I blew my initial eval *really* badly. Sigh. I was just above the noise floor of the 8560A when I got the flat looking response but failed to notice that which I should have.
The circuit is pretty standard, a diode, 3 transistor amplifier and an attenuator. It has an LM317G regulating the diode supply which on mine is set to 7.2 V. The transistor supply is the input voltage which explains the variation in behavior with input voltage.
I lined mine with RF absorbing foam. At 10 Vdc input it's flat to within 0.5 dB up to 1 GHz and then rolls off steeply reaching -3 dB at 1.35 GHz and -11 dB at 2 GHz. It rolls off smoothly up to 2.9 GHz which is the limit of the 8560A
First the foam lining job. I'm not sure this made much difference, but I had the material so I put it in. I should have taken spectra at 10, 11 & 12 V before and after but I didn't think to do that.
Next the output spectrum at 1 dB/div at various input voltages.
12 V input:
11 V input:
10 V input:
For under $30 it's actually pretty good. But something of a design fail to not have included a 2nd regulator and to specify it as 12 V input. The transistors are clearly getting rather non-linear above 10 V. And in typical cheap Chinese tradition, the spec is bogus and should read 1 GHz instead of 2 GHz.
I could not see a marking on the diode and the transistor markings had been sanded off.
I ordered a couple of RTL-SDR dongles, a V 2 and a V 3. So more fun when those arrive.
Have Fun!
Reg