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Guide · VHF/UHF propagation

Autumn tropo:
when to catch DX on 2 m and 70 cm

A few times a year the atmosphere over Poland sets up so that a signal on 2 m and 70 cm, instead of shooting off into space, bends back under a warm air layer and runs for hundreds of kilometres. In autumn such days are common, because highs linger over Europe. I show how to recognise an approaching opening, what time to switch the radio on, which frequencies to listen on and how to make your first long‑distance QSO.

high pressure + calm + fog dawn and evening SSB 144.300 · 432.200 horizontal polarisation
Live tropo map Where to listen
The ridge of the Tatra Mountains protruding above a sea of clouds at sunset, seen from a distance of about 150 km The Tatras from about 150 km, above a sea of clouds during an inversion · photo by Milan Bališin, CC BY-SA 4.0, Commons
Physics without formulae

A duct in the air, not a bounce off the sky

VHF/UHF signals usually peter out not far beyond the horizon. Tropo isn’t a reflection from the ionosphere, but bending of the wave in the lowest kilometres of the atmosphere, where the weather happens. It’s governed by how pressure, temperature and humidity change with height.

−40 N/km

A normal day

In a standard atmosphere, the refractivity of air decreases by about 40 N units per kilometre of altitude, because with height the pressure drops and water vapour diminishes. The wave bends slightly downwards, so the radio horizon lies a little farther than the visual horizon. That’s what the ITU models assume (k‑factor ≈ 4/3).

−157 N/km

Duct

When refractivity falls faster than 157 N per kilometre (i.e. the modified refractivity M decreases with height), the wave bends more than the curvature of the Earth. It no longer escapes upwards; it turns back towards the ground and runs within the layer as in a waveguide. This threshold is given by ITU‑R Recommendation P.834.

~180 m

Duct for 2 m

The duct has to be fairly thick relative to the wavelength. According to a US Navy approximation, at 144 MHz you need a layer roughly 180 m thick. It isn’t a sharp boundary: a somewhat thinner duct will still extend the range.

~90 m

Duct for 70 cm

At 432 MHz a layer about half as thick is enough. A thin duct can therefore open 70 cm while 2 m is still quiet. A thick duct will carry both bands.

How a tropospheric duct works Left: in a normal atmosphere the wave from the antenna bends slightly but escapes beyond the horizon. Under an inversion the wave is bent back towards the ground and travels far within the duct. Right: the modified refractivity M profile, which in the duct decreases with height. inversion (warm over cold) no inversion: the wave escapes upwards in a duct: the wave returns and travels hundreds of km hM duct M decreases M profile

Schematic, not to scale. A duct forms where the modified refractivity M decreases with height: most often beneath an inversion, i.e. a layer of warm, dry air over cooler, moist air.

!

Only the wave that leaves the antenna almost horizontally remains in the duct: according to ITU‑R P.834 it’s fractions of a degree, depending on the duct’s thickness and strength. What matters is an antenna that ‘sees’ the horizon, not one aimed at the sky. Mountains and tall buildings in the path are obstacles, while a high mast or a hill helps, especially when the duct lies higher above the ground.

Season

Why autumn and are we sure?

In autumn, broad high‑pressure systems often sit over Central Europe and the nights are already long and cool. These two factors create two types of inversion that matter most to us.

High‑pressure (subsidence) inversion

Under a high, air slowly descends, warming and drying as it does. Beneath it a cool, moist layer remains. The boundary between them is a ready‑made duct that can last all day, and under a stationary high for several days in a row.

Radiative (night‑time) inversion

On a clear, windless night the ground cools rapidly and chills the air just above it. Higher up it remains warmer. Such a layer is shallow, follows the terrain, and usually vanishes shortly after sunrise. Hepburn notes that a shallow nocturnal inversion can even hinder reception of long‑distance signals from a high‑pressure tropo.

What about tropo over the sea?

Honestly: according to Hepburn, maritime tropo is best in spring and early summer, not in autumn. Measurements from the island of Utö in the Baltic confirm this. Atmospheric profiles there showed ducts also in autumn, for about 40% of the time, but shallow and weak: an X‑band radar saw beyond the horizon through them only 1% of the time, compared with 37% in spring and 42% in summer. The authors write that in autumn and winter a shallow, weak duct near the sea surface often appeared in the profiles. And on 2 m you need a layer about 180 m thick. From central Poland the paths are mainly over land, though, and DXMaps states plainly that land‑based tropo is frequent outside summer.

i

The strongest argument comes from Polish data. On the PK‑UKF tropospheric distance‑record list for 432 MHz, all 12 entries are dated between 31 August and 21 December. The Polish record on 144 MHz was set on 26 October.

