Since the parasitic elements have no feedpoint, you can’t measure the resonant frequency the normal way, with an SWR meter. An example is measuring the parasitic elements of a beam antenna. This “no connection” measurement capability is handy for other purposes as well. ![]() Just bring the coil close to the tuned circuit’s coil and tune the GDO while watching for a dip in its meter reading. The external coil allows you to measure the fre- quency of a tuned circuit without any electrical connection to it. A GDO is a tunable oscillator with the coil mounted outside of the chassis. 1 While these days they no longer have a vacuum-tube “grid” to dip, the name seems to have stuck. Amateurs have been using grid-dip oscillators (GDOs) at least since 1947. ![]() It still works, although the coil socket has become intermittent over the years, which makes it a bit frustrating to use. It weighs over 2 pounds and must be plugged into the wall to obtain power for its 6AF4 vacuum tube. It’s an Eico model 710 that covers 0.4 to 250 MHz with 8 plug-in coils. By Alan Bloom, N1AL still have my first grid-dip oscillator, purchased at a flea market about 30 years ago.
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