Reg, G4FGQ wrote:
"Naturally, laboratories can differ one from another."

A lab may put its stamp of approval on your instrument, but your best
assurance may be measurement of known values. The temperature of
ice-water or the voltage of new dry cells, for example You usually can
try several dry cells for confirmation or averaging.

In antennas, one strategy for successful gain determination is
comparison with an antenna of known gain.

To determine the gain of a SW BC curtain antenna, we hung a 3-wire (to
match 600-ohms) folded dipole alongside and at the same height as the
curtain. We swiched transmission back and forth every 5 minutes between
the dipole and the curtain. We continuously measured and recorded the
signal strength for several days in the target area. We averaged
strengths of each signal and compared them for periods of the
recordings.

The HF dBd of the curtain agreed very well with that measured on the
model at 400 MHz in the lab before the curtain was built at full scale.

Best regards, Richard Harrison, KB5WZI