HF Propagation & Band Opening Calculator
One S-unit costs four times the power, so the whole legal range is under two. And a band can be dead overhead and open at 3000 km.
What the power actually buys
Every band against this ionosphere
The cheaper decibel
Antenna gain in the same units, and the power increase it substitutes for. Gain works on receive as well, which an amplifier does not.
One S-unit costs four times the power
An S-unit is 6 decibels, and 6 decibels is four times the power. That single fact reframes the whole subject. Doubling from 100 watts to 200 is half an S-unit. The entire legal range from 100 watts to 1500 is 11.76 decibels — 1.96 S-units, under two, for fifteen times the power and a four-figure amplifier. Even going from 5 watts to the legal limit, three hundred times the power, moves the needle 4.13 S-units.
Set that against what the path does on its own. Ordinary fading runs 15 to 20 decibels and deep multipath fading reaches 40 — so routine QSB takes away, and hands back, more than the biggest legal amplifier can buy. There is a neat coincidence here too: daytime D-layer absorption goes as one over frequency squared, so 80 metres suffers sixteen times the absorption of 20 — 12.04 decibels, against 11.76 for the full 100 to 1500 watt jump. Running the legal limit on 80 metres at midday gets you back to roughly where 100 watts would have been at night, and buys nothing beyond that. On 160 metres the penalty is 17.82 decibels, which exceeds the entire power range by a full S-unit.
The other thing worth internalising is that a band can be dead and open at the same moment, which sounds like folklore and is really just geometry. The maximum usable frequency is the critical frequency times the secant of the angle the wave meets the ionosphere at, and that angle depends on how far you are going. With a critical frequency of 5 MHz, 20 metres will not come back from straight overhead and comes back perfectly well from 3000 kilometres away. The skip zone is not a mystery — it is that inequality, and inside it you hear nothing because the signal is leaving the ionosphere rather than returning from it.
How to use
- Enter the critical frequency from a nearby ionosonde, in megahertz.
- Set the path distance — the same band behaves differently at each one.
- Compare a power increase against the antenna table before buying an amplifier.
- Use real propagation predictions or on-air beacons for an actual forecast.
Frequently asked questions
How much difference does more power really make?
Less than most operators expect, because an S-unit is six decibels and six decibels is four times the power. Doubling from 100 watts to 200 is half an S-unit. The entire legal range from 100 watts to 1500 is 11.76 decibels — 1.96 S-units, under two, for fifteen times the power. Even 5 watts to the legal limit, three hundred times the power, is 4.13 S-units.
Is an amplifier or a better antenna the better upgrade?
Almost always the antenna, and the comparison is unflattering to amplifiers. A three-element yagi is about 5.5 dBd — the same as running three and a half times the power — and it works on receive as well, which an amplifier does not. Height above ground often matters more than either, because it sets the take-off angle, and take-off angle decides which distances you can work at all.
Why does the signal fade so much more than my power changes it?
Because the path moves more than the amplifier does. Ordinary fading runs 15 to 20 decibels and deep multipath fading reaches 25 to 40 — so routine QSB takes away, and hands back, more than the biggest legal amplifier can buy. Switching an amplifier on during a fade and hearing the signal rise is a thing that happens, and it is usually the fade ending.
Why is 80 metres so poor during the day?
D-layer absorption goes as one over frequency squared, so 3.5 MHz suffers sixteen times the absorption of 14 — 12.04 decibels. There is a neat coincidence in that figure: it is almost exactly the 11.76 decibels of the entire 100 to 1500 watt range. Running the legal limit on 80 metres at midday gets you back to roughly where 100 watts would have been at night, and no further.
Can I power my way through daytime absorption on 160 metres?
No, and the arithmetic says so plainly. The 160 metre penalty relative to 20 metres is 17.82 decibels, which exceeds the entire legal power range by about six decibels — a full S-unit beyond what any amplifier can recover. The low bands are night bands because of the D layer, and that is a physical constraint rather than a matter of equipment.
What is the maximum usable frequency?
The highest frequency that will return to earth from the ionosphere on a given path, and the important word is path. It is the critical frequency multiplied by the secant of the angle at which the wave meets the ionosphere, and that angle depends entirely on how far you are trying to go. There is no single MUF for a place — only a MUF for a place, a moment and a distance.
Can a band be open and dead at the same time?
Yes, routinely, and it is geometry rather than folklore. With a critical frequency of 5 MHz, 20 metres will not return from straight overhead — the factor there is exactly one — but at 3000 kilometres the geometry multiplies it by more than three, so the band works perfectly to that distance. Dead locally and open for DX, at the same instant, from the same ionosphere.
What causes the skip zone?
The same inequality. A band returns only where the secant of the incidence angle is large enough to lift the critical frequency above the operating frequency, and that needs distance. Inside the skip zone you hear nothing, not because the band is quiet but because your signal is passing through the ionosphere and leaving rather than being refracted back. More power does nothing about it.
How far is a single hop?
About four thousand kilometres with a 350 kilometre layer, at which point the required take-off angle reaches the horizon and no further single hop is geometrically possible. Longer paths are multi-hop, paying another ground reflection each time — which is why a transatlantic contact is harder than the distance alone suggests, and why terrain the signal never passes near on a map still matters.
What is the critical frequency and where do I find it?
It is the highest frequency that returns from the ionosphere at vertical incidence, usually written foF2, and it is measured by ionosondes that publish readings continuously. It sets the floor for everything else: every path MUF is that number multiplied by a geometric factor between one and about 3.2, so knowing it tells you which bands are worth trying.
How reliable are the numbers here?
They are a teaching model rather than a forecast. It uses a single reflecting layer at a fixed height with no sporadic E, no ground wave, no absorption geometry and no auroral effects. It reproduces the published single-hop MUF factor of about 3.2, but that figure swings from 2.95 to 3.67 across plausible layer heights, so the principles are firm and the specific numbers are orders of magnitude.
Why do low bands work locally and high bands do not?
Because a band below the critical frequency returns from any angle including straight up, so it has no skip zone at all, while a band above it needs distance to build the geometry. That is why 80 metres is the band for a regional net at night and 15 metres is not, and why the same aerial can be excellent and useless depending only on who you are trying to reach.
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