HF Terrain Assessment

Overview

• HF Sky Wave Propagation

• What is terrain assessment?

• Why do it?

• What information and tools are used?• National Elevation Database• MicroDEM• HFTA

• Demo of the tools and process.

• Examples and comparisons of some QTHs

• Conclusion

HF Sky Wave Propagation

ARRL Antenna Book 21st pg. 23-23

Ionosphere

What is HF Terrain Assessment?

• An evaluation of an antenna’s performance given:

• The antenna’s Directivity• Gain pattern

• The required elevation angles of the desired propagation path.• Statistical averages are used

• The topographic profile of the local terrain.

Why Do Terrain Analysis?

• Determine the radio propagation characteristics of a specific location considered for a QTH.

• Find the best configuration for antennas at that location including:• Gain• Position• Height• Arrays

• Evaluation of compromises required of limited installations or multi-band antennas.• Examples:

• The location of a tower must be optimized if it will handle all azimuths.• The height of a multi-band yagi must be optimized for all bands it covers.

What do we need?

• A model of an antenna’s electrical performance.• NEC-2, EZNEC+

• 2D terrain profile data• The local topography of a specific azimuth.

• Elevation angles required for successful point-to-point communications• VOACAP provides the propagation characteristics of a circuit.

Terrain Elevation Data

• The National Elevation Dataset (NED) provides topographic elevation data for an antenna’s location and its surroundings.

• NED data can be obtained from USGS via the National Map website.• NED data is retrieved as 1 degree latitude by 1 degree longitude rectangles.

• Resolution of 1 arc-second is sufficient (~30 meters). 1/3 arc-second is available.

• Multiple formats available• ArcGrid or GridFloat – both work

• Demo

MicroDEM

• Freeware written by Dr. Peter Guth at the US Naval Academy.

• MicroDEM consumes GIS topographic data and generates 2D terrain profiles (among other things).• This would require a tremendous amount of work if done manually!

• These profiles are used by HFTA to analyze reflective and diffractive effects on signal propagation.• For HFTA, profiles are generated at 5 degree increments.

• Horizontal range is 4400 meters.

• Horizontal resolution is 30 meters.

• Demo

HFTA – High Frequency Terrain Assessment

• Software written by Dean Straw, N6BV, at ARRL.

• Available only with the ARRL Antenna Book, 19th edition and later.

• An enhanced version of NEC-2 that models an antenna and understands diffraction.

• HFTA analyzes the effects of:• antenna height• antenna position• antenna type – only the Dipole and Yagi types are supported• stacked antennas• ground conductivity

HFTA

• Uses terrain profile data to analyze reflection and diffraction of HF signals and generate a gain vs. elevation graph for an antenna• Profile data files are ASCII text. Example• Uses ray-tracing to analyze the 2D terrain effect on antenna response

• Combined with statistical elevation angle data, HFTA calculates a “figure of merit” (FOM) for an antenna configuration.• Elevation data files are ASCII text. Example• Elevations are averaged over all months, all days, all hours, over a complete

11 year sunspot cycle.

• Output (OUT.PRN) can be used with MAKEVOA to generate custom elevation statistics or coverage maps with VOAAREA.

HFTA• What can be changed in HFTA?

• What is the output graph telling me?• Elevation Statistics• Gain• Figure of Merit (FOM)

• Flat QTH• Antenna height• Antenna type• Use Flat as a reference

• KR7C QTH• Antenna height• Operating frequency• EU vs JA figure of merit• Compare to N0KE to Europe

• N0KE, KQ0C and K0ALT examples

Elevation Statisticsand Figure of Merit

Colorado to USA Colorado to Japan

Back

Antenna Type –Yagi vs. Dipole

The 3-element yagi has ~5 dB of gain over the dipole at 60 feet.

Yagi

Dipole

Antenna Height

Height inλ

HFTA angle of max gain

EZNECangle of max gain

Figure of Merit

0.5 27 23 4

1.0 14 13 8.7

1.5 9 9 10.2

2.0 7 7 10.4

HFTA and NEC produce nearly identical results over flat ground.

