@caius:
Thanks! I seem to be somehow distracted from the fact that I am already receiving planes upto 250 nm+ using my Standard (69 mm whip) Cantenna, and there are hardly any planes left un picked.
I will now attempt to use a DVB-T dongle as signal generator.
DVB-T RTL 820 as 1090 Mhz Signal Generator
Keeping signal generating dvb-t’s (trasmitting) antenna at a fixed distance from the “antenna under test”, I will compare the signal strengths of 2nd dvb-t, to which “antenna under test” will be connected, using SDR# or some “spectrum” software. I will keep the setup away from window, deep inside my apatrment where ads-b signal is practically non-existant, so that real ads-b signal do not interfere with the test.