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On Fri, Jul 12, 2024 at 12:25 AM, Steve Ratzlaff wrote:

Hi Caaarlo,

Thanks for your interesting comments. Please tell me the exact setup and equipment you used to do your own gain measurements, and what input level did you use?

Thanks,

Steve AA7U

Steve,

My setup is simple:

The VNA is a NanoVNA, which is a self-powered VNA, hence no ground influence, same goes for the laptop connected to it.
1) I did not use any balun or transformer to couple the VNA to the balanced input of the LZ1AQ.
2) The output of the LZ1AQ was also connected directly to the VNA
3) The actual power entering the amplifier is around -30dBm.
4) The turns ratio of the trifilar output transformer is 6-6-5, on a small unknown toroidal core of high ? (Mix 73 or 75).

Some words explaining each point of my setup:

1- No input transformer: First, all the setup is floating (no ground). Second, the input impedance is so low and dominant compared to the possible external couplings and interactions with the generator, the computer and ground (capacitive and/or inductive) that it ensures symmetry by itself.
Adding a transformer is not a free lunch. One has to evaluate whether the balance of pros and cons is beneficial.

2- The output of the LZ1AQ was also connected directly to the VNA, as the output coil is floating, and could be theoretically used with either a balanced or unbalanced load. In reality there is some capacitive coupling of about 20pF to ground from the primary windings that might slightly alter the "floating condition" of the VNA so perhaps a current balun would have optimized the output measurement. But 20pF have a reactance of 360 ohms at 30MHz, more than 2 orders of magnitude higher compared to 1/2 Zin (~3 ohms) that connects the ground of the vna to the ground of LZ1AQ. The influence is minimal.

So. as you can see, I consider that the ill effects of adding transformers, in this case, would outweight the light ill effects of not using them.

3) Input level: The NanoVNA has a fixed level and it is -13dBm within the HF range. But due to the huge mismatch presented by the very low Zin of LZ1AQ, the actual power entering the amplifier is around -30dBm.
This reduction of around 18dB (depends on Zin at each frequency) has to be considered when computing gain.? I have to do it manualy, as the NanoVNA software is quite basic.

4) The turns ratio of the trifilar output transformer is 6-6-5, on a small toroidal core of high ? (Mix 73 or 75) theoretically optimal for 69 ohms, but either 75 or 50 ohm loads can be connected.


73 !!