Voacap Prediction

Voacap prediction (band × hour)

🇫🇷 Cette page en français : Prédiction Voacap

The Voacap tab answers one question: which band, at what hour, between me and him? It draws a table of your bands against the 24 hours UTC for the callsign in the entry field — once for the short path and once for the long path — and colours each cell by how the contact would go.

Nothing to install, no internet needed, no account. The model runs inside OpsLog.

Getting a chart

1. Your locator must be set (Settings → Station).
2. Type a callsign in the entry strip. The chart needs two ends, and the DX end comes from the lookup or the cluster spot — a callsign with no locator yet gives the entity centre, which is good enough for a 10 000 km path and not for a neighbour.
3. Open the Voacap tab.

The line under the title is the path itself: distance, bearing, the number of hops and the take-off angle the geometry asks for. Above the chart, best now names the band with the best chance at this hour.

The current hour's column is outlined, so "what do I do right now" is one glance.

What a cell says

Pick what the cells show with the three buttons:

Reading	Cell shows
Chance	the percentage of days the contact works at that hour
SNR	the signal-to-noise ratio delivered, in dB in your chosen bandwidth
S points	what the S-meter would read — S5, S9+, or empty when shut

They are three views of the same computation, not three models. Hover any cell for all of it at once: S-meter, SNR, MUF, the chance, which layer carries it (F2 or E) and at what angle.

An empty cell is a band that is shut at that hour — either above the MUF, or the signal does not arrive above the noise.

The station block

The chart is only as honest as what it is told about your station. The table under Station used for both paths carries, per band:

• TX power — watts.
• TX gain at take-off — in dBi, at the angle shown, not the catalogue figure. A 3-element Yagi measured at 30° is a different antenna at 5°, and on a long path 5° is where the contact is.
• Height — metres above ground.
• Lowest angle — the lowest angle this band can actually be radiated at.
• Antenna — which one, for your own reference.

Everything starts from the default antenna of each band (see Maps and Antennas), so a station already configured needs nothing typed here. Changing a figure on screen is a what-if: recalculate and look. Save and recalculate writes it back.

The take-off angle is per band, and comes from the height

Height counts in wavelengths, so one figure for the whole station cannot be right — 20 m up is two wavelengths on 10 m and a third of one on 80 m.

Type the height and OpsLog works the angle out: sin θ = λ / 4h, so

Height	Lowest angle
half a wave	30°
one wave	15°
two waves	7°

For a vertical, leave the height empty and type the angle instead: its pattern comes from its radials and the ground under them, not from how far up it is. Over decent ground, 10 to 15°.

This matters more than it looks. A 16 000 km path is five hops at 3° and ten at 15° — twice the ionospheric crossings, twice the absorption — which is why a dipole at 10 m does not work the same DX as a tower, on the same band, at the same hour.

Mode presets, noise, and his antenna

• SSB / CW / Digi set the bandwidth and the SNR a contact needs in one click. They are the two numbers that decide everything: about 24 dB in 2.4 kHz for comfortable SSB, 10 dB in 500 Hz for CW, 0 or less in 50 Hz for FT8.
• Noise here — quiet rural, rural, residential or city. The P.372 noise figures differ by more than 20 dB between the first and the last, which is more than most antennas.
• His antenna — a gain in dBi for the far end. A dipole at the other end is a different prediction from a 4-square.

Set the required SNR too high and every hour reads as hopeless. A 16 000 km path delivers about 10 dB on a good day. If the whole chart is empty, check this box before concluding the band is dead.

My angles, or VOACAP 3°

Two switches at the top of the station block, and the difference is not cosmetic:

• My angles — run on the table above, which is what your antennas can really do. This is the chart to operate from.
• VOACAP 3° — run on 3° at both ends, on every band. That is the Min.TOA the ITS VOACAP programs default to, and what Log4OM and VOACAP4OM leave alone.

Use the second one when you are comparing OpsLog with another tool: the same assumption on both sides or the numbers cannot be compared at all. It is a convention and not your station — 3° is kind to long paths on the low bands, where it halves the number of hops.

What the model is, and is not

It is OpsLog's own: CCIR median ionosphere, ITU-R P.533 control points, D-layer absorption sampled where the ray actually crosses it, a full link budget and P.372 noise, over the F2 and E layers. It is not the ITS VOACAP code, and it does not claim to be — the tab is called Voacap because that is the kind of chart it is.

It is a median month, like every prediction of this kind: the figure is what the path does on about half the days of the month, with today's sunspot number and geomagnetic index applied. The header shows the month, the SSN and the hour; Refresh space weather pulls the current indices again.

What it does not model: sporadic E, aurora, TEP, grey-line enhancement, meteor scatter — nor the fact that FT8 hears 15 dB below where a voice contact gives up. A chart saying "shut" is a statement about the median ionosphere, not a promise that nothing will happen.

See also

• Propagation MUF Map — the world MUF map, which answers where rather than when.
• Maps and Antennas — the great-circle map, and the per-band antennas these figures come from.
• DX Cluster and Spots — band openings seen in real reports rather than predicted.