 ```Dear LF group, ```The directive gain of an ideal short monopole over a ground plane is 2.62dB over that of an ideal half wave dipole in free space. This sounds wrong at first, but remember that the ground plane plays a part - while the dipole is radiating both "up" and "down", the monopole radiates "up" only since the signals do not penetrate the ground plane. In this sense, the monopole-on-ground-plane is more directive than a dipole, and so has higher gain. A vertical half wave dipole would in reality be affected by the ground plane too, and would have gain over both the short monopole and a dipole in free space, the gain and radiation pattern depending on the height above ground. With regard to practical LF antennas, the directive gain takes no account of the efficiency of the antenna, so the overall gain of a real monopole, taking losses into account, will be much smaller than the theoretical ideal dipole - for the antenna I have been testing on 136k around -33dB. The defenition of ERP used in the UK amateur radio regs (and elsewhere) is the amount of power fed into an ideal dipole in free space that will produce the same field strength at the same distance as the real antenna. This is just a theoretical construct that takes no account of physical reality, and a paticular value of ERP (P watts) is just specifying that the field strength at d metres distance from the antenna is E=7sqrt(P)/d. So if you measure a field strength E at distance d, your ERP is P=(Ed)^2/49. At VHF and higher, a reasonable approximation to a half wave dipole in free space is a useful practical reference antenna, but obviously not at LF. At LF, a reasonable approximation to an ideal short monopole is a more practical proposition, so "effective monopole radiated power", EMRP, is a term sometimes used. In this case, E = 9.5sqrt(P)/d. In either case stating the ERP or EMRP is effectively stating the field strength with respect to distance from the antenna, and says nothing about how that field was produced. This is useful because it gives a measure of the signal that is received, which is, after all, what is important. ```Cheers, Jim Moritz 73 de M0BMU ```