A valley below Karpacz filled with clouds, above them the sunny slopes of the Karkonosze and a chairlift Inversion over Karpacz, view from Kopa · photo by Paweł Kuźniar, CC BY-SA 3.0, Commons
Before you switch the radio on

How to tell that an opening is coming

Tropo can be forecast far better than sporadic‑E, meteors or aurora, because it follows the weather. A few signs you can see from the window; the rest you can check online in a few minutes.

1

A high that sits

High pressure persisting for a few days, light winds, clear nights. High pressure on its own isn’t enough: what matters is that the high doesn’t move. RSGB notes that the best conditions can also be when the pressure starts to fall, i.e. the high is weakening. There isn’t a single value in hPa at which “there’s definitely tropo”.

2

Fog, haze, smog

An inversion acts like a lid. Moisture and pollutants build up beneath it, so in the morning you see fog and by day a grey layer over the city. In such weather an FM car radio on VHF‑FM pulls in distant stations. That’s the simplest indicator.

3

Beacons and distant repeaters

Beacons operate in the 144.400–144.490 and 432.400–432.490 MHz segments. Nearest to Warsaw is SR5TDM in Tarczyn (KO01KX) on 144.415 and 432.415 MHz, listed as active by PK‑UKF. Before you listen, check fresh spots in the DX Cluster. On FM the tell‑tale can be a repeater you don’t normally hear.

4

Sounding from Legionowo

A weather balloon launches from Legionowo (WMO station 12374). Results are published by the University of Wyoming. You’re looking for a layer where the temperature rises with height and the dew point drops sharply. That’s an inversion.

The Palace of Culture and Science in Warsaw disappearing in the morning fog Morning fog in Warsaw · photo by Chris Olszewski, CC BY-SA 4.0, Commons
Timing

What time to switch the radio on

A tropo opening lasts from several hours to several days, not a few minutes like a meteor‑trail reflection. Over the day the signals do ebb and flow, though, and sources don’t point to a single best time.

dawn

Morning

Hepburn says that on over‑land paths signals are usually strongest around sunrise and weakest in mid‑afternoon. That first coffee at the radio is a good habit.

dusk

Evening

After sunset the ground cools faster than the air above and a radiation inversion starts to form. DXMaps says signals are often strongest around dusk, and a few hours after dark they weaken.

all day

Under a high

A high‑pressure inversion doesn’t disappear after sunrise. Hepburn even notes that a shallow night‑time inversion can get in the way of distant signals, so with a good high conditions may be better by day.

afternoon

Over water

Over sea paths it’s the opposite to over land: the peak falls in the middle of the afternoon. From Warsaw this matters for contacts across the Baltic to Scandinavia.

Forecast on this page

How to read our tropo map

2 m / 70 cm propagation map computes ducts from the vertical atmospheric profile of the GFS model every three hours. Separately for each band, because a duct that’s enough for 70 cm may be too thin for 2 m. It’s an indicator of chances, not a guarantee of contacts.

ValueLegend on the mapWhat it means in practice
0–14normalNormal range: local repeaters and stations within the radius you’re used to.
15–29enhanced refractionThe wave bends more than usual. More distant and weaker stations are heard better than on a typical day.
30–49borderline conditionsThe duct is on the borderline for the band. It’s worth a listen, especially on SSB and FT8.
50–74duct: good tropoThe duct holds the band. Time to call CQ on 144.300 or 432.200.
75+strong ductRare days when records fall. Turn the antenna along the bright band.

Three rules

  1. Look at the path, not the spot. An opening is when a bright band links your QTH to the other station. A bright blob only over you is not DX yet.
  2. Scroll the timeline. Ducts move with the high. An evening opening is often visible already in the morning forecast.
  3. The map is a guide, not a verdict. I calculate it from pressure levels every 25–50 hPa, i.e. roughly every 200–500 m in height. A thinner, sharper inversion can disappear in these data, and the model can also misplace the duct. Under “marginal conditions” just listen to the band.

