From charlesreid1

Section 8.1: Ionosphere

  • Atmosphere gets thinner as you go further away
  • At 30 miles in altitude, gets thin enough that UV rays can knock electrons away from molecules
  • Gas is ionized by loss of electron, positively charged ion, negative free electron
  • Ion + electron respond to voltage, like electrons in conductor
  • Atmospheric layer - ionosphere - becomes weak conductor
  • Ionosphere extends to 300 miles above surface of Earth


  • ISS orbits 200 miles above Earht
  • Ionosphere arranged into multiple layers (D, E, F layers)
  • D layer - 30-60 miles, only present when illuminated by sun
  • E layer - 60-70 miles, similar to D region, lasts longer after sunset
  • F layer - 100-300 miles, least dens,e partially ionized at night
    • F1 layer/F2 layer - split during day, recombine at night
    • Height of regions vary with season, TOD, latitude, solar activity
    • F2 is highest layer, reaches highest point at noon

Reflection and absorption

  • Weak conduction of layers allows bending/refracting of waves
  • Layers of ionosphere can bend waves
  • Bending o waves depends on ionization level, and wave frequency
  • VHF/UHF waves hardly bent at all
  • HF waves bent, can be reflected back to Earth
  • Weaker bending requires lower takeoff angles, otherwise waves lost to space
  • Critical angle - angle above which all energy lost to space
  • Critical frequency - frequency above which all energy lost to space (if pointed straight up)
  • Ionosonde - device used for measuring reflection of radio waves by ionosphere
  • Absorption is the enemy of propagation
  • In D and E layers, waves pass through denser gas regions, absorbed as they are refracted
  • For HF bands, below 10 MHz, AM broadcast bands, the D layer completely absorbs radio waves
  • Absorption increases with sunlight, ionization, more UV, and lower frequencies

Sky-wave and ground-wave propagation

  • Reflection by ionosphere is called hop
  • Signals received via waves bouncing off of ionosphere called sky-wave propagation
  • Propagation via ionosphere called skip
    • Skip via higher ionosphere layers travel further
    • F2 layer skip travels up to 2,500 miles
    • E layer skip travels up to 1,200 miles
    • Sky wave propagation can also skip over Earth's surface
  • Ocean's surface reflects radio waves (salt water)
  • Skip can also travel shorter distance as angle increased
  • Short skip can indicate there is larger skip available at lower frequencies
    • Short skip on 10 m indicates long skip on 6 m
  • Ionosphere has many variations in density, turbulent, rough
  • Variations can cause signals to take multiple paths
  • Multipath signals have echo/flutter
  • Ground wave signals attenuated more (ground not good conductor)
  • Higher frequency ground waves attenuated more
  • Ring-shaped region around station forms skip zone (further than maximum ground wave and shorter than minimum sky wave)
  • Stations in skip zone can't be contacted

Long path/short path

  • Short path: shorter of great circle paths between two stations
  • Long path: travels long way around globe
  • If signal travels by both paths, short delay/echo
  • Round the world propagation: 1/7 second delay with own signal

Section 8.1 Summary

  • When making a long path contact, antenna is pointed 180 degrees from short path heading
  • If sky wave signal arriving via short and long path, a well-defined echo will be heard
  • A good indicator of possible sky-wave propagation (long skip propagation) on 6 m is, short skip skywave propagation on the 10 m band
  • Radio waves with frequencies below MUF and above LUF sent into ionosphere will be bent back to Earth
  • Approximate maximum distance covered by F2 layer skips is 2,500 miles
  • Approximate maximum distance covered by E layer skips is 1,200 miles
  • Ionospheric layer closest to surface of Earth is D layer
  • Earth's ionospheric layers reach maximum height where the sun is directly overhead
  • F2 region responsible for longest radio wave propagation because it is the highest ionospheric region
  • Critical angle in radio wave propagation refers to highest takeoff angle that will return the wave3 to Earth
  • Long distance communication on 40 m, 60 m, 80 m, 160 m, more difficult during day because the D layer absorbs these frequencies during the day
  • Ionospheric layer that absorbs most long skip signals during daylight, below 10 MHz, is D layer

Section 8.2: The Sun

Sunspots and cycles:

  • Sn generates UV rays, but a lot of variation over time
  • Variations caused by sunspots (cooler regions on Sun surface)
  • Sunspot number - number of sunspots present on solar disk
  • Sunspots vary over an 11 year period (sunspot cycle)
  • More sunspots lead to more UV radiation lead to more intense ionization
  • More ionization improves propagation on HF above 10 MHz, and into low VHF
  • Peak sunspot: bands like 10 m stay open into the evening, enabling long-distance contacts
  • High ionization increases absorption, takes a toll on 80 m and 160 m
  • Bottom on sunspot cycle: low HF bands have good propagation and higher bands (20 MHz) stay open
  • 20 m (14 MHz) supports daytime communication during the day
  • Sun rotates every 28 days, so spots change nad move
  • Propagation conditions can repeat themselves every 28 days
  • Strong daily/seasonal variations in HF propagation
  • Seasonal variation: summer, higher illumination, higher absorption, shifts HF activity to nighttime
  • Propagation around equinoxes (March/September) can be interesting

Band: 160 m / 80 m / 60 m

  • Daytime: local and regional contacts, 100-200 miles
  • Nighttime: local to long distance, best near sunset/sunrise

Band: 40 m / 30 m

  • Daytime: local and regional contacts, 300-400 miles
  • Nighttime: short and medium range to worldwide communications

Band: 20 m / 17 m

  • Daytime: regional to long distance, open at sunrise, closing at nighttime
  • Nighttime: Open to west at night, may be open 24 hours

Band: 15 m / 12 m / 10 m

  • Daytime: primarily long distance, 1,000+ miles
  • Nighttime: 10 m used for local communications 24 hours a day

Measuring solar activity:

  • Solar activity critical to propagation and communication
  • Monitored 24/7 throughout world
  • Use of data, experience, and software allows for predicting propagation and being alerted to sudden propagation changes
  • SFI - solar flux index - amount of 2800 MHz radio energy coming from sun
  • Easier to measure than solar UV, and correlates with it
  • Higher levels of SFI indicate higher solar activity and better HF propagation above 10 MHz
  • K index - values from 0 to 9, represent short-term stability of Earth's magnetic field, updated every 3 hours at NIST in Boulder CO
  • Steady K values indicate a short term stable geomagnetic field; higher K values indicate short-term disturbances in geomagnetic field, disrupting HF communication
  • A index - time-averaged K values; A index ranges from 0 to 400, 0 = stable, 400 = greatly disturbed
  • Index values and other space weather data from NIST:, WWV, WWVH

Assessing propagation

  • Software tools for predicting propagation
  • MUF/LUF = maximum/lowest usable frequency
  • MUF/LUF depend on path between 2 points, time of day, time of year, conditions, etc.
  • MUF = highest frequency at which propagation exists between 2 points
  • Waves at or below the MUF reflected back to eart
  • Waves above MUF will, at some point during the journey, penetrate ionosphere and leave Earht
  • LUF = lowest frequency at which propagation between 2 points exist
  • Waves below LUF completely absorbed by ionosphere
  • If MUF < LUF, no propagation path exists via skywave
  • Check band conditions using beacons - Northern California DX Foundation
  • Many beacons between 28.190 and 28.225 MHz
  • Reverse Beacon Network - automated receivers reporting SS for signals on HF bands

Solar disturbances

  • Common events on Sun that affect hF propagation
  • Measured by solar observatories
  • Solar flare - large eruption of energy/solar material, magnetic field disruptions occurring on surface of Sun
  • Coronal hole - weak area in Sun's corona (outer layer), plasma (ionized gases/particles) escape and stream into space at high velocities
  • Coronal mass ejection - ejection of large amount of material from corona

Sudden ionospheric disturbances

  • UV, x-ray radiation from solar flare travels at speed of light
  • 8 minutes from Sun to Earth
  • Radiation hits ionosphere, raises ionization of D layer and other layers
  • Dramatic increase in absorption, radio blackout
  • Sudden ionospheric disturbance - radio blackout due to solar flare
  • Blackouts last seconds to hours
  • Lower bands affected more strongly
  • Only day side of Earth affected

