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July 8th 04, 04:42 AM
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In article , Bill Turner
writes: On 07 Jul 2004 20:39:22 GMT, (Avery Fineman) wrote: No. 10 pFd at 1 GHz has a reactance of 15.9 Ohms. That tosses the VSWR in the bucket if that is part of a "line section." _________________________________________________ ________ A section of coax has both capacitance and inductance. Together, they form the Zo of the line. You are quoting only capacitance while ignoring the inductance. "Ignoring?" No. Spare me a lecture. 10 puffs IS a "small capacitor. You were not talking "inductance." 10 pFd IS a 15.9 Ohm reactance at 1 GHz. That's also a small reactive shunt on this mythical "line section." I grant you, a typical feedthrough capacitor is designed with large capacitance and is used for filtering, but that does not preclude one from being designed to "look" like a piece of coax if the designer wishes. And that was the original question. Sorry, no. The original question concerned testing of what LOOKED LIKE "feedthrough capacitors." The tester was thinking of "treating it like a small section of transmission line." Based on the response of actual testings, such as very low VSWR measured, the gadgets are probably just wire feed- throughs intended for something like passing through a hermetic seal. Those would have very little capacitance and thus would have very little discontinuity...the VSWR would not be raised very much. If those "feedtrhoughs" had really been capacitors, then, in all probability, they would have tested as capacitances on a bridge such as a 1 MHz bridge. A handheld C or L/C meter could have read that on a 1 nFd or 10 nFd scale. Without further ado, based on what was written by the person who has the devices, my opinion is that they are just wire feed- throughs with minimum capacitance and the inductance of a very short piece of straight wire. Those MIGHT work as "line sections" going through a hermetic seal wall of something...but not a very practical way to run a transmission line through that wall. Feed-through CAPACITORS were NEVER rated by "characteristic impedance" (Zo). Not in 1964, not in 2004, not in years between. Wire feedthroughs for sealing-type bulkheads are seldom rated for anything but current carrying capacity and withstanding or flash-over voltage. Those have just conductors going through with minimal capacity to "ground" and minimal inductance straight through. |
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