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Author Topic: Question about Vacuum Variable Capacitor  (Read 5316 times)
AI6PC
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Posts: 12




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« on: August 21, 2016, 08:14:54 PM »

Hello Guys,
               I am in the process of building a Mag Loop antenna and purchased a "Comet 100 - 1000 pf" from Ebay. However I am confused about the
setting function. In particular the screw adjustment. When lowering the capacitance the screw fitting on the capacitor starts to come out. I would have
thought that it would be permanently fitted so it doesn't start to fall out. Is this normal or have I just gotten a bad or defective one?
Your thoughts would be greatly appreciated as on this subject I do not have any knowledge.


73's
Paul 
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KJ6ZOL
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Posts: 820




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« Reply #1 on: August 22, 2016, 02:48:05 PM »

Ask on the Homebrew forum, they'll be able to help.
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AI6PC
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Posts: 12




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« Reply #2 on: August 22, 2016, 04:18:20 PM »

Bryan,
         Thanks for the heads up! I appreciate the help!

73's
Paul
 Grin
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WA7PRC
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« Reply #3 on: August 22, 2016, 04:42:30 PM »

Comet vacuum variable capacitors: http://www.comet-pct.com/Products/Capacitors/Variable-Capacitors
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AI6PC
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Posts: 12




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« Reply #4 on: August 23, 2016, 05:43:12 AM »

Thank you for the post! I will call them today as they have an office in California and speak to their tech
department. The info you gave me is helpful but still doesn't give me enough of a hint about the mechanical
workings of the capacitor.
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AA4HA
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Posts: 2630




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« Reply #5 on: August 23, 2016, 01:05:53 PM »

It is not unusual for a vacuum variable cap to have a threaded rod that moves in and out of the glass envelope as you make an adjustment to capacitance. If you can see in to the glass envelope there is often a copper colored bellows mechanism inside. The rod attaches to the end of the bellows and compresses it.

The vacuum is actually inside of the bellows (like an accordion) where the capacitor plates are. When you adjust the rod you are changing the spacing on the plates. You would think that the threaded rod would need to be vacuum proof but it is just for fine control over the dimensions of the parts inside of the bellows.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
WA7PRC
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« Reply #6 on: August 23, 2016, 04:18:45 PM »

This may give you some ideas.
Click an image below for a larger image...

A Jennings vacuum variable capacitor came to me like this:

In order to decrease C, you pull on the shaft.

A brass coupler came with it, with threads (1/4"-28) that match the shaft:


I assembled a couple of copper plumbing parts that just
happen to fit over the opening in the vacuum variable:


The depth of the copper parts ≥ the travel of the rod.

A flat ball bearing (from McMaster-Carr) significantly reduced the turning effort:


vy 73m
Bryan WA7PRC
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AI6PC
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« Reply #7 on: August 23, 2016, 09:23:01 PM »

Tisha & Bryan,
                    Thanks so much for your information!! I was worried that I might of had a defective part on my hands.
                    Your answers have calmed my fears and are greatly appreciated.

73's
Paul
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AA4HA
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« Reply #8 on: August 24, 2016, 07:45:15 AM »

Tisha & Bryan,
                    Thanks so much for your information!! I was worried that I might of had a defective part on my hands.
                    Your answers have calmed my fears and are greatly appreciated.
Glad we could help, often there are components used in certain pieces of equipment that are "unusual" and this is a great place to ask questions.

One that I have on my bench that stumps just about everyone out there is a "variometer" that is wrapped with litzendraht wire. It was originally a very low Q loading coil for 500 KHz maritime use. Maybe now with hams building stuff for 600 meters they will come back in to fashion.

http://g3ynh.info/comps/Vari_L.html
 and from a guy over the pond;
http://www.wireless.org.uk/gallery/werner.jpg
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KE6EE
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Posts: 2814




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« Reply #9 on: August 24, 2016, 09:19:13 PM »


There are some lovely inductors shown at those urls.
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WA7PRC
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« Reply #10 on: August 25, 2016, 04:02:53 AM »

Tisha & Bryan,
                    Thanks so much for your information!! I was worried that I might of had a defective part on my hands.
                    Your answers have calmed my fears and are greatly appreciated.

73's
Paul
You're welcome. Smiley
Where there's a will, there's a way.
Also, where there's a will, there's a lawyer. Wink
The most impressive variometers I recall seeing were used at a 1 megawatt VLF transmitter (US Naval Radio Station NLK - Oso, WA) in the 80'x80'x80' copper-clad tuning room. The variometers were 12 feet in diameter, sitting on 2' thick slabs of Steatite. The tuning loops were turned by geared-down 5hp motors.  I was more than a little impressed Shocked
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G3RZP
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« Reply #11 on: August 25, 2016, 05:44:27 AM »

Variometers were fairly common in marine transmitters in the 410 -512 kHz band. The biggest difficulty -and the reason some of them had taps - was the variation in Q. By using taps and restricting the amount of variation, you could get a better overall Q.
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