I believe, there ist no ultimate choice transistor. Depending on specific requirements, there are serveral suitable transistors that are available and not obsolete. Some slightly better than others with regards to low frequency noise. Steve Ratzlaffs? testing list is a perfect tool to choose from. The Noise Figure of the top performers (in the LZ1AQ type circuit) are very close. I assume that a tenth of a dB, which falls within the measurement uncertainty,? will not make a significant difference in practical operation of an active loop antenna. From the availability point of view, only BFU590 and PZT2222A are actively produced and can be purchased. Obsolete Transistors which are not longer sold do only make sense for people who have enough devices in their personal stock. The BFU580/BFU590 is an high fT 6th generation RF-Transistor diva, the Onsemi PZT2222A is a robust, proven and reliable general-purposeTransistor. As long as both do the job properly, it's up to you to? choose.
Fred DL4ZAO
"Top performers" in Steve's list
2SC5551? -92 dBm noise??? +10.55 dB gain, 0.93 dB NF
BFU590?? -93 dBm????????? +10.48 dB, 1.02 NF
ZTX327??????????????????? +10.58 dB, 1.16 NF
BFR106?? -90 dBm????????? +10.51 dB, 1.23 NF
BFG591?? -91 dBm????????? +10.39 dB, 1.28 NF
OnSemi PZT2222A -87 dBm?? +10.57 dB, 1.29 NF? (used in the new Chinese WBL-02 loop amp)
