HFTA Terrain Analysis: Optimising Ham Radio Antennas for Your Site
The ground directly under your antenna is only part of the story. Terrain stretching kilometres in the direction of your target — hills, ridges, valleys, and slopes — shapes the low-angle radiation from every HF antenna at your site. HFTA (High Frequency Terrain Assessment) models this terrain interaction and reveals whether your hill is a hidden asset or a silent 6 dB penalty on every DX path.
Every HF antenna book discusses the effect of antenna height on take-off angle and low-angle radiation. What these books rarely emphasise is that for most real-world sites, the terrain in the direction of the desired path matters as much as — and often more than — the height of the antenna above the local ground. A dipole at 15 m on the crest of a hill sloping toward Europe may outperform a dipole at 30 m on flat ground for European DX, because the downward slope effectively extends the antenna's electrical height above the terrain seen by low-angle radiation.
The mechanism is straightforward. Radio waves leaving the antenna at low elevation angles travel nearly parallel to the earth's surface. If the ground slopes downward in the direction of radiation, these waves spend their first few kilometres at a greater effective height above terrain than they would over flat ground. The ground reflection point — the location where the direct wave and the reflected wave combine before heading skyward to the ionosphere — is further from the antenna and the geometry favours a lower take-off angle and higher low-angle gain. The converse applies to upward slopes: terrain rising in front of the antenna physically blocks low-angle radiation and forces the effective radiation angle upward, reducing DX performance.
Downward slope — terrain advantage
Slope descending toward the target direction lowers the effective take-off angle and can add 3–6 dB to low-angle signal strength compared to flat ground. Contest stations deliberately seek hilltop sites with long forward slopes toward their key DX paths.
Upward slope — terrain penalty
Terrain rising within the first 1–3 km blocks low-angle radiation, pushing the effective take-off angle upward. A ridge or hill directly in front of the antenna toward Europe or Japan can cost 6–12 dB on key DX paths — more than any antenna improvement could recover.
Flat terrain — NEC2 baseline
Flat ground gives results close to standard NEC2 modelling with a real ground plane. Most amateur antenna textbooks and modelling software assume flat terrain. Real sites vary from this baseline — sometimes dramatically — in both directions.
HFTA — High Frequency Terrain Assessment — is a software tool written by Dean Straw (N6BV) and distributed by the ARRL as part of the ARRL Antenna Book CD-ROM. It combines NEC2 antenna modelling with real terrain elevation data to compute the combined antenna-plus-terrain radiation pattern for any direction from your site. The result is an elevation pattern that accounts for the actual shape of the ground your signals travel over, not just the flat-ground approximation.
Internally, HFTA works by computing the terrain elevation profile along a radial in the direction of interest — a cross-section of the ground height vs. distance from the antenna site. It then uses a technique called the Irregular Terrain Model (ITM, also known as the Longley-Rice model in its propagation form) combined with NEC2 antenna patterns to compute how the terrain modifies the free-space antenna pattern. The output is a composite elevation pattern showing the effective gain at each elevation angle, accounting for both the antenna's inherent pattern and the terrain's influence on that pattern.
Terrain data sources for HFTA
HFTA requires terrain elevation data as input. The standard source is the SRTM (Shuttle Radar Topography Mission) dataset, which provides 3 arc-second (approximately 90 m) elevation data globally and 1 arc-second (approximately 30 m) data for the United States and many other regions. SRTM data is freely available from the NASA Earthdata portal and several mirrors. HFTA reads these files in the .hgt binary format after conversion from the raw download format using a pre-processing utility.
Alternative terrain data sources include USGS National Elevation Dataset (NED) for the USA at 1/3 arc-second resolution (approximately 10 m), regional lidar surveys (0.5–2 m resolution in some areas), and hand-measured terrain profiles from topographic maps. Higher resolution data produces more accurate HFTA results, particularly for sites with complex local terrain within the first 1–2 km.
Installing and Configuring HFTAHFTA is distributed with the ARRL Antenna Book (21st edition and later) on CD-ROM or as a downloadable package for book owners. It runs on Windows XP through Windows 11. The ARRL also distributes a free limited version through its website that processes shorter terrain profiles.
Go to the NASA Earthdata portal (earthdata.nasa.gov) or a mirror such as the CGIAR-CSI SRTM portal. Download the SRTM tiles covering your location and the surrounding area in all directions of interest — typically 2–4 tiles for a continental US site, more if you are near a tile boundary. Download in the .hgt format if available, or convert using the HFTA pre-processor utility.
