El 22-11-14 3:01, Wayne escribió:


"Ian Jackson" wrote in message ...

In message , Wayne
writes


wrote in message ...


For those that do not have a firm understanding of what the chart of
dipole height over ground shows, I offer the following explanation.

The charts show, for a dipole antenna at various heights in
wavelengths
over perfect, very good, average, and extremely poor ground, the gain
and elevation angle of the antenna main lobe.

The main lobe is where the majority of the energy is radiated.

To understand what the charts mean in the real world, first you have
to understand a little bit about propagation of RF.

For a dipole antenna, there are two modes of propagation that are
relevant,
and those are NVIS (Near Vertical Incidence Skywave) and skywave which
is sometimes called skip.

Both modes depend on the RF being reflected or refracted back toward
Earth by the ionosphere.

For NVIS mode, the RF is directed straight up, that is an elevation
angle
close to 90 degrees is desired. The range of NVIS communications is on
the order of 50 - 650 km, depending on the state of the ionosphere.
The
amateur bands where this is effective is limited primarily to the 160M
to 40M band, again depending on the state of the ionosphere. It is not
impossible to have NVIS communications on the higher bands, just much
less probable to happen.

For skywave mode, a low elevation mode is desired. Most of the
literature
recommends angles of 30 degees or less. In this mode the RF "bounces"
at more obtuse angles, and with good conditions in the ionosphere,
more
than once, providing communication over global distances. Skywave
depends heavily on the condition of the ionosphere and during sunspot
peaks often occurs well past 10M.

Now since a dipole with a main lobe at 90 degrees still has some gain
at low angles, though it can be 20 to 60 dB down from the main lobe,
when conditions are very good some stations can still be heard by
skywave mode, though it is a rarity and can not be depended on.

Conversely a dipole with a low elevation angle of the main lobe has
some
gain at very high angles and can occasionly hear stations by NVIS
mode,
but again it is a rarity.

The bottom line of all this is that if you desire NVIS communications,
you should mount your dipole at a height where the elevation angle is
close to 90 degrees while if you desire long distance communications
you should mount your dipole at a height where the elevation angle is
less than 30 degrees, or higher if possible.

If the required height is impractical at your location, then the
alternative is a ground mounted vertical or a close to ground mounted
ground plane antenna, which will have an elevation angle in the 20
degree range.

Along the lines of a "testimonial"...
I once lived in the center of a state that had an active 75 meter
net. At one point I was asked to be one of the net control stations
because of my consistent strong signals within the net.

The secret? A 75 meter dipole at 20 feet with 100 watts.
On longer paths, of course, the "big boys" kicked my butt big time.

# Despite the obvious theory, and over 50 years in amateur radio, I still
# find it hard to believe that, in real life, an 80m dipole at (say) 20'
# ever really outperforms (at any distance) one at (say) 100'. Given the
# choice, I know which one I would choose!

A 20 foot high 75 meter dipole wouldn't be my first choice for an
antenna. At the time, that was the highest supports I had available.

I just dusted off EZNEC and out of curiosity ran the plot for a 75
meter dipole at 20 feet over "real" ground.
The max lobe was 9.36 dbi straight up at 90 degrees and a 3 db down
beamwidth of 99.4 degrees. The 3 db down points were at 40.3 degrees
and 139.7 degrees.

Maybe you confused directivity (D) with gain (G), or used a wrong
simulation paramater. A 20' high half wave dipole for 75m over
average soil has about D = 9 dBi. However because of the heat
dissipation into the soil below the antenna, the actual gain will be
around 3 dBi. In other words about 75% of the RF energy is dissipated
into the ground.

Is this problem? frequently not, as the link budget on 75/80m has
lots of margin under average conditions. Only onder worse conditions
(large D-layer absorption and/or high local noise level at the target
location) the one with the highest EIRP (=gain*power) will make the QSO.

Several years during JOTA we had a better then average NVIS antenna
for 80 m (with elevated reflection wires and a well fertilized
production field). With 100W input we get complaints about why we were
using a PA (we don't have one!). We decided to use a QRP TRX (10W) so
the FT101ZD could be used for 40 m. We had no complaints about the
signal.

So on average there is nothing wrong when using an NVIS antenna with
say 6 dB less performance (compared to an optimized one).

Nowadays we use a "downgraded version". about 3..4 dB loss of gain,
but it can be installed within 20% of the time required for the big one.



So at least according to EZNEC, and my own personal experience for
short range HF communication on 75 meters, a low dipole is a pretty
good choice.




--
Wim
PA3DJS
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