Terrain Analysis Brian McGinness N3OC

1
Terrain AnalysisBrian McGinness N3OC
2
Terrain Analysis

• Now that we know what angle signals arrive at,
  and what effect antenna height has on takeoff
  angle, what about the effects of the terrain?

3
Terrain Analysis

• We have been showing you a scientific approach to
  designing
• your contest station antenna systems.
• Know what arriving signal angles must be covered.
• Model the antennas and their height and design
  your system
• to cover those angles.
• Be aware of the effects of terrain and take
  corrective action,
• if required.

4
N3OC to Europe

• N3OC to Europe 10 miles out, using
• Delorme TopoQuads.

5
N3OC to Europe
Delorme Topo3D shows desired downhill terrain out
to one mile. But at 2-3 miles out, there is
undesirable uphill terrain. What effect does
this have on HF signals?
6
ARRLs HFTA Software

• One way to answer the question is to purchase the
  ARRL Antenna Book, which includes HF Terrain
  Assessment software.
• HFTA uses public USGS terrain data to model the
  effects of terrain on your HF signal.

7
Effects of Terrain
Generally speaking, flat terrain will produce an
even, bell-curve plot on HFTA, on all bands. This
is really the lobes of the antenna pattern that
you see when modeling an antenna, shown
vertically.
8
Effects of Terrain
Here is the same antenna, shown on EZNEC. Note
the main lobes at 12 degrees and 32 degrees.
9
Effects of Terrain
Terrain that slopes down from the antenna will
enhance the low angles. This antenna now needs
to be lowered a bit on the tower to compensate
for the terrain.
10
Effects of Terrain
This really becomes a problem on 10
meters! Beware of mountain QTHs with too high
of an antenna, especially on 10 meters.
11
Effects of Terrain
As you might now expect, uphill terrain enhances
the high angles, and impairs the low angles.
This slide shows the effects on 20 meters, where
the gain at low angles has dropped by 5db!
12
Effects of Terrain
Here are the effects on 10 meters. Our antenna
that was too high for flat and downhill terrain
is starting to look a little better! (But still
needs to be lowered or stacked).
13
HFTA Case Studies
The first example will be a real-world example of
a somewhat compromised antenna system, a
tribander stack, with average terrain. Since the
antennas have to cover three bands, they cannot
all be at the optimal height for each band. In
this example, the antennas are of the
typical multi-monobander type of tribander,
and are located at 100, 63 and 33. These are
not ideal heights, but they are what work on a
100 tower due to the constraints of the guy
wires.
14
Obtaining Terrain Data for HFTA
Data must be downloaded and prepared before you
can use HFTA. First, you must download the
terrain data, centered on your antenna
location. Then the street map data is merged
with the terrain data. Use of the street map data
is optional. Finally, terrain azimuth files are
created from the terrain data that you have
assembled. There is one file for each five
degrees of the compass, from the base of your
tower out to 4400 meters.
15
Obtaining Terrain Data for HFTA
Alternatively, you can create your own terrain
data files by using topographical maps and a text
editor. These files contain the terrain
elevation in meters, every thirty
meters. Terrain data is available for download
in the DEM (digital elevation model) or NED
(national elevation dataset) formats. Each
format has its pros and cons. We will use the
NED format in the following examples.
16
Downloading Terrain Data
NED data is available at http//seamless.usgs.gov
/
17
Defining the Data Limits
Limits are defined 1/10th of a degree each
direction from the base of your tower.
18
Request Summary Page
Once the limits and output format (tiff) are
defined, the data is downloaded to your computer.
19
Downloading Saving the Data
The data is then saved to your hard drive. NED
Data is saved in the C\mapdata\DEMs directory.
20
Opening the Data in MicroDEM
The NED data is unzipped, then opened using
MicroDEM.
21
Opening the Data in MicroDEM
There is no street data yet, just raw elevation
data.
22
Download Street Map Data (optional)
