VOACAP on Linux — Complete HF Propagation Prediction Setup and Guide

Live propagation predictor

Run VOACAP propagation predictions directly from your browser — enter your location and a target, choose a band and time, and get instant signal reliability forecasts based on current solar conditions.

What is VOACAP and how does it work

VOACAP is an HF propagation prediction engine originally developed by the Voice of America and the Institute for Telecommunication Sciences (ITS). It models the ionosphere using empirical data accumulated over decades of ionospheric research and calculates the probability that a signal on a given frequency will successfully propagate between two points at a given time.

Unlike simple MUF charts that only show the maximum usable frequency, VOACAP models the entire signal path — transmission power, antenna gain and pattern, ionospheric absorption, receiver noise floor, and atmospheric noise — and outputs a Signal-to-Noise Ratio (SNR) and a reliability percentage for each hour of the day on each HF band. This tells you not just whether propagation exists but whether it is good enough for your particular mode and equipment.

The model runs monthly median predictions based on the 11-year solar cycle. It does not predict actual real-time conditions — it tells you what the median propagation is likely to be for a given month and solar activity level. Think of it like a weather climate model rather than a weather forecast.

Who uses VOACAP

• DX operators — finding the best bands and times to work rare entities from their location
• Contesters — planning band change strategies and estimating which multipliers are workable
• Broadcasters — scheduling shortwave broadcasts to reach target audiences
• Emergency communications — planning NVIS links for regional emergency networks
• Military and government — the original users of the underlying propagation models

Installation on Linux

VOACAP itself is a Fortran-based command-line engine. Most amateur radio operators access it through web interfaces or GUI front-ends rather than running the raw engine directly.

Method 1 — voacapl Python package (recommended)

The voacapl Python package wraps the VOACAP engine and provides both a command-line interface and a Python API for scripting propagation predictions:

Method 2 — VOACAP Linux binary

Method 3 — Web interface (no installation)

For most amateur radio propagation planning the online VOACAP interfaces at voacap.com and soundbyte.com/cgi-bin/voacap require no installation. Enter your parameters in a browser and get results instantly. The live predictor on this page also provides a convenient no-install option.

Verify installation

Key concepts — MUF, SNR, reliability, solar flux

Understanding these concepts makes VOACAP output much more useful for operating decisions.

MUF — Maximum Usable Frequency

The MUF is the highest frequency that will reflect off the ionosphere for a given path at a given time. Signals above the MUF pass through the ionosphere into space — no HF propagation. The MUF varies with solar activity (higher during solar maximum), time of day (higher during daylight), and path geometry (longer paths have higher MUFs).

VOACAP calculates the median MUF — 50% of days the actual MUF will be above this value, 50% below. It also calculates the 90% MUF (exceeded 90% of days) which is a more conservative planning tool.

SNR — Signal-to-Noise Ratio

VOACAP's SNR output in dB tells you how much signal power is available above the noise floor at the receiver. The SNR required for reliable communication depends on mode:

Reliability percentage

VOACAP's reliability output (also called circuit reliability or REL) is the percentage of days in the reference month that the circuit is expected to be usable. A reliability of 90% means the path should be workable on 27 out of 30 days. The reliability is calculated for a specific SNR threshold — VOACAP uses different thresholds for different services.

For amateur radio planning: reliability above 70% indicates a regularly workable path, 50–70% means the path is open on most days but not all, below 50% means conditions are marginal and the path will frequently be closed.

Solar flux and sunspot number

VOACAP requires a solar activity input to model the ionosphere. Two indices are used:

• SSN (Smoothed Sunspot Number) — the traditional index, values from 0 (solar minimum) to 200+ (solar maximum). Higher SSN = better HF propagation on higher bands.
• SFI (Solar Flux Index) — measured at 10.7 cm wavelength, easier to measure in real time. SFI and SSN are related: SFI ≈ 49.9 + 0.59 × SSN. Most VOACAP interfaces accept either.

For current solar flux check the solar banner at the top of this site or NOAA's Space Weather Center at swpc.noaa.gov. Use the actual current SFI for short-term planning and the smoothed monthly value for longer-term predictions.

