General Course, Module 2: The Sun Is Your Repeater

2.1Reading the Star

The sunspot number is the oldest gauge on the dashboard. More sunspots mean more of the ultraviolet and X-ray radiation that ionizes the upper atmosphere, and higher sunspot numbers indicate a greater probability of good propagation at higher frequencies. That single sentence explains the rhythm of the hobby: at solar maximum, 15, 12, and 10 meters roar; at solar minimum, those same high bands are the least reliable for long-distance work, and everyone migrates down. The 20-meter band is the great exception, supporting worldwide daylight propagation at any point in the solar cycle, which is why it is the workhorse band of amateur radio.

The modern gauge is the solar flux index: a measure of solar radiation at a wavelength of 10.7 centimeters, a radio measurement of the Sun that tracks ionization-producing activity and works even when clouds hide the sunspots.

The Sun also misbehaves, on two different clocks:

• A solar flare blasts out ultraviolet and X-rays that travel at the speed of light, reaching Earth in about 8 minutes. The result is a sudden ionospheric disturbance, and it disrupts signals on lower frequencies more than higher ones (hold that thought for Section 2.3, where the D region explains why).
• A coronal mass ejection hurls charged particles, which are matter, not light, and take 15 hours to several days to arrive. Charged particles also stream from coronal holes, and when either arrives, HF communication is disturbed.

That arriving material stirs Earth's magnetic field into a geomagnetic storm: a temporary disturbance in the geomagnetic field. Its signature effect on radio is degraded high-latitude HF propagation, which is why polar paths die first when the Sun acts up. There is one consolation prize: high geomagnetic activity creates auroras that can reflect VHF signals, handing 6-meter operators a strange and wonderful propagation mode while the HF operators complain.

Two indices measure the shaking, and the exam wants them kept straight:

• The K-index: short-term stability of Earth's geomagnetic field.
• The A-index: long-term stability of the same field.

Ray's mnemonic: K is quick, A is averaged. And one more clock: propagation conditions repeat on a 26-to-28-day cycle because the Sun's surface layers rotate around its axis, carrying active regions back around to face Earth like a lighthouse beam.

2.2MUF, LUF, and the Window Between

For any two points on Earth at any moment, ionospheric propagation is a window between two frequencies, and every HF operating decision is an attempt to sit inside it.

The MUF, Maximum Usable Frequency, is the highest frequency that supports communications between two specific points. Above it, signals pass through the ionosphere and into space. The LUF, Lowest Usable Frequency, is the floor for those same two points. Below it, signals are attenuated (absorbed) before reaching the destination. Between them, radio waves are refracted back to Earth, and the sweet spot is just below the MUF, where long-distance skip suffers the least attenuation.

What moves the MUF? Path distance and location, time of day and season, solar radiation and ionospheric disturbances. All of it, which is why the MUF is a weather report, not a constant. And when conditions get bad enough that the LUF rises above the MUF, the window closes: ordinary skywave communication over that path is simply not possible. It happens, and knowing it happens saves you an evening of blaming your antenna.

How far does one hop carry? Via the F2 region, about 2,500 miles maximum along the Earth's surface; via the E region, about 1,200 miles. Long paths are chains of hops, ionosphere to ground and back up.

Sometimes the ionosphere hands you the planet's far side twice. A signal can travel the short path (the direct great circle) or the long path (the rest of the great circle, 180 degrees from the short-path heading). Hear both at once, and the extra distance shows up as a slightly delayed echo on the signal, one of HF's genuinely eerie sounds.

How do you actually check conditions from your own station, tonight? The modern answer: transmit and use a network of automated receiving stations on the internet (the Reverse Beacon Network for CW, PSK Reporter for digital) to see exactly where you are being heard. Propagation forecasting used to be astrology; now it is a map with your call sign on it.

Two seasonal notes finish the group: summer on the lower HF bands means high levels of atmospheric noise and static from continental thunderstorms, and it is the price of July on 80 meters.

2.3The Layers That Do the Work

Time to meet the machinery. The ionosphere is not one mirror but a stack of regions, each with a personality.

• The D region is the lowest, closest to Earth's surface. It is not a mirror at all but a sponge: during daylight it absorbs signals below about 10 MHz. The D region is why 40, 60, 80, and 160 meters are difficult for long distance during the day, and why they come alive when the Sun sets and the D region fades. It is also why a solar flare's X-ray burst, which supercharges the D region, silences the low bands first.
• The E region sits above it: modest hops (that 1,200-mile figure), plus the sporadic-E surprises you met at Technician.
• The F region, splitting into F1 and F2 by day, is the high mirror. Skip via F2 is the longest precisely because F2 is the highest region: simple geometry, a higher mirror throws a longer shadow.

Two "critical" definitions govern whether a signal comes back at all:

• The critical frequency is the highest frequency refracted back to Earth at a given incidence angle. Fire straight up: signals below it return, signals above it leave.
• The critical angle is the highest takeoff angle that will return a wave to Earth under the conditions of the moment. Steeper than that, and the wave punches through.

Those two definitions collide usefully in NVIS, Near Vertical Incidence Skywave: short-distance MF and HF propagation using high elevation angles. Aim 40 or 80 meters nearly straight up, below the critical frequency, and the ionosphere rains your signal down in a few-hundred-mile circle around you, filling in the gap between ground wave and the first skip hop. NVIS is the emergency communicator's best trick, regional coverage with no infrastructure, and your Field Record notebook will meet it again.

Between the hops lies the skip zone: too far for ground wave, too close for the first hop. Scatter propagation is what lets signals be heard there anyway, energy scattered out of the main beam by irregularities along the path. Its fingerprints: scatter signals are weak, because only a small fraction of the signal energy scatters into the skip zone, and they have a characteristic fluttering, distorted sound, because the energy arrives via several different paths at once.