Receiver Status

[NZ0I]

Yesterday I installed the components for the Dual-Band Receiver 2m LNA preamp. Today I tested it. It seems to work.

I have not taken careful measurements, but applying a purely subjective test, it makes the received signal much louder. I'll run some experiments using a step attenuator to see if the gain is the predicted +15dB, but based on the observed loudness increase it seems like it could easily be that much.

Perhaps the most encouraging thing about this preamp is that no tweaking of the parts values was necessary. All components are exactly what is shown in the schematic diagram, and there is nothing to adjust. Turn it on and things get louder.

I did bypass the 2m band-pass filter components with a blue wire. So the predicted loss of the BPF is not present, meaning that all the gain of the preamp is reaching the input of the (already sensitive) matching network of the SA605. We were seeing a sensitivity estimated at better than 1uV prior to installing the preamp. With the preamp we should (theoretically) be very close to the 0.1uV we were aiming for.

So do we need the BPF or not? The matching network provides some selectivity. The preamp matching components should also provide a tiny amount of band-pass filtering. Is it enough? We need to test.

[NZ0I]

Hold the presses. 

The subjective results described below are now called into question. It is true that the signal strength does change dramatically between preamp on, and preamp off. Likely the difference is about 20 dB or a bit more. But the more important comparison is between preamp in circuit, and preamp out of circuit. It is proving tricky to determine how much of an improvement the preamp is providing, versus feeding the antenna directly into the SA605 matching circuit. Part of the problem is the lack of shielding - just moving my hand near the circuit board can cause a large signal strength change. Also, attaching the preamp output to the SA605 input matching circuit affects the match. Attaching an antenna to the matching circuit while the preamp output is also attached there, affects the match even more. So there isn't a simple way to go back and forth to compare performance with and without preamp.

So the jury is still out regarding the benefits of having the preamp installed. All I can say for sure is that the preamp provides a convenient way to shift the receiver gain by ~20dB, and that the receiver still seems to be pretty sensitive with the preamp installed. More experiments and testing needed.

-Charles