Street data can be downloaded at http//www.censu
s.gov/geo/tiger99/tl_1999.html
23
Find the FIPS Number Download Data
Montgomery is 24 031 and Howard is 24 027
24
Save the Street Data
Tiger street data is saved in the
C\mapdata\tiger subdirectory
25
Return to MicroDEM Merge Map Data
Click on Vector Overlay icon. N3OC QTH needs two
counties.
26
Completed Terrain for N3OC with Street Data
27
Entering Weapons Viewshed Parameters
Click the Weapons Fan icon, then double-click
anywhere on map
28
Enter Tower Location
Enter the coordinates of your tower base.
29
Enter ViewShed Parameters
Enter the parameters for the radial files. These
settings will produce radial files every 5
degrees out to 4400 meters from your tower base.
30
Specify Radials File Name
Give a meaningful name to your radial files.
MicroDEM will append the degree bearing to this
name for each file.
31
MicroDEM Creates 71 Radial Files
These radial files contain elevation data every
30 meters from the tower base out to 4400 meters,
every 5 degrees.
32
MicroDEM Creates 71 Radial Files
HFTA will use these files to model the effects of
this terrain on your antennas.
33
Setting up HFTA Analysis
Select a radial elevation file for your location
and the direction of interest, and enter your
antenna type and height.
34
Setting up HFTA Analysis
Also you can select the profile for flat terrain
to use as a comparison.
35
Setting up HFTA Analysis
Select an elevation file to use as a reference
for arriving signal angles. We are using W3LPLs
angle data instead of the data that comes with
the program.
36
Resulting Terrain Profile
Profile of the terrain as specified in the
N3OC-45 terrain radial file. Note the antenna
heights are shown too.
37
HFTA Terrain Plot for N3OC to Eu on 20m
The blue plot shows gain (in dbi) of N3OCs
terrain, and the red plot shows flat terrain,
using a 3/3 stack at 100 63 feet, on 20 meters.
38
HFTA Terrain Plot for N3OC to Eu on 20m
Purple bars show arriving signal angles that need
to be covered to Europe, using W3LPLs data.
39
HFTA Terrain Plot for N3OC to Eu on 20m
Normally the program uses angle data referenced
to the frequency that a particular angle produces
propagation. Some of these angles appear
unreasonable.
40
HFTA Terrain Plot for N3OC to Eu on 20m
Conclusion is that my terrain slightly helps the
signal to Europe on 20m, compared to flat
terrain, on the lower angle paths. Not enough to
worry about, and may not be noticeable.
41
HFTA Terrain Plot for N3OC to Eu on 15m
Lets start on 15m by having a look at the stack
compared to flat terrain, to evaluate the effects
of the terrain on this band.
42
HFTA Terrain Plot for N3OC to Eu on 15m
The downhill terrain has shifted the angles a
little to the left, and chewed up the plot a bit,
but probably not enough to worry about.
43
HFTA Used to Evaluate Stacks
HTFA can also be used to evaluate the angle
coverage of individual antennas, and stacks,
referenced to the arriving signal angles that
need coverage.
44
HFTA Used to Evaluate Stacks
This complicated slide shows the plots for the
stack (blue), the upper antenna (red), the middle
antenna (green), and the lower antenna (cyan).
Lets look at them one at a time for simplicity!
45
HFTA Used to Evaluate Stacks
First, the upper antenna at 100. Note the deep
nulls at 14 degrees. This antenna covers the low
angle paths nicely, but is no good for the high
angles.
46
HFTA Used to Evaluate Stacks
Next, the middle antenna at 63. This antenna
covers the middle angles, except at 10 degrees
thanks to the terrain. If you had to pick one
antenna, this would be the one, mounted a little
higher.
47
HFTA Used to Evaluate Stacks
Here is the bottom antenna at 33. This is
obviously high-angle antenna, probably best
suited for sweepstakes!
48
HFTA Used to Evaluate Stacks
Finally, the entire stack compared with flat
terrain. The stack produces a few db of gain
over the individual antennas. Gain is achieved
by redirecting the energy to the desired angles.
49
HFTA Used to Evaluate Stacks
Just for reference, here is the stack using just
the upper two antennas.
50
HFTA Used to Evaluate Stacks
And here it is using the lower two antennas.
51
