You've got a SDR so have at least basic spectrum analyser capability, even though its uncalibrated. You'll need a fully screened matched terminator for your coax cable - probably 50 ohm, and a M-M coupler Start off with a full spectrum scan with the SDR input terminated to get a baseline for the noise floor of the receiver. Then connect the cable and power injector, with the DC source disconnected and its input terminals shorted out. Terminate the far end of the cable and do another full spectrum scan. Investigate any suspicious peaks - they are unwanted pickup that's breaking through the supposedly fully screened coax's shielding. Now connect the LNA to the cable and terminate its input and do another scan. It shouldn't be doing anything because its unpowered. Remove the short on the DC injector and connect its power supply. Now the LNA is active so the noise floor will go up a bit, but with its input disconnected and terminated it shouldn't be picking up much. Investigate any suspicious spikes that weren't there with just the cable connected. Finally hook up the antennae and see what you get.
That's really good advice, this will be immensely useful for me in the future.
I wish I could do proper spectrum analysis and bode plotting, a DS1054z is all I got. Well, admittedly expensive equipment like that would be a little wasted on me, maybe I will buy a cheap Chinese function gen like the yf6600 that can do sweeps and play around with that
[EDIT:
THIS might be interesting to look into.]
Anyway, I somewhat changed my plan. While researching the topic I found
LNA4ALL, which seems to be really popular, nicely designed and supports bias-t. While the board is not that expensive either, the custom CNC enclosure is understandably pricey.
Does anyone have some advice on getting adequate, internationally available enclosures for such a thing that are on the less expensive side?
I searched around on AliExpress where I get most my stuff, but its swamped with the usual aluminium project boxes.
I will probably still get the parts for the bias-t mod on my next Mouser order and compare the two LNAs.
Will post results, but it could take me some time to do all that.