Hi Wa4gch
Your tests show a huge 5dB difference in sensitivity between the two transistors and I think something surely isn't right there.
I have actually built the front end of a 148 receiver on a separate PCB and tested it for noise figure. i.e. I tested the input amplifier stage.
I tried it with the AGC diodes and without and also with and without the input L match and L4.
I always got consistent results showing the transistor in this circuit was giving about a 4dB noise figure.
I also tried putting loads of different transistors in the front end amp in the actual radio and always got about the same sensitivity.
Your 5dB improvement seems a lot. Is there any way your radio has been modified in some way or maybe the original transistor was duff?
You can see by my test results in my video that the standard radio can lose 4dB of gain at the top/bottom channels because of the L8, L9 BPF but the S/N only degrades by about 2dB.
This tells me the gain of the first stage in the centre of the band is already good enough to pretty much overcome the noise contribution of the rest of the radio path. So the only way to get better sensitivity would be to lower the front end NF somehow (rather than just pile on lots more amplifier gain).
There's no way you can get the NF 5dB lower just swapping out the transistor. I'd be amazed if it could be 1dB lower NF with just a transistor swap.
However you CAN get a very big change in NF if the input transformer is not set to the best setting for the before and after comparisons.
i.e. it is possible to get poor signal to noise by doing nothing other than adjusting the input transformer L7 slightly to the noisy side of the peak.
i.e. I can make the same radio look bad even though nothing has changed apart from the L7 setting is now 'noisy'
Can I ask, are you testing in the middle of the band for your 5dB improvement or at the outer band edges?
