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Author Topic: Conductive grease for homebrew butterfly capacitor  (Read 2968 times)
KF7ZFC
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Posts: 124




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« on: March 13, 2018, 10:26:27 AM »

I am building a butterfly capacitor for a 14 to 30 mhz mag loop. I live in Arizona where it is dry though we have heavy short rains in our summer.

Several sites took about TIG welding to keep the resistive losses low. I can't do TIG welding.  No equipment or knowledge.

Are there  any conductive "greases" I can use to keep the resistive losses low?
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G3RZP
Member

Posts: 168




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« Reply #1 on: March 13, 2018, 10:58:48 AM »

Surely the idea of using a butterfly capacitor is that there isn't any current through the bearing? To keep the Q up, use brass or copper sheet for the vanes and make sure they are at least .007 inches thick to minimise skin resistance.
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WD4HXG
Member

Posts: 325




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« Reply #2 on: March 13, 2018, 03:32:47 PM »

TRW built runs of Linear Master Oscillators
for the Heathkit SB receiver and transceiver
lines which utilize conductive grease at the
shaft bearings of the variable cap they used.
It was notorious for losing volatiles and
congealing causing crashing when tuning the
radio. I spoke with a Heath tech in the late
70's when mine started giving problems and
he advised me of the problem. He stated it
contained silver particles which increased the
conductivity between the shaft and mount
thus allowing the Q to increase. The grease
turned into a big problem for owners as it
was not readily available and Heath did not
provide instructions on getting into the LMO.

I did open mine and used white lithium grease
to add to the existing lube on the shaft ends.
It solved my problem insofar as I could tell but
I also had no metrology to measure the phase
noise of the LMO nor the residual FM of the
oscillator at the time. It stopped the crashing
sound but it is up in the air if it improved
performance which I could not measure.

MG Chemicals sells 8463-7G Carbon Conductive
Grease in a syringe. They mention it is used
with o-rings were conductivity is needed. They did
not say if it was for thermal conductivity.


Also conductive lube with copper particles is
available. See the PDF on Electrolube's
product here:

https://www.electrolube.com/core/components/products/tds/044/HCG.pdf

https://www.digikey.com/catalog/en/partgroup/circuitworks-series-silver-conductive-grease/1199

https://www.2spi.com/catalog/documents/SilverGrease-04993-AB.pdf      (This one claims it is mostly conductive silver)
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K8BYP
Member

Posts: 185




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« Reply #3 on: March 21, 2018, 01:44:46 PM »

1 Grease is an INSULATOR Why put an insulator between conductors?
2. Conductive grease is a hoax, its insulating grease with metal particles that will play grinder..
3. MUST solder or weld, especially with RF current, even using similar metals, oxidation and corrosion are severe in antenna systems. Electrical current wrecks mechanical joints especially at VHF.
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KG4RUL
Member

Posts: 3207


WWW

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« Reply #4 on: March 21, 2018, 07:29:40 PM »

I believe that you are asking about fastening aluminum plates to the shaft without the use of a TIG welder?  You could try an aluminum solder provided the shaft is brass or aluminum.  The bearings do not conduct any current and using conductive grease on them would be of no benefit.
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N7EKU
Member

Posts: 872




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« Reply #5 on: March 22, 2018, 07:50:13 AM »

I am building a butterfly capacitor for a 14 to 30 mhz mag loop. I live in Arizona where it is dry though we have heavy short rains in our summer.

Several sites took about TIG welding to keep the resistive losses low. I can't do TIG welding.  No equipment or knowledge.

Are there  any conductive "greases" I can use to keep the resistive losses low?

Hi,

I would build the loop and capacitor out of copper and brass.  This way you could solder everything and not worry about the joints.  I would do the moving vanes and stationary plates of the capacitor all out of bass since it would be easy to obtain sheet brass and brass bolts and nuts.  The frame could be aluminum for strength, and the loop out of copper pipe.

73,


Mark
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Mark -- N7EKU/VE3
K8BYP
Member

Posts: 185




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« Reply #6 on: April 19, 2018, 09:46:55 AM »

Conductive grease is 90 percent insulating grease so how is that "conducting?"

Absolutely not, the connections MUST be soldered or welded, bonding is not enough with RF currents.

Sadly, Aluminim is a pig to work with and oxidizes horribly.

RF currents greatly accelerate corrosion and oxidation.

I use galvanized steel tubing, soldering to the galvanized coating. Not easy but it works and is cheap.
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K1ZJH
Member

Posts: 3660




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« Reply #7 on: April 19, 2018, 11:57:42 AM »

Surely the idea of using a butterfly capacitor is that there isn't any current through the bearing? To keep the Q up, use brass or copper sheet for the vanes and make sure they are at least .007 inches thick to minimise skin resistance.

Worth rereading. Bearing and shaft should be floating.
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N4MQ
Member

Posts: 206




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« Reply #8 on: April 19, 2018, 05:21:26 PM »

I have built a few mag loops and the largest is 1 kw rated on 160 n 80 meters.

To build a proper cap from scratch, you have no current to the frame/shaft.  Each half of the loop connects to half of the stationary plates of the cap.  The rotor couples energy between the capacitor stators, and there are NO MOVING CONNECTIONS.  This requires the cap to have twice the intended capacity as the pair of caps are in series electrically.  This is the ideal capacitor short of using a vacuum variable cap which also has no moving 'contacts', BUT they are more complicated to mount and adjust and are CUBIC BUCKS even from Russia.  If you go vac variable, go to Ebay as they have everything, I got a couple of 45,000 volt units rated at 400 pf.

Enjoy, W Shocked  Roll Eyes D Y

More loop stuff from me: https://sites.google.com/view/n4mq-site/home
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G3RZP
Member

Posts: 168




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« Reply #9 on: April 20, 2018, 01:55:41 PM »

I would argue that potentially, the butterfly approach with suitably thick vanes and welding/brazing has the chance of being lower loss than the vacuum cap.

But I read about people making butterfly caps from cut up biscuit tins and expect high Q....
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