A Grid Dip Oscillator
constructed in 1984
based on a balanced push-pull FET oscillator
based on a design first published by the RSGB in the RSGB VHF/UHF Manual

This  Grid Dip Oscillator is biult directly into an old die case box.  The coils which consist of nothing more than assorted hairpin inductors are mounted on banana plugs or for the UHF "coil" is nothing more than a bent piece of 1/8th brass brazing rod jammed into the banana plug receptacles.

The little PCB carrying the oscillator is directly mounted on the banana plug receptacles so as to minimise every possible source of parasitic inductance.

Choice of FETS is quite critical to the performance of the unit. I used MPF102s which was a marginal choice. A proper UHF rated FET would have worked better.  The feedback capacitors need some attention. Too little feedback will result in poor performance at the the low frequencies.  The benefit of a balanced oscillator is that the  resonant circuit  needs no inductive of capacitive taps, there are very few spurious responses across the span of the variable capacitor which is an inevitable consquence of an unbalanced oscillator.

The variable capacitor used here is completely unobtainable in this day and age and was originally recovered from some scrapped radio equipment. It still is possible to find this kind of capacitor at ham fests.  The capacitor should not have a maximum capacitance greater than 50pF or UHF operation will be ineffective.

The maximum frequency of this GDO was about 470Mhz.  It could have been higher with a better thought out PCB layout.

The balanced oscillator was very successfull with a very nice smooth operation and consistent performance. The next GDO project for HF will be based on it again.

the banana jack to pcb trace junction needs more thought to minimise parasitic inductance

include circuit here