UHF oscillator 1

This was an attempt to make a UHF oscillator, specifically one that oscillates in the hundreds of MHz range and eventually one that is tunable around 300MHz. The purpose was to use it as a starting point for developing a frequency source for the testatika Linden experiment under the assumption that the experiment's long wire is a lecher line that is 1 metre long, the wavelength corresponding to 300MHz which in turn corresponds to the speed of light (approximately 300,000,000 metres/second.)

The source for the design is this UHF oscillator here. I first noticed it because the author used it to demonstrate measuring the frequency of a lecher line here. From my understanding it is basically a resonant LC circuit where the inductor coil (the L) is a single turn and the capacitor (the C) is needed to be so small that it is provided by the capacitance of the circuit and the transistor. Amplification is then provided by a 12volt power supply.

As the testing section below states, I did not have any success making this work. I'm not an electronics expert and have reached the end of my analysis abilities. Any help would be most welcome.

Construction of the UHF oscillator

The backing was made by taking single sided PC board, scraping away the unwanted parts and assembling the results along with supports and sockets for the antenna...

Next, the components were soldered on...

From left to right: 820ohm, 3300ohm and 6800ohm resistors, NTE107 high frequency UHF transistor (replacement for C717), 1nF capacitor, ferrite bead.
The capacitor and transistor in place.
The 3300ohm and 6800ohm resistors in place.
The 820ohm resistor with the ferrite bead on one leg.
The end result with single turn coil in place. Other coils that were tried are included.

Testing the UHF oscillator

The following photos illustrate the testing. With the analog meter on the 1VDC scale, the needle did not move at all. One thing that may or may not be a clue to the problem is that after the 12V power supply is turned off at the end of the test, the voltage meter on the face of the power supply does not immediately drop to 0V but instead slowly drops to 0V over about 30 seconds, indicating that the voltage is being held somewhere or there is no load to dissipate it.

Three different coils were tried: 15mm diameter, 22mm, 25mm. Also, two different transistors were tried:

  • NTE107 (replacement part for C717) - T-NPN, Si, High Frequency UHF,
    VCBO=30V, VCEO=12V, VEBO=3V, IC=50mA, PD=200mW, hFE=75 Typ, fT=1000MHz Min
  • NTE123AP - T-NPN, Si, Amp, Audio to VHF Sw (Compl to NTE159),
    VCBO=75V, VCEO=40V, VEBO=6V, IC=600mA, PD=625mW, hFE=200 Typ, fT=300MHz Min

The diode used in the pickup loop (see photos below) is NTE109 - Ge. General Purpose Diode Fast Switching, PHV=100V Max, IF=60mA Max, IFSM=500mA Max, VF=1V @ 200mA.

The diode on the pickup loop which is connected to the analog meter probes.
The pickup loop being held near the single turn coil, hoping for inductance.
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