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 > > > -- ------------------------------------------------------- ___ _ Doug Smith \ / ) P.O. Box 1457 ========= Los Gatos, CA 95031-1457 _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: doug@xxxxxxxxxx \ _ / ] \ _ / Website: http://www.dsmith.org ------------------------------------------------------- ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu