Propagation Basics

SimTX simulates realistic radio propagation between stations, including the ionosphere, time of day, season, solar and geomagnetic activity, ground quality and receiver noise. This page explains, from the operator's point of view, what determines whether a transmission is heard at a given distance. It intentionally describes behaviour, not the internal models.

Two ways a signal travels

• Groundwave: the signal follows the ground outward from the antenna. It dominates short range (out to a few tens of kilometers on all bands, farther on lower frequencies) and favours vertically polarized antennas and good ground (sea water is best). Groundwave range shrinks quickly as frequency rises; on the higher HF bands it is essentially local.
• Skywave: the signal refracts off the ionosphere and returns to Earth hundreds to thousands of kilometers away. This is how HF covers distance. Multiple hops extend the range further at the cost of extra loss.

Between the end of groundwave and the first skywave return there can be a skip zone where the station is inaudible even though it is heard both closer and farther away.

The frequency window: LUF and MUF

For any skywave path at any moment there is a usable frequency window:

• Above the maximum usable frequency (MUF) the ionosphere no longer bends the signal back; it escapes into space.
• Below the lowest usable frequency (LUF) absorption in the lower ionosphere eats the signal.

The window moves constantly:

• The MUF rises with solar activity and during daytime, and is generally higher on paths toward the equator than at high latitudes.
• The LUF rises during daylight (absorption is a daytime phenomenon) and drops at night, when very low frequencies open up.
• Frequencies just below the MUF usually propagate best; deep inside the window absorption grows toward the low end.

Rules of thumb that follow directly:

• Low bands (below about 5 MHz): night-time and short-to-medium range in daytime. Daytime long-distance work is largely absorbed.
• Middle bands (5 to 12 MHz): workable day and night; the default 4625 kHz falls here.
• High bands (above about 14 MHz): daytime and high solar activity; they close at night.
• Around dawn and dusk (the grey line), low and middle band paths along the terminator are enhanced.

Short range via the ionosphere: NVIS

Coverage from roughly 50 to 500 km needs radiation that goes almost straight up and reflects back down (near-vertical incidence skywave). It requires a frequency comfortably below the vertical MUF (typically 2 to 8 MHz depending on conditions) and a high-angle antenna: a low dipole, inverted-V or loop. This is the regime where "worse" antennas outperform beams.

Long range

Distances beyond about 1000 km leave the antenna at low elevation angles. What helps: a low takeoff antenna (Yagi, vertical, high dipole), a frequency near the path MUF, and daylight positioning appropriate to the band. Multi-hop paths add loss per hop, so more power or better antennas are needed as distance grows.

Noise at the receiver

Whether a signal is readable depends on how far it sits above the receiving station's noise floor:

• Atmospheric noise dominates the low bands and is much stronger at night and in disturbed conditions.
• Man-made noise depends on the receiver environment; a city location can be tens of decibels noisier than a quiet rural one.
• Noise grows with receiver bandwidth: narrow modes hear deeper.

Mode sensitivity

Approximate signal-to-noise ratios needed for readable copy, from most to least sensitive:

Mode	Approximate required SNR
CW	around 0 dB
SSB (USB/LSB)	about 6 dB
AM	8 to 10 dB
NFM / WFM	about 12 dB

The spread matters: a path hopeless for FM can still carry CW comfortably. Narrow modes are the best probes of marginal propagation.

Disturbances and fading

• Geomagnetic storms raise absorption, depress the MUF, destabilize high-latitude paths and add Doppler drift and spreading to signals. During storms, ionospheric signals can slowly march in frequency and fade rhythmically.
• Even on quiet days skywave signals fade continuously (multipath fading); deep fades of many decibels are normal and average out over time.
• Sporadic E can open surprising mid-range paths on high frequencies, mainly in local summer.

Checklist: will I be heard at distance X?

1. Under about 50 km: almost any setup works; vertical polarization helps most.
2. 50 to 500 km: use a low band frequency below the day's vertical MUF and a high-angle antenna (NVIS).
3. 500 to 3000 km: pick a band whose window is open for the time of day (lower at night, higher by day); a dipole at good height works.
4. Beyond 3000 km: choose a frequency near the path MUF, use a low-takeoff antenna aimed at the target, and prefer sensitive modes.
5. Always compare the required SNR of the mode with realistic noise at the receiving end; when in doubt, drop to a narrower mode rather than raising power.