It’s worth comparing the forecast with the maps by Hepburn (multi‑day forecast, updated daily) and F5LEN (French ARPEGE model, four times a day). When all three maps say the same, your odds are high.

IARU Region 1 bandplan

Where to listen and where to call

Long‑distance tropo is mostly worked with narrowband modes: SSB, CW and FT8. Activity clusters around a few frequencies, so you don’t have to sweep the whole band.

scroll the table sideways →

BandMHzModePurpose
2 m144.050CWCW activity centre. The weakest signals on the noise floor.
2 m144.174FT8It isn’t in the bandplan table, but it’s the WSJT‑X default and that’s where the activity is. The easiest way to start with a modest antenna.
2 m144.300SSBSSB activity centre. This is where you listen and call; once you’ve made a contact, QSY a few kHz off.
2 m144.400–144.490beaconsSegment for beacons only. The simplest test of whether the band is open and in which direction.
2 m145.500FMFM calling. In good tropo you’ll hear stations here from several hundred kilometres away.
70 cm432.050CWCW activity centre.
70 cm432.174FT8As on 2 m: a community convention; among other things, the VHF‑UHF FT8 Activity runs on this frequency.
70 cm432.200SSBSSB activity centre on 70 cm.
70 cm432.400–432.490beaconsSegment for beacons only.
70 cm433.500FMFM calling frequency.

Based on the IARU Region 1 VHF Handbook (version 10.03, February 2026). FT8 on 144.174 and 432.174 MHz is a community convention rather than a separate bandplan entry. Both frequencies sit within segments where the bandplan allows digital modes (MGM). Full band breakdown: IARU BandPlan.

Most common mistake

Horizontal, not vertical

Most of us have a vertical on the roof for repeaters. For long‑distance SSB and CW, that’s bad news.

On VHF/UHF it’s customary that SSB, CW and digital modes use horizontal polarisation, and FM repeaters use vertical. IARU Region 1 specifies vertical polarisation for repeaters, and describes horizontal as common practice for operation outside the FM channels.

When one station has a vertical antenna and the other a horizontal one, you have to allow for a loss of 20 dB or more (ARRL Antenna Book). That’s more than three S‑units. A signal that would be readable with matching polarisation disappears into the noise.

What to do about it

  1. Cheapest: FM and repeaters. A vertical works in tropo too. Listen on 145.500 and 433.500, and check distant repeaters.
  2. Better: a small horizontal Yagi. Even a few elements turned horizontal get you onto SSB. Designs with dimensions are in the article Yagi antenna 2 m / 70 cm.
  3. Direction matters. A directional antenna and a rotator let you aim along the duct. A duct doesn’t carry the signal equally in all directions.
  4. Coax counts double. At the threshold of audibility every dB of loss is the difference between a QSO and silence. Work out your link budget in the loss calculator.
A large stack of Yagi antennas for 144 MHz on a lattice mast at dusk DK5LA’s 144 MHz antennas (horizontal and vertical polarisation) · photo: Reinhard Kuehn, CC BY-SA 4.0, Commons
Step by step

Your first long‑distance QSO via tropo

A plan for a morning when the map is glowing and there’s fog outside.

1

Check the forecast

Open the tropo map and switch the band. See which way from Warsaw the bright band runs and how long it’s going to last.

2

Listen to the beacons

Tune slowly through 144.400–144.490 MHz in USB or CW. A beacon you don’t normally hear confirms the opening and hints at the direction. On 70 cm it’s the same in 432.400–432.490.

3

Listen before you call

Set 144.300 MHz on USB and listen for a few minutes. In an opening you’ll hear calls and QSOs from distant locators. Answer someone before you start calling CQ yourself.

4

Call briefly, with your locator

On VHF/UHF the locator is as important as the callsign, because distance is computed from it. Example: “CQ tropo, CQ tropo, this is SQ5CBA, locator KO02”. After making contact, QSY from 144.300 to a free frequency nearby.

RS report + locator · e.g. 57 KO02ME
5

Too weak for SSB? FT8

If you only hear noise on SSB, move to 144.174 MHz for FT8. The decoder pulls out signals your ear can’t hear, and PSK Reporter will show where you were received.

