NEC-LIST: Enhancing low-angle radiation for h.f.

For h.f. communication over long distances via ionospheric (F-layer)
skip, radiation at low elevation angles (below about 15 degrees) is
most important. Unfortunately, low-angle radiation is canceled by
ground reflection if the antenna is not high enough -- as is well
known.

E.g., an EZNEC simulation shows that a horizontal half-wave dipole
operating at f = 14 MHz and height = 0.75 wavelength (wl) above
average ground radiates best (with gain = 7.3 dBi) at an elevation
angle of 19 degrees, and its gain drops to 4.3 dB at 9 deg.

Increasing antenna height to 1 wl would improve the low-angle gain,
but in practice this is often impossible. An alternative method of
improving low-angle gain is to add a second dipole 0.5 wl *below* the
first (in this example, at a height of 0.25 wl) and to drive the lower
dipole in phase with the first, with half the current.

This trick increases the power radiated at 19 deg elevation, and also
at 9 deg, by 1.7 dB. These numbers are also from EZNEC, whose "Fast"
and "High Accuracy" methods of computing "real" ground effects yield
the same results (for this example).

Enhancing low-angle radiation by adding a radiator 0.5 wl *below*,
when it's impossible to go higher, must be an old trick; but I've
never heard of it. Has any NEC-LIST reader? Can anyone improve on
it? (Obviously, beam antennas could be substituted for dipoles.)
I've done a little numerical experimentation, which suggests that 0.5
wl vertical separation and 0.5x current in-phase drive are near
optimal, and that the trick fails if the lower dipole is too close
(e.g., 0.1 wl) to the ground.

73 de Chuck W1HIS
Received on Wed Oct 08 1997 - 08:23:56 EDT