Poor practice, but does it create a voltage error?

I may have answered my own question, I shorted the cable to 12" and lowered the frequency to 70kHz, the HP3400A still reads the same. I think the error is in my Q meter. Adding a 50Ω feed thru termination at the HP3400A input makes no difference. Still open to input. Mikek

I'd expect the error at 1 MHz to be tiny, much smaller than the 7.5% you see.

You could Spice it for the actual numbers.

Because 50 ohms is so much larger than the 0.02 ohm source impedance, the source will drive the full output onto the cable. This voltage will run the length of the cable, and be reflected by the high impedance, with the two adding. So the load should see the driving voltage doubled. This reflection will run to the driver and, the driver appearing to be nearly a short, will have virtually no impact on the driving signal strength. The only trouble is, since the frequencies are low, you may not see any of this happen.

So if the Q meter is outputting 20 mV and the meter is reading 21.5 mV, that would make sense, since the impact of the reflection is greatly muted by the low frequency of the signal. It takes a lot less current to drive the voltage ramp of a 1 MHz signal than a faster one. Try the same measurement at 10 MHz, and 100 MHz. I would expect a higher frequency to produce a higher reflection.

I raised the frequency to 10MHz, the '20mV' signal increased to read 23.6mV on the HP3400A, installing a 50Ω termination dropped it to 23mV. Putting a 1x scope probe on the HP3400A, the '20mV' measurement reads 21mV @1MHz. Even the very shortest connection I can make to the HP3400A, it still reads 21mV. (less than 1 inch)

this point, I think the Q meter 1x multiplier setting is reading low. Mikek