From: J. Forster (jfor@quik.com)
Date: Thu Apr 03 2003 - 19:38:48 PST
Joe Foley wrote:
A measurement at 10 MHz will be meaningless.
> +++++++++
> Hmm,..... better tell the military! The ZM-11 has been around since the early
> '50's! 10.79 MHz is the test freq. on that unit.
>
> Where'd you find that formula?
>
> Joe
If you put a capacitor at the end of a 20 foot long wire and measure it at 10
MHz, the measurement will be nonsense. At 10.79 MHz, a piece of wire is a
transmission line.
The wavelength of 10.79 MHz is about 90', so 1/4 wave is about 22.5 ft. A
quarter wavelength transmission line transforms an open circuit into a short
circuit. Just think of a whip antenna for a HF radio. That tester is likely for
checking components and the leads must be kept short.. My original post (below)
is correct. I'm sure the military knows this too.
-John
> The wiring is significant fractions of a wavelength and will not yield
> anything meaningful. If you want to measure the capacitance it should be done
> at some audio frequency, like 60, 400, or 1000 Hz. I'd still suggest
> disconnecting the battery and using the capacitance range of a cheap DMM.
> (Check for voltage before connecting the capacitance function)
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