Electronics > RF, Microwave, Ham Radio

Battery powered LNA after 20v power injector and down converter


Ok so right out of my SDR stick I use an LNA for all or a self made LNA with the same mini circuits ps-4blah mmic. These work awesome because there noise floor is so much lower than the RTL stick you barely have to use any of the sticks gain, I usually just power with a 9 volt or something. Now if im running a decent feed line and trying to pick up anything over 1ghz I use a cheaper e-bay LNA at the antenna to make up for the coax loss, these dont even have an option for bias tee. So I recently modified a few DirectTV down Converters to get to 4ghz, but they require a power injector from 12 to 20v, which I have a 20v around. My question is if I run antenna -> self powered LNA  -> 20v injector with down converter ->feedline -> LNA4ALL -> RTL-SDR am I going to blow up the LNA right behind the antenna? Even if it didnt blow up wouldn't I be pumping a bunch of DC noise through the mmic?

Im hoping I can run my lines this way but if not, I could use two female sma connectors back to back with a DC blocking cap between them how would I size that cap and what kind of cap should I use, also is it ok to use a through whole cap with the lead practically cut off or should I really be using an smt cap? How would I calculate the loss, just calculate the capacitive reactance and thats the resistance but how do I turn that from R/Z to DB?

The other option would be to put the LNA before the down converter/injector but then i am incurring the insertion loss from the combo, and im only amplfying the down converted signal, not the acuall signal im trying to recive, so im guessing this is kind of a bad solution?


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