HFTA Terrain Plot for N3OC to Eu on 10m
Things change quite a bit on 10 meters. This
example shows the result of an stack that is too
high note the deep null at 12 degrees, which is
an angle that needs to be covered!
52
HFTA Terrain Plot for N3OC to Eu on 10m
Adding a lower antenna to the stack almost fixes
this problem. This example shows a 5/5/5 stack at
100, 63 and 33. The ridges out at 2 miles are
effecting the signal.
53
HFTA Terrain Plot for N3OC to Eu on 10m
The downhill terrain in close helps at 3-4
degrees, but the ridges at 2 miles out hurt the
signal at 5 9 degrees. But we still need to
check the coverage of the individual antennas.
54
HFTA Used to Evaluate Stacks
We have only looked at the 45 degree path to
Europe. When evaluating your station with
difficult terrain, you need to check the entire
path to target contest audiences.
55
HFTA Case Studies K4VV
The next example will be a real-world example of
a mountaintop QTH, where we might get
into trouble with antennas that are too high for
the local terrain. I chose to use K4VVs QTH,
since he has a hilltop QTH with complicated
downhill terrain, and he is the process of
building a station at this location. Lets see
what works at his location!
56
HFTA Case Studies K4VV
Here is K4VVs MicroDEM data. Note the ridge
line running northeast southwest.
57
HFTA Case Studies K4VV
The street data is added, but this is not
required. There is Jacks street.
58
HFTA Case Studies K4VV
K4VV terrain to Europe, at 45 degrees. This
terrain is complicated and runs along the ridge
line.
59
HFTA Case Studies K4VV
K4VV terrain to Japan, at 330 degrees. This will
have more of an effect. It is downhill all the
way.
60
HFTA Case Studies K4VV
Lets start with a 100 yagi on 20 meters to
Europe, and compare it with flat terrain. This
antenna is already showing the effects of being
too high because of the terrain.
61
HFTA Case Studies K4VV
This is the same antenna, now looking towards
Japan. Note the ugly null at 9 degrees caused by
the terrain. We need more antennas to fix this,
and may need to lower the antennas a bit.
62
HFTA Case Studies K4VV
First lets try a stack, at the traditional
heights for a 20 meter stack. Its getting
better Lets try lowering the antennas a bit.
63
HFTA Case Studies K4VV
Here are the antennas at 90 and 45. Now lets
look at the coverage of the individual antennas.
64
HFTA Case Studies K4VV
Note that the upper antenna alone (red) is about
3db better than the stack at the null at 6
degrees caused by the terrain.
65
HFTA Case Studies K4VV
There is no magic fix for the effects of the
terrain. All you can do is move the effects
around by varying the antenna heights and using a
stack to help control the angles.
66
HFTA Case Studies K4VV
Here is K4VVs 15 meter path to Japan, again
compared with flat terrain. The terrain is
working in our favor on this path at this height.
67
HFTA Case Studies K4VV
Lets look at some other directions.
68
HFTA Case Studies K4VV
The same height looks about right for Jacks path
to Europe. What about South America? We havent
looked there yet.
69
HFTA Case Studies K4VV
Here is Jacks terrain to the south, very
different from his other directions. It is
actually slightly uphill for the first mile.
70
HFTA Case Studies K4VV
The path to South America is the most complex,
due to variations in propagation, and needs
coverage over a wide range of angles. Note the
nulls at 6 and 19 degrees that need fixing.
71
HFTA Case Studies K4VV
The null at 6 degrees is caused by terrain and
may not be fixable. The null at 19 degrees can be
fixed with a lower antenna and a stack.
72
HFTA Case Studies K4VV
Stacking with a lower antenna removes the null at
19 degrees and produces a little gain. The gain
is probably not noticeable, the angle coverage is
the real benefit of a stack.
73
Conclusions
Downhill terrain enhances the lower
angles. Uphill terrain enhances the higher
angles. Irregular terrain introduces peaks and
valleys in the antennas vertical pattern that
are hard to control. Minor variations in terrain
have little effect on the antenna pattern. You
will probably only notice problems with terrain
that has wide variations.
74
The End.