Using the VOACAP web interface

The easiest way to run VOACAP predictions is through the web interfaces. The live predictor on this page and the external tools at voacap.com both provide point-to-point and area predictions without installation.

Point-to-point prediction — what to enter

Interpreting the 24-hour reliability graph

The main VOACAP output for amateur radio use is a 24-hour bar chart showing the circuit reliability percentage for each band across each hour of the day (in UTC). Reading this chart tells you which bands are open and when:

• Dark green / 90%+ — excellent propagation, this band should be open on almost every day
• Green / 70–90% — good propagation, regularly usable
• Yellow / 50–70% — marginal propagation, open on most days but unreliable
• Orange / 30–50% — poor propagation, more closed than open
• Red / below 30% — very poor, rarely open

Command line operation

For scripted predictions or batch processing VOACAP can be run from the command line using the voacapl Python package.

Basic point-to-point prediction

Multi-band prediction script

Batch predictions for contest planning

Script VOACAP to predict paths to multiple target regions simultaneously — useful for CQWW or other DX contests where you want to know which zones are workable on which bands and at what times. Output the results to a CSV file for easy reference during the contest.

Input parameters explained

Antenna selection

The antenna parameters have a large impact on VOACAP results — getting them right makes predictions much more accurate:

Transmit power

Enter your actual transmit power in watts. Every 3 dB increase in power doubles the signal strength. Going from 100W to 1500W is a 12 dB improvement — roughly equivalent to adding a 3-element Yagi. VOACAP shows how much the SNR changes with different power levels, helping you decide whether an amplifier is worth the investment for a specific path.

SSN input — which value to use

• Current planning (next few weeks) — use the current smoothed SFI from swpc.noaa.gov and convert: SSN ≈ (SFI - 49.9) / 0.59
• Contest planning (specific month) — use the predicted monthly SSN from NOAA's solar cycle predictions
• Long-term planning — run predictions for a range of SSN values (50, 100, 150) to see how results change across the solar cycle

Noise figure

The noise figure represents how much noise the receiver adds to the incoming signal. For amateur HF operation values of 10–20 dB are typical. A higher noise figure (noisier receiver or poor antenna location) reduces the effective SNR. In urban environments with high local noise the effective noise figure can be much higher than the radio's specification — this is why a receiver that performs excellently in the lab sounds noisy in a suburban shack near power lines.

Understanding VOACAP output

The reliability-vs-time table

The primary VOACAP output is a matrix showing reliability % for each band at each hour of the day. Read it as follows:

• Each row is a band (80m through 10m)
• Each column is an hour of the day in UTC (00-23)
• The value in each cell is the probability (0-100%) that the circuit is usable at that hour
• A cell value of 90 means propagation should be present on 27 out of 30 days

SNR output

Alongside reliability VOACAP can output the median SNR for each hour and band. This is useful for mode planning — knowing the SNR lets you determine whether SSB, RTTY, or FT8 is the appropriate mode for a given path and time of day.

MUF output

The MUF output shows the predicted Maximum Usable Frequency for each hour. If your operating frequency is above the MUF at a given time, the signal passes through the ionosphere and there is no sky-wave propagation. Planning your frequency just below the MUF gives the shortest path and lowest absorption — but also the highest risk of the path closing as the MUF drops.

FOT — Optimum Traffic Frequency

The FOT (Frequency of Optimum Traffic, sometimes called the OWF — Optimum Working Frequency) is typically 85% of the MUF. Operating near the FOT gives good propagation with a reasonable safety margin below the MUF. VOACAP outputs the FOT alongside the MUF in detailed output mode.