HFTA requires your antenna location in decimal degrees latitude and longitude to at least four decimal places (approximately 10 m accuracy). Use a GPS receiver at the antenna base for best accuracy, or extract from Google Maps (right-click on the antenna location and select the coordinates). Height above mean sea level is also required — find this from a topographic map or Google Earth.
HFTA has built-in models for common antenna types — horizontal dipoles, verticals, Yagis, and stacked arrays — parameterised by height and element configuration. Select your antenna type and enter the height above ground. For more complex antennas, HFTA can import a NEC2 pattern file computed externally by EZNEC or 4NEC2, allowing any antenna type to be analysed over terrain.
Enter the compass bearing toward your target region — for example, 45° for Northeast (Europe from the US West Coast), 310° for Northwest (Japan from the US East Coast). HFTA extracts the terrain profile along that azimuth from the SRTM data and computes the composite elevation pattern. Run for multiple azimuths to build a complete picture of your site's terrain advantage or disadvantage in each direction.
HFTA produces a modified elevation pattern — a polar plot showing gain vs. elevation angle that combines the antenna's inherent pattern with the terrain effect. The most important features to read are:
The main lobe position
Compare the take-off angle of the main lobe in the HFTA terrain-modified pattern versus the flat-ground NEC2 pattern. A downward slope typically shifts the main lobe to lower angles — sometimes dramatically. A 5° slope descending for 3 km can lower the effective take-off angle by 3–7° and add 3–6 dB of gain at the new lower angle. An upward slope shifts the main lobe upward and reduces low-angle gain.
Terrain-induced gain enhancement
HFTA computes a terrain gain figure — the difference in gain at the optimal take-off angle between the terrain-modified pattern and the flat-ground pattern. This is the terrain bonus (positive) or terrain penalty (negative) in decibels. A site with +4 dB terrain gain toward Japan effectively has 4 dB more antenna gain in that direction than its physical height alone would suggest. This is why hilltop contest stations so dramatically outperform flat-terrain stations at comparable antenna heights.
The first Fresnel zone
The terrain feature that matters most is what lies within the first Fresnel zone — the elliptical region of ground around the reflection point where the reflected wave has significant amplitude. At low elevation angles on HF, the first Fresnel zone reflection point is typically 1–5 km from the antenna. Terrain within this zone has the greatest influence on the HFTA result. Terrain beyond 15–20 km has diminishing effect.
For a dipole at 15 m and a target elevation angle of 10°, the ground reflection point is approximately 15 / tan(10°) = 85 m from the antenna base. At 5° elevation, it is 170 m. This means the terrain within the first 200–500 m from your antenna base dominates the low-angle radiation behaviour — close-in terrain improvements (levelling, removing obstructions) have measurable effects at these angles.
Interactive Calculator: Terrain Reflection Point & Fresnel ZoneGround Reflection Distance Calculator
555;margin-bottom:14px;font-family:Arial,sans-serif">Calculates the distance from the antenna base to the primary ground reflection point at a given elevation angle — the terrain location that most affects your signal at that angle.
The ideal DX site profile
The ideal HF DX site has a gradual downward slope in the direction of the target — ideally 2–5° of descent sustained over at least 2–5 km. This slope geometry causes the antenna's radiation pattern to be "redirected" downward relative to the local terrain, effectively simulating a much higher antenna. The classic contest station sites in the American Southwest (toward Asia and the Pacific) and in Europe (toward North America) almost invariably have this characteristic — an ocean cliff or a hillside descending toward the target continent.
The valley site — often misunderstood
A station in a valley between two ridges has complex terrain interactions. If the ridge in the direction of interest is close (under 2 km) and high (more than 5° elevation angle above the antenna), it blocks low-angle radiation and the station pays a significant DX penalty in that direction. However, if the valley opens in a favourable direction — the ridge is only in the unimportant rear direction, and the station looks out across a long descending slope toward the target — the valley site may actually outperform a hilltop site due to the favourable slope geometry in the open direction.
Urban and suburban sites
Most amateur operators cannot choose their site. For a typical suburban garden, the terrain within 200–500 m is dominated by buildings, streets, and local drainage features rather than natural landform. HFTA is less useful in dense urban environments because the terrain model does not include buildings, and the real near-field environment is dominated by structures that are not in the SRTM dataset. For suburban sites, HFTA is still useful for assessing the terrain beyond the immediate neighbourhood — the presence of a ridge 3–5 km away in the direction of Europe, for example, is a real constraint that HFTA quantifies accurately.
