Quote:
Originally Posted by rabbit73
It would be necessary to compare then two systems at the same instant because OTA signals constantly vary in strength...

That would be good if was practical. For accuracy, the two antennas would need to be located at the same physical place. In many cases at UHF, a separation of a foot or so, and the two antenna combos can receive different signals.
Quote:
...I have seen OTA signals vary as much as 10 dB in 30 minutes.

Yes, at 100 mile path have seen variation of 1015 dB in a minute or so.
Quote:
...I have more confidence in the results when using a constant strength transmitted test signal for a comparison...

That can allow measurement of signal strength related to antenna gain.
But doesn't address realized noise figure, or realized signal quality.
Quote:
...Keeping the preamp close to the antenna will minimize the mismatch loss...

In general, short transmission line minimizes transmission line losses.
Transmission line losses contribute directly to signal loss & degrade overall system noise figure.
However, even with virtually no transmission line, mismatch loss between antenna and preamp may still be present.
Mismatch has two main effects:
1. Signal loss which is is a result of reflection and reduces realizable antenna gain.
2. Degrades realizable noise figure of preamp.
The noise figure of the first active device in a preamp is dependent on the active device seeing a specified termination impedance.
Low noise preamps rarely have an input impedance of *real 75 Ohms*.
An antenna rarely has an impedance of real 75 ohms.
A preamp only has a specified noise figure when loaded with a specified complex impedance.
Preamps are usually tested for noise figure when terminated with 75 Ohms real impedance.
In reality, impedances of antennas are not real 75 Ohms and preamps input impedances are not real 75 Ohms.
So when combined;
 The realized antenna gain is less than optimum and
 The realized noise figure of the preamp is not as predicted.

*real 75 Ohms*

rabbit73, mainly for others benefit, realizing you know this:
Impedances generally are complex numbers, consisting of a real part and an imaginary part.
Usually denoted a R+jX or RjX.
Where R=real and X=reactance, either inductive (+) or capacitive ().
For perfect 75 Ohm match, the real part must be 75 Ohms and the reactance or imaginary part must be zero Ohms
.