As an antenna is raised, the main lobe moves to lower angles.

0.5λ

1.0λ

1.5λ

2.0λ

The effect of terrain on KR7C’s QTH toward Europe

The effect of the high horizon (Red Mountain) is clearly seen in the cutoff at 12 deg.

My dipole has a figure of merit of minus 5.1!!

I have positive gain for only about 15% of the incoming signal elevations!

No wonder I hear Europe poorly.

The terrain of KR7C

My QTH has a 2000 ft. rise in the first 10,000 ft. at 40 degrees azimuth, which has a huge effect on propagation toward Europe.

Would it help to raise my dipole?

Not much.

Maybe ~3 dB or ½ an S-unit improvement in Figure of Merit, but only for ~15% of signals.

The 12 degree horizon still dominates.

Flat @ 60 ft.

Flat @ 30 ft.

Aspen @ 60 ft.

Aspen @ 30 ft.

Effect of operating frequency

KR7C to Europe at 28 MHz

Higher frequency allows low dipole to function better at low angles above the 12 degree horizon angle.

Approximately 1 wavelength high now so main lobe is lower.

8.2 dB difference in Figure of Merit due to the 12 degree horizon.

KR7CEurope vs. JapanMuch lower horizon to JA. FOM is only 3.5 dB better, but I should be able to hear signals below 12 degrees!

Compare KR7C and N0KE to Europe.

Maximum gain of 16 dBi at N0KE is at about 14 deg.

The figure of merit for Phil’s KT36XA 20m antenna is 12.7, approximately 17 dB better than KR7C!

ON AVERAGE N0KE has > 3 S-unit advantage.

N0KE

KR7C

Flat

N0KE’s terrain to Europe compared to KR7C

N0KE has a slight slope away from the antenna which serves to increase low angle gain because of ground reflections.

Terrain effect at N0KE for 4 azimuths

60

270

45

0 degrees

Compare N0KE, KQ0C and K0ALT to Europe

N0KE Figure of Merit is about 4 dB better than a tribander over flat terrain.

KQ0C’s cliff improves the response at low angles. FOM is about 2 dB better.

K0ALT looks into the hogback, which limits responses to > 7 degrees.

The terrain comparison is very telling.

KQ0CN0KE

K0ALT

Comparison of terrain at N0KE, KQ0C and K0ALT to Europe

Profiles are highly exaggerated vertically due to different scales.

The previous propagation results correlate with the characteristics of the individual terrains.

20% slope at KQ0C dominates the terrain effects.

Hogback dominates antenna response at K0ALT with an 8 degree line of sight.

Stacking Effects at N0KE

Adding a second yagi at 120 feet improves the low angle > 9 dB at 2 degrees.

Second configuration suffers from a deep null between 20-22 degrees.

Adding a third yagi at 30 feet improves the higher angles markedly and improves FOM to 15.8 dBi.

60 ft.

120/60

120/60/30

Conclusion

• Terrain data can be obtained via the National Elevation Database.

• MicroDEM is used to produce terrain profiles.

• HFTA consumes those profiles and VOACAP propagation elevation statistics to calculate the resulting antenna response.

• Terrain Assessment is an important tool when picking a QTH or designing antenna structures.

• Questions?

References

• USGS National Map Viewer:• http://nationalmap.gov/viewer.html

• MicroDEM:• http://www.usna.edu/Users/oceano/pguth/website/microdem/microdemdown.htm

• HFTA:• ARRL Antenna Book, 19th edition and later

• N6BV presentations:• http://nccc.cc/misc/HFTA.wmv• http://wwrof.org/webinar-archive/what-ive-learned-in-two-decades-of-terrain-assessment/

Additional Slides

• KQ0C QTH Compare

KQ0C QTH Comparison to Europe

Silt QTH has a FOM > 12 dB over Aspen Glen towards Europe.

Silt

Aspen Glen

Example terrain profile

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Ray Tracing – positive and destructive interference• Reflection • Diffraction direct

diffracteddirect

reflectedterrain

terrain

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Example elevation statistics file

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HFTA “OUT.PRN” output file

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