This is a design based on 2 well regarded circuits sited in Amateur Radio (Australia) magazine, (see ref's below.)
It is essentially the Impedance bridge design (ref 2).
However: by observation it is apparent that the "variable frequency oscillator" section is essentially the same circuitry as the earlier published "Fet Dip Oscillator" (ref 1)
LINK TO FET DIP METER (my construction)
Additionally a diode "clamp" is now shown b/w JFET gate and gnd.
The diode clamp limits +ve going excursions which aids oscillator stability and reduces harmonic o/p by limiting device transconductance on peaks.
This reduces changes in junction capacitance which restricts varactor action.
The diode is most effective when a source bias resistor of 100 ohm paralleled with 10nF of capacitance (Ref ARRL h/book 1986 pg 4.35)
Therefore it was easy to COMBINE both designs, and add a "changeover" switch (for the meter circuit) which would then allow a dual- purpose of being a FET Dip Oscillator, OR an AC bridge (impedance) measuement device. (consult schematics below)
In the FDO position the meter indicates Oscillator relative output and you should look for a "dip" in meter reading as Freq control is rotated; (showing the resonance of the nearby tuned circuit)
In the Z-bridge position the meter is used as a null detector the reading (at best null) on the calibrated Z potentiometer indicating the Impedance of the "unknown" connected to the input (i.e. antenna etc)
The dial calibration of frequency (for each of the 4 plug-in coils) is determined by listening for the swish/plop of the VFO in a good receiver. The working calibration is then flatbed scanned and a colourful artworked dial scale printed using the PC.
Likewise the Z (impedance) dial scale is determined by empirical measurement of fixed value carbon type resistors 10 thru 220 ohm.
Likewise an artworked and printed scale is printed from the PC.
Ref 1: Amateur Radio June 1990 Dip oscillator 1.8 - 30 MHz VK3XU
Ref 2: Amateur Radio Aug 1991 Portable Impedance Bridge for HF VK3XU