[SI-LIST] Re: Using Current Probes to Measure Cable Resonance

Hi Orin,

Thanks for the reply. I have a B&K grid dip meter that I still 
occasionally use. Your discussion brought back many memories.

I have been doing some research on measuring the resonant frequency of 
physical structures lately and will publish a subset of the results over 
the next few months on my website. The first article from this work is 
the January 2008 Technical Tidbit on my site.

Doug

olaney@xxxxxxxx wrote:

>An echo box is a different kind of gadget -- a passive, resonant cavity
>tuned to the center frequency of a radar pulse, also called a phantom
>target.  What you described is a grid dip oscillator, or GDO, called that
>because it was based on a small vacuum tube back when these were first
>used.  The meter monitors current through the control grid, a milliamp or
>thereabouts when the circuit is oscillating vigourously.  If the GDO is
>tuned to the freq. of a nearby resonant structure, the resonator will
>soak energy from the GDO, causing it to resonate less vigorously, hence
>the dip in grid current.  Eico made one, and so did Heathkit, among
>others.  There was a follow on version based on a tunnel diode ("tunnel
>dipper").  These days it's just called a dip oscillator, and probably
>uses a JFET.  I don't know if these are still offered commercially,
>though I'm sure there are ham radio DIY articles for it.  The "external
>antenna" is not an antenna per se, but the externally mounted coil for
>the LC resonant circuit of the dipper.  Energy is coupled magnetically to
>whatever you are checking.  Any EM radiation in the antenna sense is
>coincidental.  I have an old Eico 710 in my personal museum; it only goes
>to 250 MHz. These days I'd probably try a small magnetic probe on a
>network analyzer and monitor S11.  The probe has to be lightly coupled to
>get a good, sharp resonance.  A small loop works best.   If you try the
>classic split toroid it will be overcoupled, and you get messy results. 
>Doug's technique is a good approach for multi-conductor cables and for
>low Q resonances.  The two techniques should be considered complementary
>-- they have similarities, but Doug is measuring coupling (S21) rather
>than resonance as such.  It's a better technique for the intended
>purpose, IMHO.
>
>Orin
>
>  
>
-- 
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