6

Log it and calculate

Log the contact with the locator and check the distance in the locator calculator. Openings can shift, so every so often swing the antenna and check the spots.

i

Where you can see the band is alive: spots on DXMaps, FT8 reception reports on PSK Reporter and the ON4KST chat, room “144 & 432 MHz” for Europe. On the chat you arrange direction and frequency; the QSO itself is done over the radio.

Identifying by sound

Tropo, Es, meteors or aurora?

A distant station on 2 m doesn’t always mean tropo. Each type of propagation sounds different. Below is a summary based on DXMaps’ description.

scroll the table sideways →

TypeWhenRange on 2 mWhat it sounds like
Tropoanticyclones, autumntypically 500–1000 kmSignal stable and clean, opening for hours or days. Point the antenna at the station.
Sporadic Emid-May – mid-August800–2500 kmVery fast, deep QSB: from a huge signal to silence.
Meteor scatterall year, showers800–2300 km“Pings” lasting a fraction of a second and, less often, reflections lasting a few seconds. Normally MSK144; during showers also CW and SSB.
Aurorageomagnetic storms800–2300 kmRaspy, distorted tone. Point the antenna towards the aurora, usually north, not at the station.

Aurora occurs more often around the equinoxes, so in autumn too. Details in the article Aurora on 2 m and 6 m. Sporadic E, meteors and aurora cannot be computed from weather data, so they are not on the tropo map.

From Polish record lists

Big openings, almost always in autumn

The list of tropo distance records from Poland, maintained by PK‑UKF, is a good indicator of what you can expect over land. Below are selected QSOs from the tropo list.

DateBandkmContact
26.10.19852 m2087SP9EWO – OY7JD/P (Faroe Islands), Polish record
10.11.202470 cm1990SO3Z – RD6F (southern Russia), Polish record
16.12.20232 m1969SO3Z – EB1LA (Spain)
21.12.200770 cm1872SP8CUR – MM5AJW (Scotland)
27.09.19752 m1740SP5JC – G3CHN (England)
26.10.198570 cm1569SP1FPG – RB5EU, the same day as the 2 m record

Data: PK‑UKF, 144 MHz ODX records and 432 MHz.

Not just hams

Tropo can be heard on ordinary radio too. 27–28 September 2017 a huge high over north‑eastern Europe allowed Finnish VHF‑FM broadcast stations from Kolari to be received in Tomaszów Mazowiecki (1798 km), Góra Kalwaria and Płock. 9–10 October 2018, during the Polish golden autumn, stations from Moscow were received in Poland via tropo for the first time. Both reports were covered by the RadioPolska service.

World records are over the sea

IARU Region 1 records exceed 5,000 km on 2 m and 70 cm, but per the IARU list these are sea paths from the Canary Islands across the Atlantic to Puerto Rico. From central Poland such distances are unattainable. A realistic target in a good autumn anticyclone is a few hundred to a thousand kilometres, and more in an exceptional opening.

Autumn 2026 calendar

Contests, i.e. guaranteed activity

During contests, big stations with good antennas come on the band. Even without tropo it’s the best time to see how far your station reaches, and when there’s an opening the contacts just pour in.

3–4.10

IARU UHF/SHF

IARU Region 1 contest on 70 cm and higher bands, from Saturday 14:00 to Sunday 14:00 UTC. On 70 cm the whole of Europe at once.

Tuesdays

SPAC

SP Activity Contest PK‑UKF, always 19:00–23:00 Polish time. 2 m: 6.10, 3.11, 1.12. 70 cm: 13.10, 10.11, 8.12.

7–8.11

Marconi (MMC)

Marconi Memorial Contest on 144 MHz, CW only, 14:00–14:00 UTC. In 2026 it’s the first full weekend of November.

FT8

FT8 activity

VHF‑UHF FT8 Activity according to the PK‑UKF calendar, 17:00–21:00 UTC. 2 m: 7.10, 4.11, 2.12. 70 cm: 14.10, 11.11, 9.12.

Dates according to the PK‑UKF contest calendar for 2026 (as at 25 September). Check the rules and the exchange before you start. Full calendar with countdown and a step‑by‑step guide to your first contest: VHF/UHF contests.

Where these numbers come from

Sources

Next step

Check what the map shows today

The tropo map is recalculated every three hours. Have a look in the morning and evening, and when you see a bright band from Warsaw, switch the radio to 144.300.

Tropo map 2 m / 70 cm Build a horizontal Yagi