Hi.

I have a long term goal of building my own HAM transmitter. But first, I need to master the basics.

Right now I am trying to build a ~100MHz Hartley oscillator. I have drawn a circuit which simulates OK. Making it work in real life is more difficult.

If I use my scope probe as an antenna (not in contact with any wire), I see it oscillates at the predicted frequency, but it stops oscillating when I load the circuit with my probe. Why is it so fragile? I have had success with oscillators up to ~40MHz, but higher frequencies seem to be more difficult.

The circuit is built with the "ugly construction" method, with as short wires as I could make, and with a big ground plane under the components. The inductor is a 10 turns air coil with 4mm diameter.

Is this a construction problem or is it a design problem? Any practical things I need to consider when building things at these frequencies?

Here is schematic and simulations (C3 and R2 represent my scope probe):

 

100MHz transmitters do not derive there output frequency directly from an oscillator. The oscillator operates at a frequency of much lower magnitude and is multiplied before reaching the driver and finals. Doing it directly from the osc. will probably create the most unstable transmitter known to man and not make you any friends, including the FCC.

BTW, what Amature band is at 100MHz?

 

You don't need all those parts.
Your scope probe capacitance is loading the oscillator. You can add a source follower.
See below.

BTW, I don't claim to be an expert on oscillators, so this is not optimum. I was just trying to make your design work.

 

Ron H. is right -- the probe is loading the oscillator in such a way as to absorb all its output. A low power amplifier stage (e.g. emitter follower) is called for.

At 100 MHz, the Hartley oscillator is going to be very touchy about mechanical vibrations, ambient temperature, and almost everything else in the known universe. Better to use one of the tried and true techniques of creating a super stable low frequency signal and multiplying up into the VHF region. Don't try to put a 100 MHz signal on the air unless that frequency is authorized for amateur use in your country.

 

I got the impression that he was doing this as a learning exercise, and not necessarily as a building block for his transmitter. I may be wrong.

 

Yes Ron, this is just a learning exercise. I have no use for a 100MHz oscillator, I just want to push the limits to see where things break down. And do not worry, I will not put an antenna on anything I make unless I am sure the base frequency is in an amateur band and harmonics are within legal limits.

I will try Rons suggestion when I get back from work today. But I did model the probe load in my simulation, so I expected it to work in real life also. I understand from your answers that I should not trust my simulations too much at these frequencies?

I note from your comments that VHF hartley is not something people usually builds, instead I should use multipliers. What would be the highest practical frequency for a hartley if I actually want to use it for something real?

 

LC oscillators are too unstable to use as a reference for a transmitter. You would need a crystal oscillator.

 

I tried the circuit I got earlier in this thread. It works

I learned several things from this, so thank you all for your comments. I will now go on and try something else.

Asbjørn