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
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