Operating guide — using predictions for DX and contesting

Reading a VOACAP chart for band selection

When you open a VOACAP prediction for a specific path the most useful information for operating decisions is immediately visible in the reliability chart. Here is how to use it in practice:

1. Identify which bands show reliability above 70% — these are your primary operating bands for that path
2. Note the hours when each band is open — 20m to Japan from Europe may be open 06-12 UTC, while 40m may be better 14-22 UTC
3. Look for transitions between bands — when 15m is opening but 20m is still strong is the ideal time to move up a band
4. Note which bands are completely closed (reliability near zero) — do not waste time calling on a closed band

Example — planning a 20m contact from USA to Europe

Running VOACAP from a US Midwest location to central Europe at SSN 120 (moderate solar activity) for October typically shows:

• 10m — may show 60–80% reliability from 12-18 UTC — excellent when open but closes by evening
• 15m — good 13-20 UTC, peak around 16-17 UTC
• 20m — strong and reliable 12-22 UTC, the workhorse band
• 40m — opens strongly from 20 UTC onward as the greyline hits Europe, stays open all night
• 80m — good all night, peaks around 02-05 UTC

Gray line operating

The gray line — the terminator between day and night — is a zone of enhanced HF propagation. At the gray line the D-layer (which absorbs HF signals during daylight) is minimal while the F-layer (which reflects HF) is still strong. VOACAP models this effect and shows peaks in the reliability chart around local sunrise and sunset. Plan high-band DX operating to coincide with gray line conditions at either end of the path.

DX planning with VOACAP

Finding the best time to work a specific entity

1. Enter your location and the target entity's approximate location (capital city or main operating area)
2. Set the current month and smoothed SSN
3. Use 100W and a dipole for both ends — this gives a conservative baseline
4. Read which bands show reliability above 70% and at what UTC hours
5. If you have better antennas or more power the actual reliability will be higher than predicted

Evaluating whether a DXpedition is workable

When a rare DXpedition is announced, run VOACAP before it starts to understand:

• Which bands have the best propagation to your location
• What time of day each band opens and closes
• Whether the path supports high bands during solar maximum periods
• Whether low bands (40m/80m) are needed as a backup for poor high band conditions

Understanding why some DX is harder from your location

VOACAP reveals structural propagation asymmetries — why some operators consistently find certain paths easier than others. A station in the Pacific Northwest USA finds Japan much more accessible than a station on the US East Coast, despite similar distances on paper. VOACAP quantifies this by showing higher reliability and better SNR values for the more favourable path geometry.

NVIS planning for regional communications

NVIS (Near-Vertical Incidence Sky-wave) is the use of HF for short-range communication (50–500 km) by reflecting signals nearly straight up off the ionosphere. VOACAP models NVIS propagation when the path length is short. For emergency communications planning NVIS is critical — 40m and 80m with horizontal antennas are the primary NVIS bands. VOACAP shows the best hours and frequencies for NVIS links in your region.

Contest preparation

VOACAP is an essential tool for serious contesters. Running pre-contest propagation analysis helps you develop a band-change strategy that maximises QSO rate and multipliers.

Pre-contest analysis workflow

1. Identify the target zones and entities that are multipliers in your contest
2. Run VOACAP predictions from your location to each major target area (Europe, Asia, Pacific, South America, Africa)
3. Use the actual contest month and current SSN prediction
4. Create a band opening schedule showing which bands are open to which regions and when
5. Identify conflicts — times when multiple valuable regions are open simultaneously on the same band
6. Plan band changes to maximise multiplier opportunities

Example CQWW band strategy from USA east coast

A rough guide for high solar activity (SSN 150+) in October from the US East Coast:

• 00-03 UTC — 40m and 80m to Europe, excellent conditions both bands
• 03-06 UTC — 80m long path to Asia and Pacific, 40m Europe still going
• 06-10 UTC — greyline into Europe on 40m/20m, start checking 15m and 10m toward the end
• 10-14 UTC — 10m and 15m open to Europe and beyond, 20m still solid
• 14-18 UTC — 10m and 15m to Asia and Pacific long path if band is open
• 18-22 UTC — 20m strong to Europe and Africa, 15m fading
• 22-24 UTC — 40m opening to Europe, 20m still useful

Rate vs multipliers trade-off

VOACAP helps quantify the rate vs multiplier trade-off. If 20m shows 90% reliability to Europe but only 40% reliability to Asia at the same time, staying on 20m for Europe contacts is higher rate but you will miss the marginal Asia opening. VOACAP data lets you make informed decisions about whether to chase the marginal path or stay on the high-rate band.

