RF amplifier from ebay
Are they any good? The idea is that I can use it as LNA to amplify signals from parabolic antenna and use it with RTL SDR for some amateur radio astronomy.
I do not see any NF specification, so I doubt it can be used a a LNA. Similar amplifiers are used as IF amplifiers after satellite TV LNBs.
Radio astronomy can be done at UHF up to 2 GHz but there is a lot of interference by TV, mobile phones, etc. You will need to find a "clean" section of spectrum at your location, and use good filters to reject the interference.
For instance, I use a C-band LNB with a 50-cm dish for solar noise observation, but the local WiFi must be switched off to sense solar noise.
For this one http://www.ebay.com/itm/NEW-0-1-2000...8AAOSw3ydV5QZo, for 1.4GHz frequency (Hydrogen line) NF is around 2.7 dB. Is that too much for listening to the hydrogen line?
Also, I have been looking at Avago RF amplifiers, this one http://www.farnell.com/datasheets/1904392.pdf. They provided demo board and analyzed that board (page 8) and it seems that it has NF of about 0.4dB, which is much better than the one from ebay. But the drawback is that I would have to either buy demo board or make it myself. And I would rather not attempt to make it myself.
If you check radio-amateur literature on low-noise preamps for e.g. 1.4 GHz, you will find very good MESFET amplifiers with NF less than 0.3 dB. Such preamps require careful adjustment and again, any interference will degrade receiver performance.
I think NF of 2.7 dB is too poor for 1.4 GHz hydrogen line receiver. But a good RF filter to reject the interference will help even better.
I would buy this board:
http://www.ebay.com/itm/20MHz-2-4GHz...oAAOSw3ydVkf-X
and install my own chip into it, the minicircuits SAV-581+.
would give me 0.5 dB nf. You might be able to find a better chip if you look.
The board you link to does not have a manufacturers name. Some people build things roughly in line with the transistor manufacturers application notes and sell them with the original specification. You have no guarantee on the performance at all. My ex company researched some of these and found them to be a waste of time.
Frank