Jim Lux wrote:

In free space, there is not much difference in pattern between a dipole
that is perfectly horizontal and one that is drooped or sags (or for
that matter one that is bent horizontally). What changes is the
feedpoint impedance (72 ohms for perfectly flat, getting towards 50 ohms
with a 120 degree included angle).


The other thing that changes is the apparent height above ground. If
you model a drooped dipole and fiddle with the height to try and match
the vertical pattern for a perfectly flat dipole, you find that the
"effective center" of the drooped dipole is around 1/3 of the droop
distance. That is, if you had a dipole where the center were at 100ft,
and it drooped 30 feet on either end, the pattern is about the same as a
flat dipole at 90 ft.

The height above ground has a HUGE effect on the dipole pattern
(especially at low elevation angles) and probably dominates any small
changes.

After all, a perfect dipole has a gain of 2.15dBi and a infinitely short
dipole has a gain of 1.6dBi, and a drooped or sagging dipole pretty much
has to be somewhere in between.

Those gains are for dipoles in free space. A dipole over ground
typically has several dB greater gain.

The absolutely worst gain any efficient antenna can have in free space
in its best direction is 0 dBi. So nothing you can possibly do to the
pattern can reduce it below this value in free space, as long as you
don't introduce loss. But as Jim says, you can mess with a dipole a fair
amount without changing the pattern much -- except for filling in or
moving the nulls, which can be very sensitive to small changes.

Roy Lewallen, W7EL