Owen wrote:

BTW, 5 to 10 degrees elevation is probably below most propagation paths
for DX, let alone local contacts. Lowest angle doesn't necessarily
translate to optimal for distant stations (it depends on the
propagation mechanism at the time), and the most desirable DX might not
be at the greatest distance. Best performance at low angles might
assure maximum pickup of local noise sources, and in my case QRM
because I am not at all interested in DX.

Recent editions of the ARRL Antenna Book cover this topic very
thoroughly. They note that arrival angles from a given location vary
with time of day, time of year, time in the sunspot cycle, whether the
ionosphere is stable or disturbed, and so on...

Then they produce statistics for major paths (eg "G-VK"), averaged over
all these factors. OK, so these figues are "only computed"... but they
are computed using VOACAP (the Voice of America's propagation program
that has has years of development and a lot of verification) so this is
the best information we're ever likely to get.

The big lesson is that there is ALWAYS A SPREAD OF POSSIBLE ANGLES.

As Owen says, 5-10deg is not always the most likely angle. It all
depends on how many F2/F1/E hops will fit into the required path length.
That means the arrival angles tend to jump, especially as the band is
coming on or going out. (However, it turns out that a substantial
fraction of signals on the G-VK path come in at angles of only a degree
or two. These paths are workable, of course, but the antennas at both
ends will usually have very little gain unless the land slopes down to
lower the reflection angle.)

Hams became fixated on "5 to 10 degrees" a very long time ago, and we
still tend to wave this figure around like some kind of flash-card.
Professionals have moved on to a much better understanding of the range
of possible angles that they need, and this is now starting to filter
into ham radio too.

If anyone is interested in this subject and hasn't read a recent edition
of the ARRL Antenna Book, it's time for an update.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek