General/Chapter 8 Study Guide

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

Regions:

• 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