At this frequency with both antennas resonant on 136 khz on the same site, the
induced rf from the small vertical into the larger one would distort any
meaningful tests. The antennas would need to be sited several kilometres apart,
when co located one would need to be dismantled while the other one is tested.
I have already tested this by starting off with a 60 ft vertical and progressing
to 120 ft and it certainly made a big difference for the same given rf power
output. Signal reports all over europe confirm this.
One fairly basic approach is that the far field is a function of antenna
"amp metres". If the amps are the same, and the effective height is
increased, then there is an obvious gain. It is far easier to arrive at
a given product of amps and metres in a tall antenna! Changing from 60
to 120 feet, with the same pattern of current distribution, would give a
gain of 6 dB for the same applied current. This is for one antenna in
the environment, with no mutual coupling to something large nearby.
An amateur regulatory constraint of 1 watt eirp limit starts to bite in
to the transmitter power budget for taller (more efficient) antenna