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[TowerTalk] Re: Matching Capacitors

 To: [TowerTalk] Re: Matching Capacitors DAVED@ctilidar.com (Dave D'Epagnier) Thu, 12 Aug 1999 08:50:13 -0600
 ```Scott, Capacitors made from G-10 material (dielectric constant 4.8 and 1/16" thick) have about 17pF per square inch of capacitance and a breakdown voltage of about 50kV not including edge effects. The breakdown voltage at the edges is considerably less and one should allow about 1/8"/kV of dielectric to extend out past the plate conductors (copper clad). This makes it hard to trim pc board capacitors for high voltage applications. Instead of just trimming the board edge to reduce capacitance, one must peal up or mill away the copper clad while leaving the dielectric intact. The dissipation factor for G-10 is about 2% at 1MHz, which means it's got a Q of about 50 at 1MHz. Much lower loss capacitors can be made of teflon board material which commonly have dielectric losses of only about 0.1% (Q=1000). Teflon is more expensive and somewhat harder to find but it's often available at surplus electronic parts dealers for a song. The formula for calculating capacitance of other board materials is: C = 0.2249 * k * A / d (picofarads/in.) where: k is the dielectric constant of the material A is the plate area of the capacitor (in) d is the board thickness (in) One other note. The equivalent circuit of this capacitor can be modeled as a series RLC combination. The magnitude of R is equal to the capacitive reactance at a given frequency multiplied by the dissipation factor. So if Xc is -j100 ohms, then R=2 ohms if the dissipation factor is 2%. Also, the inductance is roughly about 30nH per inch of capacitor radius it turns out. If lower series inductance is needed, one can stack boards (connecting alternating layers at the edges) and thereby reduce area (and inductance) for a given capacitance. Hope this helps. Dave K0QE -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
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