From the EZNEC plots for the original MM0FMF-12.50 m inv-L below, it can be seen that the mast height, radiator and radial lengths chosen for the multiband antenna result in only modest radiation characteristics, especially on the 17 to 10 m bands.
The main reason for the very pronounced steep radiation on the higher bands is the “radial” length of 5.18 m, which is far too long relative to the radiator length and, very unfavorably, one half-wave length long at 10 m.
A significant improvement in the order of ~11/8/3/2.8 dB @ 28/24/21/18 MHz @ 15 deg elevation could easily be achieved with the radial arrangement shown below. With this geometry, the MM0FMF-rev-12.50 m inv-L would have some similarity to my 10.90 m multiband inv-L, which was originally developed for the needs of Bruno, HB9CBR, and which is therefore connected directly to the transceiver.
