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Author Topic: ethylene glycol as a dielectric  (Read 5591 times)
KE7BZI
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« on: June 01, 2011, 03:10:30 PM »

All

I'm thinking about building a small loop antenna.  I've been looking into option for the HV tuning cap.  I see that some people are putting a standard air variable caps in oil to increase the capacitance and voltage.  I started looking around at other fluids that could be used. The dielectric constant of ethylene glycol is 38 instead of 3 for mineral oil.  Anyone see an obvious flaw in using standard air variable submerged in ethylene glycol?  It seems like it would provide an impressive increase in performance.  I found a reference that the breakdown voltage is very high (60Mv/mm?).  The auto combustion temp is about 750F.  I also found a reference to it's use in electrolytic caps, but not as the main dielectric.

I'm aware of the butterfly / fixed stator vs. cap with wiper issue. 

Thanks
Marc
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W5FYI
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« Reply #1 on: June 01, 2011, 09:05:42 PM »

Check its hygroscopic qualities; it may absorb moisture from the air and change its dielectric constant over time. It is also attractive to animals and insects, and is poisonous. Other than that, you may have hit upon a good idea. Let us know how it works out.
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KA4POL
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« Reply #2 on: June 01, 2011, 10:12:14 PM »

I do have some doubt on the results of your plan. In one point you are right, the higher the permittivity (=dielectric constant) the higher the capacitance. However, the lower the permittivity the higher the voltage. I.e. your best solution for high voltage would be a vacuum cap. It may not be the best solution for your wallet. Wink
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W8JX
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« Reply #3 on: June 01, 2011, 10:31:39 PM »

Check its hygroscopic qualities; it may absorb moisture from the air and change its dielectric constant over time. It is also attractive to animals and insects, and is poisonous. Other than that, you may have hit upon a good idea. Let us know how it works out.

Actually you might want to try propylene glycol. It is non toxic and it has it highest boil point and lowest freeze point at 100% concentration while pure ethylene glycol freezes at about 9F or so.  Liquid cooled aircraft engines have long used pure propylene glycol.
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KE7BZI
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« Reply #4 on: June 01, 2011, 11:20:37 PM »

I found a PHD paper that lists the breakdown voltage of 95% ethylene glycol 5% water as 270 kV/cm.
Seems like it might be worth a try.

Marc 
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G3RZP
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« Reply #5 on: June 02, 2011, 02:54:22 AM »

What about its dielectric loss tangent at radio frequencies? Unless it's very low (like Teflon) you may not win anything.
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KE3WD
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« Reply #6 on: June 02, 2011, 06:48:44 AM »

What power level do you intend to run? 

Use of oil or glycols may be overkill here. 

If wanting to design for more than about a hundred watts, I'd recommend biting the bullet money-wise and investing in a vacuum-variable cap suited to the purpose. 


73
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KA4POL
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« Reply #7 on: June 02, 2011, 07:39:29 AM »

What power level do you intend to run? 

Use of oil or glycols may be overkill here. 

If wanting to design for more than about a hundred watts, I'd recommend biting the bullet money-wise and investing in a vacuum-variable cap suited to the purpose. 


73

Good and justified question concerning the power levels. We have not been told the value yet. And thanks for supporting my suggestion  Wink on the vacuum cap: http://www.surplussales.com/vaccumvarcaps/VVC2.html would be one possibility. He may be lucky on some auction as well.
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GRADY
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« Reply #8 on: June 02, 2011, 10:03:57 AM »

I think just plain ethylene glycol is very corrosive as well. They have to add other properties with it for antifreeze purposes if I am not mistaken.
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KF5KZX just the new guy on the block!
N3OX
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« Reply #9 on: June 02, 2011, 02:14:22 PM »

What about its dielectric loss tangent at radio frequencies? Unless it's very low (like Teflon) you may not win anything.

Agreed.  

I found a reference (Zahn et al. Proceedings of the IEEE, Sept 1986, Vol 74 Issue 9 Dielectric properties of water and water/ethylene glycol mixtures for use in pulsed power system design) that puts the loss tangent of ethylene glycol at 0.03 at 1MHz and 0.008 at 10MHz.

That puts it somewhere around wood or dirt at 1MHz and somewhere around G-10 fiberglass (FR4 circuit board material) at 10MHz.  I cooked a piece of fiberglass that was probably like that by using it as a spacer between the plates of a 17m magloop I was playing with, and that was with 30W.

Your capacitor Q is going to be much too low if you use even something with loss tangent of 0.008.  That's Q=125 and most of your power will go into heating the dielectric.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KE3WD
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« Reply #10 on: June 02, 2011, 04:19:07 PM »

Design is never a one-input problem. 


73
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KE7BZI
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« Reply #11 on: June 02, 2011, 05:44:30 PM »

Well I suspected there might be a flaw and ESR may well be it.  I don't have the math skills to understand tangent loss but I understand it is one component of ESR.
I'll see if one of the guys at work will walk me through it.

My rig is in the area of 100w so the standard solutions of a vacuum variable or an air variable are options. 
The affordable vacuum caps are either old and unknown,  or tend to be sold by people from the capital of internet fraud (Ukraine), and still cost $200 with shipping.
Butterfly or fixed stator caps in the 5KV range tend to be expensive. I'm not saying they aren't the best option by it seemed interesting to look for options.

Since I have a complete machine shop, I tend to like mechanical problems so building some kind a of an air variable may the way I go.
After all, when you have a hammer, everything looks like a nail.

Marc
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N3OX
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« Reply #12 on: June 02, 2011, 06:29:38 PM »

The affordable vacuum caps are either old and unknown,  or tend to be sold by people from the capital of internet fraud (Ukraine), and still cost $200 with shipping.
Butterfly or fixed stator caps in the 5KV range tend to be expensive. I'm not saying they aren't the best option by it seemed interesting to look for options.

I don't know your definition of "affordable" and I don't know what capacitance you need but Max Gain Systems has a couple of Comet vac variables (200pF max and 60pF max) that clock in around $100:

http://www.mgs4u.com/RF-Microwave/vacuum-variable-capacitors-200.htm

Don't know what shipping adds to that, but it would just be regular UPS.  Anyway, not trying to discourage you from other approaches, but if you weren't aware of Max Gain I just wanted to point it out.  I haven't purchased a vac variable from them, but have been happy with their service with fiberglass and other doodads and they'd be one of the first places I'd look for a vac. variable just because of their stated policies about hi pot testing and returns coupled with their reasonable prices.  They do have a lot of $200 caps too, of course.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KA4POL
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Posts: 1927




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« Reply #13 on: June 02, 2011, 10:07:41 PM »

Quote
My rig is in the area of 100w so the standard solutions of a vacuum variable or an air variable are options. 

Air is between 2 and 5 kV/mm so there is absolutely no need for any effort to improve the dielectric strength. Just take a capacitor with sufficient distance between the plates.
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G3RZP
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« Reply #14 on: June 03, 2011, 04:15:43 AM »

N3OX,

You just confirmed my suspicions......
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