Terrain Analysis by N36PqYC

VIEWS: 19 PAGES: 74

• pg 1
```									 Terrain Analysis
Brian McGinness N3OC
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?
Terrain Analysis
We have been showing you a scientific approach to designing

1. Know what arriving signal angles must be covered.

2. Model the antennas and their height and design your system
to cover those angles.

3. Be aware of the effects of terrain and take corrective action,
if required.
N3OC to Europe

N3OC to Europe – 10 miles out, using
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?
ARRL’s HFTA
Software

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.
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.
Effects of Terrain

Here is the same antenna, shown on EZNEC. Note the
main lobes at 12 degrees and 32 degrees.
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.
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.
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!
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).
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.
Obtaining Terrain Data for HFTA

use HFTA.

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.
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.

(digital elevation model) or NED (national
elevation dataset) formats.

Each format has it’s pros and cons.

We will use the NED format in the following examples.

NED data is available at: http://seamless.usgs.gov/
Defining the Data Limits

Limits are defined 1/10th of a degree each direction from
Request Summary Page

Once the limits and output format (tiff) are defined,

The data is then saved to your hard drive. NED Data is
saved in the C:\mapdata\DEMs directory.
Opening the Data in MicroDEM

The NED data is unzipped, then opened using MicroDEM.
Opening the Data in MicroDEM

There is no street data yet, just raw elevation data.

http://www.census.gov/geo/tiger99/tl_1999.html
Data

Montgomery is 24 031 and Howard is 24 027
Save the Street Data

Tiger street data is saved in the C:\mapdata\tiger subdirectory
Data

Click on Vector Overlay icon. N3OC QTH needs two counties.
Completed Terrain for N3OC with
Street Data
Entering Weapons & Viewshed
Parameters

Click the Weapons Fan icon, then double-click anywhere on map
Enter Tower Location

Enter the coordinates of your tower base.
Enter ViewShed Parameters

Enter the parameters for the radial files. These settings
will produce radial files every 5 degrees out to 4400 meters

will append the degree bearing to this name for each file.

These radial files contain elevation data every 30 meters from
the tower base out to 4400 meters, every 5 degrees.

HFTA will use these files to model the effects of this
Setting up HFTA Analysis

of interest, and enter your antenna type and height.
Setting up HFTA Analysis

Also you can select the profile for flat terrain
to use as a comparison.
Setting up HFTA Analysis

Select an elevation file to use as a reference for arriving
signal angles. We are using W3LPL’s angle data instead
of the data that comes with the program.
Resulting Terrain Profile

Profile of the terrain as specified in the N3OC-45 terrain
radial file. Note the antenna heights are shown too.
HFTA Terrain Plot for N3OC to Eu on 20m

The blue plot shows gain (in dbi) of N3OC’s terrain, and
the red plot shows flat terrain, using a 3/3 stack at
100 & 63 feet, on 20 meters.
HFTA Terrain Plot for N3OC to Eu on 20m

Purple bars show arriving signal angles that need to be
covered to Europe, using W3LPL’s data.
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.
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.
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.
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
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.
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!
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.
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.
HFTA Used to Evaluate Stacks

Here is the bottom antenna at 33’. This is obviously
high-angle antenna, probably best suited for sweepstakes!
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.
HFTA Used to Evaluate Stacks

Just for reference, here is the stack using just the upper
two antennas.
HFTA Used to Evaluate Stacks

And here it is using the lower two antennas.
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!
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.
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.
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.
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 K4VV’s QTH, since he has a hilltop QTH
with complicated downhill terrain, and he is the process
of building a station at this location.

Let’s see what works at his location!
HFTA Case Studies – K4VV

Here is K4VV’s MicroDEM data. Note the ridge line running
northeast – southwest.
HFTA Case Studies – K4VV

The street data is added, but this is not required.
There is Jack’s street.
HFTA Case Studies – K4VV

K4VV terrain to Europe, at 45 degrees. This terrain is
complicated and runs along the ridge line.
HFTA Case Studies – K4VV

K4VV terrain to Japan, at 330 degrees. This will have
more of an effect. It is downhill all the way.
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.
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.
HFTA Case Studies – K4VV

First lets try a stack, at the traditional heights for a 20 meter stack.
It’s getting better… Let’s try lowering the antennas a bit.
HFTA Case Studies – K4VV

Here are the antennas at 90’ and 45’. Now let’s look at the
coverage of the individual antennas.
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.
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.
HFTA Case Studies – K4VV

Here is K4VV’s 15 meter path to Japan, again compared with
flat terrain. The terrain is working in our favor on this path
at this height.
HFTA Case Studies – K4VV

Lets look at some other directions.
HFTA Case Studies – K4VV

The same height looks about right for Jack’s path to Europe.
What about South America? We haven’t looked there yet.
HFTA Case Studies – K4VV

Here is Jack’s terrain to the south, very different from his other
directions. It is actually slightly uphill for the first mile.
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.
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.
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.
Conclusions
Downhill terrain enhances the lower angles.

Uphill terrain enhances the higher angles.

Irregular terrain introduces peaks and valleys
in the antenna’s 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.
The End.

```
To top