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Author Topic: TRombone capacitor formula  (Read 3645 times)
N2LK
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Posts: 71




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« on: September 17, 2010, 05:35:46 AM »

Looking to build a trombone cap out of copper pipe and just wondering if there is a capacitance calculation out there? Say example: 3/4" pipe with a 3/8" slider inside...still trying to figure out a dialectric that is workable...


Want to tune a 10 ft diam. loop 12-30 meters...need up to about 65 pf range.
Any thoughts appreciate!
73
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LU2DFM
Member

Posts: 91




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« Reply #1 on: September 17, 2010, 06:16:48 AM »

Hi.
Here's the explanation: http://www.davidpace.com/physics/em-topics/capacitance-cylinders.htm

And here is a calculator: http://www.ajdesigner.com/phpcapacitor/cylindrical_capacitor_equation.php

73 de Fer
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KI4VEO
Member

Posts: 166




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« Reply #2 on: September 20, 2010, 08:34:18 PM »

Use some sheet teflon.
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DXZEPP
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Posts: 20




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« Reply #3 on: September 21, 2010, 09:31:31 AM »

Down load Hamcalc, here is a link ->http://cq-amateur-radio.com/cq_ham_calc/cq_ham_calc.html 

It has the capacitance calculator your looking for, it's called telescoping variable. 

Have fun!
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KW5B
Member

Posts: 21




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« Reply #4 on: January 27, 2012, 08:18:42 AM »

I know this is an old posting, but considering the difficulty in finding suitable material for the dielectric for a trombone someone might be interested.

Use 12 x 3/4 in repair links for the outer part of the capacitor.  You can transition to the 3/4 in part of the loop easily since the repair links are exactly the same as regular couplings.  Then for the dielectric 1/2 in pex will slide into the coupling.  Relatively easily.  Then 1/2 inch copper pipe will slide perfectly into the pex.  PEX has a dielectric constant of about 2.25.  

A two legged trombone made this way will give you plenty of capacitance for a 8.5 foot loop that will cover 14 to 29 mhz.  A faraday loop is far superior to a gamma match.  

By the way PEX  is cross linked polyethylene and is superior to using sheet Teflon in  this instance.

Also some of the capacitor calculators just calculate the value for an air capacitor.  Go ahead and use it, but multiply the value by the dielectric constant.  In this case 2.25, but then you'll have to divide by 2 since the trombone is in parallel.

Or use this:  http://www.66pacific.com/calculators/capacitance_calc.aspx


ljg
« Last Edit: January 27, 2012, 08:25:11 AM by KW5B » Logged
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