Following is copy of today’s post, addressed to me on eham.net forum, from Martin - G8JNJ (g8jnj.net). I have not yet tried his proposal, but it sounds very promising, and if found successful, will prove a low-cost valuable tool for testing antennas & filters.
*"Anyway here’s (yet) another suggestion. You could use a spare RTL 820 dongle as a low power signal test source transmitting on 1090MHz to build a crude antenna test range.
Connect the RF input of the spare dongle to your dipole antenna, and set up the dongle with SDR sharp set to ‘center tuning’ mode, set the AGC off and set the RF gain to approx 12.5dB (this gives maximum LO leakage).
To produce an output near to 1090MHz tune SDR Sharp to either 268.93MHz for a plain vanilla RTL820 dongle or 266.5MHz for a RTL820T2 dongle (LO divided by four, minus the low first IF frequency which is usually either 3.57MHz for the 820T or 6MHz for the 820T2, depending on the exact tuning commands sent by SDR Sharp. This formula is valid from approx 220MHz to 880MHz which equates to an output frequency of 904MHz to 3544MHz). e.g. 1090MHz / 4 = 272.5MHz subtract 3.57MHz (for 820T) = 268.93MHz
This will bring the local oscillator up on a frequency very close to 1090MHz and produce an output level (at the dongle RF input connector) of about -80dBm.
This should be a high enough level for short range testing, but you could always ‘pep’ it up a bit further by using one of your satellite TV amps (if required).
Placing the test source antenna some distance away from your ‘antenna under test’ would allow you to make simple gain comparisons from a stable signal source, which should be repeatable on a day to day basis."*