Extracting Terrain Profiles Without HFTA SoftwareEven without access to the full HFTA software package, you can obtain useful terrain profile data for your site using freely available web tools. This is valuable for a preliminary site assessment before investing in the full HFTA analysis.
Google Earth terrain profiles
Google Earth Pro (free) allows you to draw a path on the map and display its elevation profile. Draw a path from your antenna location in the direction of your primary DX target — for example, a 50 km line toward Europe — and view the elevation profile. This immediately shows whether terrain rises or falls in that direction, where the highest obstacle is, and at what distance from your site major terrain features appear. While not as precise as HFTA's integrated analysis, this visual assessment takes five minutes and gives a good qualitative picture of your site's terrain environment.
HeyWhatsThat terrain path profiles
The HeyWhatsThat.com website provides viewshed and path profile analysis based on SRTM terrain data. Enter your location, select a direction, and the tool displays the terrain elevation profile and the line-of-sight horizon in that direction. The horizon elevation angle shown by HeyWhatsThat directly corresponds to the elevation angle at which terrain begins to block low-angle radiation from your site — a horizon angle of 3° means terrain is blocking all radiation below 3° in that direction, regardless of antenna height.
Radio Mobile terrain analysis
Radio Mobile (ve2dbe.com) is a free propagation analysis tool that uses SRTM terrain data and supports HF path analysis. While primarily designed for VHF/UHF link budgets, its terrain profile display and horizon calculation functions are directly applicable to HF terrain assessment. Radio Mobile can display terrain profiles for paths of any length and in any direction from your site.
Interactive Calculator: Slope Gain EstimatorTerrain Slope Gain / Penalty Estimator
555;margin-bottom:14px;font-family:Arial,sans-serif">Estimates the terrain-induced gain change relative to flat ground for a uniform slope in the direction of the target. Based on the terrain gain model from Dean Straw N6BV's HFTA documentation.
List the compass bearings from your location toward the DX areas you most want to work. For a US East Coast station: Europe ~55°, Caribbean ~140°, South America ~160°, Japan ~330°, West Africa ~100°. For a UK station: North America ~280°, Japan ~30°, Australasia ~120°, Africa ~170°. These are the azimuths you will run HFTA analyses for.
Before downloading terrain files and running HFTA, do a 10-minute Google Earth assessment. Draw elevation profiles in each target direction out to 30–50 km. This immediately identifies any obvious terrain obstacles (a ridge 5 km away toward Europe) or advantages (a valley opening that descends toward Japan). This pre-assessment guides which HFTA runs will be most informative.
SRTM tiles are 1°×1° geographic blocks named by their southwest corner coordinates — for example, N51W002.hgt covers a tile with its southwest corner at 51°N 2°W (part of southern England). Download all tiles needed to cover 50 km in each target direction from your site. Each tile file is approximately 25 MB for 3 arc-second data.
Configure HFTA with your station coordinates, SRTM data folder, and antenna type/height. Run the analysis at your planned antenna height and at 2× your planned height — this shows how much improvement additional tower height would provide at your specific site. A flat-terrain site sees dramatic improvement with height; a hilltop site with a good descending slope may show diminishing returns above a certain height because the terrain is already providing effective height.
The most useful HFTA output is the comparison between the terrain-modified pattern and the flat-ground NEC2 pattern at the same antenna height. This quantifies the terrain bonus or penalty at your site in each direction. Document these numbers — they are invaluable when deciding whether to invest in a taller tower, a different antenna location on the property, or antenna types optimised for a particular elevation angle.
If your property has multiple possible antenna sites — the front garden vs. the rear, a higher corner of the plot vs. a lower corner — run HFTA for each candidate location. Even a 30 m shift can change the terrain profile within the critical first Fresnel zone significantly. The HFTA analysis may reveal that one location has a 4 dB terrain advantage over another for your most wanted DX direction.
Horizontal antennas (dipoles, Yagis)
Horizontal antennas benefit from terrain slope in a manner directly analogous to increasing antenna height. The slope-induced gain improvement depends on the antenna's height above ground, the slope angle, and the distance over which the slope is maintained. A horizontal antenna at moderate height (0.3–0.5λ) on a 3° downward slope sustained for 5 km toward the target can achieve the low-angle performance of a flat-ground antenna at twice the height. This is the fundamental physics behind why the best DX stations combine good antenna height with favourable terrain.
