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,

Terrain AnalysisBrian 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 AnalysisWe have been showing you a scientific approach to designing

your contest station antenna systems.

1. Know what arriving signal angles must be covered.

2. Model the antennas and their height and design your systemto 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, usingDelorme TopoQuads.

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

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.

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 yousee when modeling an antenna, shown vertically.

Effects of Terrain

Here is the same antenna, shown on EZNEC. Note themain lobes at 12 degrees and 32 degrees.

Effects of Terrain

Terrain that slopes down from the antenna will enhance thelow 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 wastoo high for flat and downhill terrain is starting to look alittle better! (But still needs to be lowered or stacked).

HFTA Case Studies

The first example will be a real-world example of asomewhat 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 ona 100’ tower due to the constraints of the guy wires.

Obtaining Terrain Data for HFTA

Data must be downloaded and prepared before you canuse HFTA.

First, you must download the terrain data, centered on yourantenna 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 theterrain data that you have assembled. There is onefile 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 filesby 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 it’s pros and cons.

We will use the NED format in the following examples.

Downloading Terrain Data

NED data is available at: http://seamless.usgs.gov/

Defining the Data Limits

Limits are defined 1/10th of a degree each direction fromthe base of your tower.

Request Summary Page

Once the limits and output format (tiff) are defined, the data is downloaded to your computer.

Downloading & Saving the Data

The data is then saved to your hard drive. NED Data issaved 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.

Download Street Map Data (optional)

Street data can be downloaded at:http://www.census.gov/geo/tiger99/tl_1999.html

Find the FIPS Number & Download 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

Return to MicroDEM & Merge Map 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 settingswill produce radial files every 5 degrees out to 4400 meters

from your tower base.

Specify Radials File Name

Give a meaningful name to your radial files. MicroDEMwill append the degree bearing to this name for each file.

MicroDEM Creates 71 Radial Files

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

MicroDEM Creates 71 Radial Files

HFTA will use these files to model the effects of thisterrain on your antennas.

Setting up HFTA Analysis

Select a radial elevation file for your location and the directionof interest, and enter your antenna type and height.


Also you can select the profile for flat terrain to use as a comparison.


Select an elevation file to use as a reference for arrivingsignal 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 terrainradial 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, andthe red plot shows flat terrain, using a 3/3 stack at

100 & 63 feet, on 20 meters.


Purple bars show arriving signal angles that need to becovered to Europe, using W3LPL’s data.


Normally the program uses angle data referenced to thefrequency that a particular angle produces propagation.

Some of these angles appear unreasonable.


Conclusion is that my terrain slightly helps the signal to Europeon 20m, compared to flat terrain, on the lower angle paths.

Not enough to worry about, and may not be noticeable.


Lets start on 15m by having a look at the stack compared toflat terrain, to evaluate the effects of the terrain on this band.


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.

HFTA Used to Evaluate Stacks

HTFA can also be used to evaluate the angle coverage ofindividual antennas, and stacks, referenced to the arriving

signal angles that need coverage.


This complicated slide shows the plots for the stack (blue), theupper antenna (red), the middle antenna (green), and the lowerantenna (cyan). Lets look at them one at a time for simplicity!


First, the upper antenna at 100’. Note the deep nulls at14 degrees. This antenna covers the low angle paths

nicely, but is no good for the high angles.


Next, the middle antenna at 63’. This antenna covers the middleangles, except at 10 degrees thanks to the terrain. If you had topick one antenna, this would be the one, mounted a little higher.


Here is the bottom antenna at 33’. This is obviouslyhigh-angle antenna, probably best suited for sweepstakes!


Finally, the entire stack compared with flat terrain. The stackproduces a few db of gain over the individual antennas. Gainis achieved by redirecting the energy to the desired angles.


Just for reference, here is the stack using just the uppertwo antennas.


And here it is using the lower two antennas.


Things change quite a bit on 10 meters. This example showsthe result of an stack that is too high – note the deep null

at 12 degrees, which is an angle that needs to be covered!


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.


The downhill terrain in close helps at 3-4 degrees, but the ridgesat 2 miles out hurt the signal at 5 & 9 degrees. But we still

need to check the coverage of the individual antennas.


We have only looked at the 45 degree path to Europe. Whenevaluating your station with difficult terrain, you need to check

the entire path to target contest audiences.

The next example will be a real-world example of amountaintop QTH, where we might get into

trouble with antennas that are too highfor the local terrain.

I chose to use K4VV’s QTH, since he has a hilltop QTHwith 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 runningnortheast – southwest.


The street data is added, but this is not required.There is Jack’s street.


K4VV terrain to Europe, at 45 degrees. This terrain iscomplicated and runs along the ridge line.


K4VV terrain to Japan, at 330 degrees. This will havemore of an effect. It is downhill all the way.


Lets start with a 100’ yagi on 20 meters to Europe, andcompare it with flat terrain. This antenna is already showing

the effects of being too high because of the terrain.


This is the same antenna, now looking towards Japan. Note theugly null at 9 degrees caused by the terrain. We need more

antennas to fix this, and may need to lower the antennas a bit.


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.


Here are the antennas at 90’ and 45’. Now let’s look at thecoverage of the individual antennas.


Note that the upper antenna alone (red) is about 3db betterthan the stack at the null at 6 degrees caused by the terrain.


There is no magic fix for the effects of the terrain. All you cando is move the effects around by varying the antennaheights and using a stack to help control the angles.


Here is K4VV’s 15 meter path to Japan, again compared withflat terrain. The terrain is working in our favor on this path

at this height.


Lets look at some other directions.


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


Here is Jack’s terrain to the south, very different from his otherdirections. It is actually slightly uphill for the first mile.


The path to South America is the most complex, due to variationsin propagation, and needs coverage over a wide range of angles.

Note the nulls at 6 and 19 degrees that need fixing.


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.


Stacking with a lower antenna removes the null at 19 degreesand 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.

Documents

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,