Art Unwin wrote:
"My ARRL books go back a decade or more and the graphs showing the gain
per boom length has several curves based on different measurements
e.t.c."

I haven`t encountered similar curves based on NEC, but I`ve found thye
old ARRL curves on page 163 of my 1970 edition of the "A.R.R.L. Antenna
Book" (cover price=$2.50).

Variation between curves from 3 different groups of observers are within
"1 S-unit" (6 dB). Gain ranges from a minimum of about 7 dB for a Yagi
with a 1/2-wave boom length (pessimistic) to a 19 dB gain for a
6.5-wavelength boom length (optimistic).

The optimistic curve is labeled "Greenblum", and seems too good to be
true. The other two curves have flattened out as the number of
wavelengths increases. The Greenblum curve is almost a straight line. As
Kraus showed in his solution to the Deutsche Welle antenna gain problem,
you often need to double the total number of elements to get a 3 dB
antenna gain.

Best regards, Richard Harrison, KB5WZI