Vertical antennas
Vertical antennas have an inherently low take-off angle over a good ground system. Terrain slope interacts with verticals differently from horizontal antennas. A downward slope toward the target still helps, but the improvement is less dramatic because the vertical's radiation pattern already extends to very low angles over flat ground. An upward slope in front of a vertical can still cause significant shadowing of the lowest radiation angles, raising the effective take-off angle and reducing DX performance. HFTA models vertical antennas correctly — run separate analyses for each antenna type you are considering at a new site.
Stacked arrays and phased verticals
Stacked horizontal arrays (two or more Yagis on the same tower) have narrower elevation patterns than a single antenna — the stack concentrates radiation in a tighter range of elevation angles. If the stack's main lobe angle aligns with the terrain-advantaged elevation angle at your site, the combination is particularly powerful. HFTA handles stacked arrays through its antenna import feature — compute the stack's pattern in EZNEC or 4NEC2, export the pattern file, and import it into HFTA for terrain-modified analysis.
Using HFTA Results to Make Station DecisionsHFTA is a decision support tool, not a guarantee of performance. Its results are most useful when framed as comparative questions rather than absolute predictions. Common decision scenarios where HFTA provides clear guidance:
| Decision | How HFTA helps | Typical finding |
|---|---|---|
| Tower at 18 m vs. 24 m | Compare terrain-modified patterns at both heights for key DX azimuths | On flat sites, 6 m extra height is significant. On a good slope, the gain may already be saturating. |
| Dipole vs. vertical for DX | Run both antenna models over actual terrain in the target direction | A dipole on a descending hilltop often beats a vertical on flat ground — terrain changes the comparison dramatically. |
| Front vs. rear garden antenna location | Run HFTA from each candidate location for target azimuth | Even 30–50 m positional shift can change the terrain profile within the Fresnel zone. |
| Evaluating a potential new QTH | Run HFTA before signing the lease or purchase contract | Identifies sites with terrain obstacles that no antenna investment can overcome, versus sites with genuine DX potential. |
| Understanding poor DX performance in one direction | Run HFTA for the problem azimuth and compare with good-direction performance | Often reveals a ridge or hill at the critical reflection distance — explains why the problem is structural, not an antenna fault. |
How much can terrain actually affect signal strength?
Terrain effects of 3–8 dB are common and well-documented. Extreme cases — a hilltop station with a long descending slope toward a low-angle DX path — can show terrain gains of 10–15 dB compared to flat ground at the same antenna height. This is equivalent to increasing transmitter power from 100 W to 1,000–3,000 W, which explains why contest stations prioritise site selection above almost every other factor.
Is HFTA accurate for urban and suburban sites?
HFTA is less accurate in dense urban environments because buildings — which are not in SRTM terrain data — dominate the near-field environment within the first 200–500 m. For suburban sites where homes and trees are 5–10 m tall but the surrounding terrain is otherwise natural, HFTA gives useful results for distances beyond 500–1,000 m from the antenna. For rural sites, HFTA accuracy is very good.
What SRTM resolution do I need for HFTA?
3 arc-second data (approximately 90 m resolution) is adequate for terrain features beyond 1 km. For the critical first Fresnel zone within 500 m, 1 arc-second (30 m) data gives noticeably better results. If 1/3 arc-second USGS NED data is available for your US site, it provides the best accuracy for close-in terrain. For most HFTA analyses, 3 arc-second global data gives useful results.
Can I use HFTA to compare multiple antenna heights?
Yes — this is one of HFTA's most useful applications. Run the same terrain profile with antenna heights of 10 m, 15 m, 20 m, and 30 m and compare the resulting patterns. The diminishing returns of height on a good sloping site versus the continued improvement on a flat site clearly motivate or justify (or not) the investment in a taller tower.
Does HFTA work for NVIS antennas?
HFTA is designed for low-angle DX analysis and is less relevant for NVIS (Near Vertical Incidence Skywave) antennas, which deliberately target high elevation angles (60–90°). Terrain effects at 70–90° elevation are very small. For NVIS antenna selection, standard NEC2 flat-ground modelling with real ground parameters is the appropriate tool.
Are there alternatives to HFTA for terrain analysis?
The ARRL's PropLab Pro includes terrain analysis functions. Radio Mobile (free) provides path profiles from SRTM data. The online tool at heywatsthat.com gives terrain horizon profiles. For a full HFTA-equivalent analysis, HFTA itself remains the most widely used and documented tool specifically designed for amateur HF terrain assessment. Some commercial programs (EZNEC Pro, NEC-Win Pro) also offer terrain import functionality.