But in practical terms, how do you get a "perfect" earth ground reference
for the analyzer to use when making the measurement?

===================================

You don't need a "perfect earth" because that is precisely the quantity you
are trying to measure.

The only "perfect" thing is the 36 ohms which is the calculated value of the
1/4-wave antenna's radiation resistance.

But it is not essential to have an antenna as tall as a quarterwave to find
the earth-loss resistance. There's not many people who can erect a 1/4-wave
vertical at 160 meters.

The 'perfect' radiation resistance of an antenna shorter than 1/4-wave is
given with sufficient accuracy by -

Rrad = Square( 24 * Height / Wavelength ) ohms.

Of course, an antenna shorter than 1/4-wave has reactance. But this can be
tuned out with any coil which has a negligible loss resistance compared with
antenna resistance.

Make the measurement at the resonant frequency of coil + antenna whatever it
may be. The antenna analyser will tell you when you are at resonance. Vary
frequency for the minimum pure resistance. And that is the resistance you
are looking for.

The lower the measurement frequency the more easy and accurate the
measurement provided it is not overdone. The earth resistance measurement
doesn't have to be made at the same frequency the antenna is to be used at.
It can be appreciably lower.

The great advantage of these one-hand held, battery-driven, antenna
analysers is that the connecting wires can be kept very short, ie., much
shorter than a 1/4-wavelength.

They are sufficiently accurate for this sort of job. And don't be afraid of
lying on your belly to read the meter. The best things since sliced bread.
---
Reg, G4FGQ.