I'm working on an old school VFO for the 15 meter band based on a JFET Hartley oscillator. The plan is to follow the oscillator section with a JFET buffer, then a CB amplifier (for increased reverse isolation) and finally, a feedback amplifier for gain.
The oscillator and buffer stage are complete and working well. But, I'm having some issues with the CB amplifier.
In the schematic above, the buffer output is represented by V1. I measured it's output impedance to be ~80 ohms (it has quite a high trans-conductance). The impedance seen at the CB emitter should be about 26mV/4mA||510 or ~ 6.7 ohms.
The (theoretical) gain of this amplifier should be (50 ohms x (n^2))/6.6 = 68 approximately. So, you would expect to see 20mV p-p x (68/3) across the 50 ohm load or something just shy of 0.5 of a volt.
Instead, having built the circuit and after taking some measurements, I get about 240mV or half the gain. What seems to be happening is that the transformer is not "transforming" like it should. If I test the transformer using a signal generator, I get a transform ratio of 3:1, as expected. However, in-circuit it's more like 1.66.
Interestingly, the BJT is fulfilling its duty nicely: if you consider that a turns ratio of 1.66 makes 50 ohm look like 138 ohm at the collector. Then 138 / 6.6 = 21 approx. Then (20mV x 21) / 1.66 = 250mV which is very close to what I see across the load.
I'm just a bit stumped as to why the transformer is not behaving itself. I've used these type 43 cores before without issues and I even tried a bigger core size (same material) and I experimented with bigger values of C2. No change.
Also, the transistor is not a BC546 but rather a high frequency transistor pulled from a radio front-end. I've tried various HF transistors in its place with the same result.
Is there something about the CB amplifier that I'm overlooking?