Solar cycle effects on propagation

The 11-year solar cycle has a profound effect on HF propagation, particularly on the higher bands. Understanding where we are in the cycle helps calibrate VOACAP predictions and set realistic expectations.

Solar maximum vs solar minimum

Current solar cycle position

Solar Cycle 25 began in December 2019 and has been significantly more active than early predictions suggested — solar maximum is expected around 2025–2026 with SSN values potentially exceeding 150. This is excellent news for 10m, 12m, and 15m propagation. Run VOACAP with SSN values of 150–200 to see what high-band propagation will look like at the peak of the current cycle.

Using the solar banner data with VOACAP

The solar banner at the top of this site shows the current Solar Flux Index (SFI). Convert this to SSN for VOACAP input: SSN ≈ (SFI - 49.9) / 0.59. For example if the SFI reads 180: SSN ≈ (180 - 49.9) / 0.59 ≈ 220. Use this value in VOACAP for predictions that reflect current actual conditions rather than a monthly average.

Limitations of propagation models

VOACAP is the best available HF propagation model but it has important limitations to understand before relying on its predictions.

What VOACAP does not predict

• Short-term variations — VOACAP predicts monthly medians. Actual conditions on any given day may be better or worse. A geomagnetic storm can wipe out HF propagation that VOACAP predicts as excellent.
• Sporadic-E — the model does not include sporadic-E propagation which can open 10m and 6m unexpectedly and dramatically at any time of year
• Transequatorial propagation (TEP) — not reliably modeled
• Aurora — aurora-induced propagation disturbances are not included
• Local noise — VOACAP uses a standard atmospheric noise model. Your actual local noise floor may be much higher due to QRM, power line noise, or electronic interference
• Specific antenna effects — the model uses idealized antenna patterns. Real antenna performance varies with height, ground conductivity, and nearby structures

Confidence in VOACAP outputs

Treat VOACAP reliability percentages as planning guides rather than certainties. A path showing 80% reliability will frequently be open but not always. A path showing 20% reliability will occasionally be open — sometimes spectacularly so due to enhanced conditions not captured in the model. Use VOACAP to identify your primary and backup bands, then adapt in real time based on what you actually hear on the bands.

Complementing VOACAP with real-time data

Use VOACAP alongside real-time data sources for best operating decisions:

• DX cluster spots — live evidence of which bands are actually open to which regions right now
• PSKReporter — real-time FT8 reception map showing actual propagation paths
• WSPR spots — wsprnet.org shows beacon spots providing continuous propagation data
• RBN (Reverse Beacon Network) — CW skimmer network showing real-time band openings
• Solar weather alerts — NOAA Space Weather Center alerts for geomagnetic storms that disrupt propagation

Alternative propagation tools

Troubleshooting and common questions

VOACAP predicts good propagation but I hear nothing

Several explanations for this common experience:

• Geomagnetic storm — check the planetary K-index at swpc.noaa.gov. K-index above 4 significantly degrades HF propagation that VOACAP predicts as good. The K-index is shown in the solar banner at the top of this page.
• Absorption event — solar X-ray flares cause sudden HF fadeouts (SID — Sudden Ionospheric Disturbance) that can last minutes to hours
• Monthly median vs today — VOACAP predicts median conditions. Today may simply be below the median
• Antenna pointing — if using a directional antenna check it is pointing correctly toward the target region
• Band activity — a band being propagation-open does not mean stations are active on it. Check the DX cluster for activity reports

VOACAP predicts poor propagation but I worked the DX easily

This is sporadic-E, aurora-E, or a particularly enhanced ionospheric day. VOACAP is a statistical model based on typical conditions — exceptional conditions happen regularly and produce propagation well above the predicted median. This is a feature of HF radio, not a flaw in the model.

Results seem optimistic for low solar activity

Check that your SSN input reflects actual current conditions. During solar minimum operators often use SSN values from a previous active cycle by mistake. An SSN of 10 versus 100 dramatically changes predictions for 10m, 12m, and 15m. Also verify your antenna gain input — overestimating antenna performance gives optimistic results.

Frequently asked questions

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