>From: K3BU <K3BU@aol.com>
To: <topband@contesting.com>
>Date: Sat, 14 Mar 1998 10:13:20 EST
>
>In a message dated 98-03-13 19:15:00 EST, N7CL writes:
>
><< Ground level (far field) field strength measurements are
> perfectly adequate to verify and compare the performance of these
> systems. The screen density affects only the radiation
> efficiency of the system. It does not significantly modify the
> radiation pattern of the antenna in the elevation plane. So a
> change in the "ground wave" field strength is accompanied by a
> proportional change in the intensity of the radiation in the
> antenna's main lobe which is launching the sky wave. >>
>
>
Snip...
>On your above statement, I have seen in the books that radiation
>diagrams for verticals, with poor ground would have the part of
>vert. radiation pattern "missing" at low angles, better ground
>(salt water) seem to fill that pattern all the way down to almost
>0 deg. Isn't the situation between few and lot of radials
>analogous to poor and good ground? Therefore affecting the signal
>strength of "mile away on the ground measurements"? Woudln't the
>difference at some higher angle like 30 deg. be less pronounced?
Hi Yuri,
In all those books, when they changed the ground conductivity,
they changed it everywhere. Not just for a 1/4 wavelength radius
around the base of the radiator. Unless the ground conditions
from around 1/2 wavelength to 3 wavelengths (and more) radius
from the radiator are also changed, improving the close in ground
conditions at smaller radius than 1/2 wave will not materially
affect the shape of the elevation pattern. It will only vary its
relative size (if size is proportional to field strength).
>From amateur point of view we might be more (90% or so) times
>interested in the efficiency at that higher angle due to
>propagation modes on 160.
Efficiency is efficiency. The losses we are preventing with the
1/4 wavelength radius ground screen are losses that happen before
the radiated wave gets launched. It is energy that is removed
from the system before radiation happens.
The shape of the radiation pattern is mostly a far field effect
of the interaction of the radiated field with the earth
relatively distant from the base of the antenna. On topband,
there is little most hams can do about the far field effects
other than to move to a seaside location.
>I can see why broadcast stations are interested in
>omnidirectional pattern and maximum signal at the ground level
>(why don't they use crossed low dipoles?).
They don't use crossed dipoles for a number of reasons. To
present only two:
1. Virtually 100% of their audience is listening with receivers
connected to vertically polarized antennas. And they are
located within a zone where the dominant propagation mode
preserves the polarization of the transmitting antenna.
2. They want to minimize short hop (nearly vertical takeoff
angles) skywave multipath interference at the edges of their
daytime coverage area. The null in the vertical radiator's
elevation pattern at the zenith is most useful for preventing
this.
>But for our ham radio purpose, I am looking more for launching
>my signal at the best optimum angle in the most favorable
>direction at the time. Has anyone done A - B tests on DX
>signals between big screen and few radial verticals (or a half
>wave sloper) ?
>
>73 Yuri, K3BU, VE3BMV
I don't know if anyone has. But making signal strength
comaprisons of antennas on the basis of long distance multihop
sky wave propagated signals would be very nearly menaingless at
worst and extremely laborious at best. I would be very
suspicious of any data quantifying antenna performance on that
basis. I would have to read the details of how the study was
performed and what was controlled for VERY carefully before I
could even form an opinion. And then, what I would have would be
an opinion, not knowledge (IMHO).
73, Eric N7CL
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