This is the JFET based dip resonance meter as described by Drew Diamond VK3XU in Amateur Radio (Australia) magazine, June 1990. (see scan below)
FET DIP METER
I essentially followed the design as printed: however, for the dipping coils, I used octal cable plugs and (my children's) re-cycled "crayola" pen casings as coil formers.
By sawing two perpendicular hacksaw cuts in the base of same they become a snug inteference fit in the locating spigot of the 8 pin "octo-plug"
Also being of different colours allows the corresponding dial scale to be printed in the same color. (see pix)
I would like to restate the following from the original article:
The "dipping" phenomenon is produced when the coil of the "dipper" is coupled to the coil of the tuned circuit under test.
This is what I think actually happens.
When the field from the primary coil energizes the passive coil it will if the exciting frequency is at or near the resonant frequency of the passive circuit set up its own field in opposition to the primary field, which tends to lower the amplitude of oscillation in the primary circuit.
If the coupling is sufficiently tight oscillation may actually cease.
The higher the Q of the passive circuit the sharper the dip for the same degree of coupling.
This explanation is at odds with that given in the standard texts which generally state that energy is absorbed or sucked from the dipper coil, but this does not explain why a higher Q circuit produces a deeper dip than a low Q one.
For another informative article scanned from Practical Wireless mag (UK) Dec' 1985 :
LINK: Using the FET dip oscillator
TO ORIGINAL MAGAZINE ARTICLE - next page