Geomagnetic disturbances

  • Solar wind - stream of charged particles
  • Interaction between solar wind and Earth's magnetic field creates magnetosphere
  • Charged particles from coronal holes/coronal mass ejections can take 20-40 hours to reach earth
  • Charged particles can affect/become trapped in Earth's magnetosphere, create disturbances
  • Charged particles depositing in magnetosphere increase ionization of E layer
  • Geomagnetic storm, auroral displays
  • Changes in geomagnetic field disrupt upper ionosphere layers, some HF paths passing near poles are completely wiped out
  • Auroras are glow of bases ionized by incoming charged particles
  • Conductive sheets that glow also reflect radio waves above 20 MHz
  • Auroral propagation strongest on 6 and 2 m, modulates signas with hiss or buzz

Section 8.2 Summary

  • Significance of sunspot number with with respect to HF propagation is, higher sunspot number indicates better propagation at higher frequencies
  • Sudden ionospheric disturbance in daytime affects ionospheric propagation of HF waves by disrupting signals on lower frequencies more than signals on higher frequencies
  • Increased UV/x-ray from solar flares take 8 minutes to reach earth
  • For long distance communication during periods of low solar activity, the least reliable bands are 15 m, 12 m, 10 m
  • Solar flux index is measure of solar radiation at 10.7 cm wavelength (2.4 GHz)
  • Geomagnetic storms are temporary disturbances in Earth's magnetosphere
  • For 20 m band, worldwide propagation supported at any point in solar cycle
  • A geomagnetic storm can degrade high-latitude HF propagation (near the poles)
  • A high sunspot number enhances long-distance communication on the upper HF and lower VHF bands
  • HF propagation conditions vary on a 28-day cycle due to the sun's rotation on its axis
  • The typical sunspot cycle is 11 years
  • The K-index indicates the short-term stability of the Earth's geomagnetic field
  • The A-index indicates the long-term stability of the Earth's magnetic field
  • Charged particles escaping from coronal holes affect radio communications by causing an HF blackout (HF communications are disturbed) (due to higher absorption)
  • Charged particles from coronal mass ejections affecting radio propagation take 20-40 hours to reach Earth
  • Periods of high geomagnetic activity benefit radio propagation by creating conductive sheets that reflect VHF signals
  • When selecting a frequency for lowest attenuation on HF, select a frequency just below the MUF (best frequency for low attenuation)
  • To determine if MUF is high enough to support skip propagation between two distant stations for 14-30 MHz, use beacons (NC DX F beacon network)
  • Radio waves with frequencies below the MUF and above the LUF, when sent into the ionosphere, are bent back to the Earth's surface
  • Radio waves with frequencies below the LUF are completely absorbed
  • LUF = lowest usable frequency for communicating between 2 points
  • MUF = maximum usable frequency for communicating between 2 points
  • When LUF exceeds MUF, there is no HF radio frequency that supports sky wave communications over the path
  • Factors that affect the MUF include all of the following:
    • Path distance and location
    • Time of day and season
    • Solar radiation and ionospheric disturbances

Section 8.3: Scatter Modes

  • HF signals via scatter are weaker than HF signals via sky-wave propagation (reflection is not efficient)
  • Scatter may make a signal arrive from multiple paths, causing fluttering sound or wavering sound
  • Waves close to MUF can be reflected by mountains and ocean, and can return back to transmitter (backscatter)
  • Waves can also be scattered within ionosphere inside skip zone (if too distant for ground wave and too high a frequency for short hop skywave)
  • scatter or backscatter can fill in this skip zone


  • below critical frequency, ionosphere reflects waves arriving at any angle (including vertical)
  • critical frequency is always above 5 MHz, often above 7 MHz (40 m)
  • For HF signals below that critical frequency, reflected signals are scattered over a wide area
  • This scatter mode is called NIVS, near vertical incidence skywave
  • Critical frequency increases with solar illumination
  • Antenna should be 1/8 to 1/4 wavelength high
  • Skip will be good over 200-300 miles

Section 8.3 Summary

  • A characteristic of HF scatter signals is a wavering sound
  • HF scatter signals sound distorted because they are arriving via multiple paths
  • HF signals arriving via scatter in skip zone are weak because only a small part of the signal energy is scattered into the skip zone
  • Radio wave propagation that works for stations too close for ground wave propagation and too far for sky wave propagation is scatter propagation
  • On HF bands, indication of signal received via scatter propagation is a signal being heard above a max. usable frequency
  • Antenna type most effective for skip communication on 40 m during day is horizontal dipole 1/8 to 1/4 wavelength above ground
  • NVIS propagation is short distance HF propagation using high elevation angles