadia tongue.gif

Keep in mind the trim/padding caps on the side might not have the same umpfh, and also make sure you don't have the shaft hot, unless you've got a good insulated knob for it..

I'm thinking of 40's octal sets too, late run mini tube ones with PC boards shrunk the caps down somewhat..

I got a big honking one at the local surplus joint, I can check if they have some more sometime...


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Geek
Posted: March 15, 2012 11:34 am
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QUOTE (Colt45 @ March 14, 2012 11:31 pm)
At least in the desert where we live, maybe not in sub-tropical cascadia tongue.gif

laugh.gif thumbsup.gif

Indeed! 50% derating for 100% humidity 10 months of the year.


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CWB
Posted: March 15, 2012 02:33 pm
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yep ... normally the major concern is breakdown voltage .
there are cases where dissipation is/can be a factor ...
tuning the tank circuit in a decent sized output rf final or the ATU for a high powered transmitter .


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MacFromOK
Posted: March 15, 2012 02:58 pm
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You can submerge it in mineral oil and increase both voltage and capacitance. beer.gif

"A 100 to 365pF variable capacitor with a 1KvDC breakdown voltage (i.e. a plate spacing of 1mm) becomes a 270- to 985pF unit wit 7500 VDC breakdown rating."

From this page:
http://www.sentex.ca/~mec1995/circ/hv/hvcap/hvcap.html


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Sch3mat1c
Posted: March 15, 2012 04:19 pm
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I think the average radio size cap is 500 or 1000V rated. Should be able to put more than a few amps through them, which translates not to real power, but to reactive power. I mean, you can see the whole thing and it's all metal, so there should be no problem sinking 20A or more through one -- that's a good 10kVAR right there, but you probably can't even push it to melting because the capacitance is small, so the frequency would be high, and parasitic inductance would become an issue (the rotor mounts themselves would have notable inductance at low UHF where you'd actually be able to draw that much current through the capacitance!).

Tim


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tekwiz
Posted: March 15, 2012 10:20 pm
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AA5 caps were the 'load' cap of choice for the little 120W CB linear amps I used to make. Single sweep tube, ~700VDC B+, 52Ω output impedance.
So, yeah, figure on 750-1000 volts, for a typical cap with ~.020" plate gap.


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