THE SOUND OF SILENCEAND OTHER MYSTERIES OF HF PROPAGATIONLarry Ledlow, Jr. [email protected]
AGENDA
• Basic HF• Ionization• The ionosphere• The geomagnetic field• Solar wind
• Polar Propagation• Data
THE BASICS
”Two atoms are walking along…”
One atom stops and says, “Oh no, I’ve lost an electron!”
“Are you sure?”“I’m positive!”
In the upper atmosphere, the main source of ionization is the sun: X-rays, UV, charged particles.
The ionosphere extends across multiple layers of the atmosphere.
INC
REASIN
G D
ENSITY
The rarer the atmosphere, the more ionized a gas will be.
There are fewer opportunities for electron-ion re-combination.
Some layers exist only during daylight.
Radio wave interaction with the ionosphere depends on the frequency and degree of ionization. D absorbs medium wave transmissions, but long distance is possible at night.
Daytime DX on higher frequencies via F layers
Hams enjoy DX on 20, 15, and 10 meters during day.At night the F1 and F2 layers are no longer so distinct.
Solar activity follows 11-year cycles.Low bands are favored during lulls.
THE NOT-SO BASICS
”But seriously, folks…”
A Quick Re-cap1. The sun ionizes the atoms in the upper
atmosphere.2. The amount of ionizing radiation from
the sun varies constantly.3. Some ionospheric layers disappear at
night – no solar radiation for ionization.4. Low bands (160-40m) are favored at
night. DX is possible on 20-10m during daylight.
5. But it’s more complicated than that…
GEOMAGNETIC FIELD
“Aurora’n good time!?
The earth’s magnetic field simplified, but…
…is not actually uniform due to solar wind.
The measure of geomagnetic disturbance is the planetary K index (Kp), ranging over 0-9.
The solar wind constantly buffets the earth and distorts the magnetic field. See animation here.
The solar wind streams off of the Sun in all directions at speeds of about 400 km/s (about 1 million miles per hour)
The resulting turbulence in both the ionopshere and magenetosphere cause propagation problems.• Absorption• Multipath• Flutter/rapid fading• Skewing
You need to learn to read the “tea leaves” to know what is happening.• Sound of signals• Sound of noise• Space weather indicators
FUGGETABOUTIT!
A lot of what they teach you in radio school is bunk when it comes to polar propagation.
Why are conditions at high latitudes special?
• The geomagnetic field becomes nearly vertical in the high latitudes.
• Magnetic field lines connect the high latitudes to the outer part of the magnetosphere, which is directly impacted by solar wind.
• That also means the ionosphere is tilted as well as more complex and turbulent.
• The atmosphere is thinner near the poles, and the active regions of the ionosphere are nearer the ground.
• Arctic radio wave coupling to the ionosphere is inefficient.• Paths are not necessarily reciprocal! Direction can be
significantly skewed.
In short, polar propagation is far more sensitive to solar-induced disturbances than any other area on the planet.
• Rapidly changing conditions may force a change of antenna, frequency and/or mode.
Conditions you will encounter
Long-term, “flat” fading – slow changes in signal strength across a band. Often accompanied by increasing noise.
Conditions you will encounter
Selective fading – A narrow band of frequencies fade
Conditions you will encounter
Multipath and Aurora flutter
OH1ML - LB1AH QSO on 10 meters via aurorahttps://www.youtube.com/watch?v=gB1nTxxkkbw
Conditions you will encounter
Higher absorption on the path out of Alaska.
EXPECT YOUR SIGNAL TO BE 6-10 DB WEAKER AT THE OTHER END (1-2 S-UNITS) UNDER “NORMAL” CONDITIONS.
Specific effects are highly localized. Two stations close together can endure completely different propagation.
RX LEVEL AT WA7LNW
http://reversebeacon.net/analysis/
Conditions you will encounterSkewed paths
NORMAL PATH
ACTUAL
Special CircumstancesSporadic E (Es)
Occasionally, patches of the E layer become highly ionized. At high latitudes, Es tends to occur in the fall and spring.Contacts on 10 and 6 meters and higher is possible.
Special CircumstancesGray Line Enhancement
As sunrise and sunset occur, signals on any HF band can be enhanced between two stations near the day/night terminator. Openings are usually +/- 2 hours before sunrise/set.Try short and long paths. “Follow the sun” is a good guideline.
DX
In practical terms
• Try changing antennas• Receive diversity is a powerful tool• The best rx path is not necessarily the best tx path.
• Be patient.• If the Kp is 3 or higher, you will be able to hear stations,
but they won’t hear you. Openings occur periodically, so keep trying.
• Try changing bands.• Lower is not necessarily better. 15 meters can open
before 20m.• Try changing modes.
• CW is easier to receive than SSB in many conditions• Use a digital mode with good error correction; e.g.,
JT65 vs. PSK31
REAL-TIME TOOLS
Handy data to assess conditions
Websites• NOAA Space Weather Prediction Center
http://www.swpc.noaa.gov/communities/radio-communications
• Space Weather dot com http://spaceweather.com/
• Australian Space Weather Serviceshttp://www.sws.bom.gov.au/
What some of the numbers mean• A index (Ap): Used to compute daily
average of geomagnetic activity.
• Sunspot number (SSN): an arbitrary numerical value that is used to describe the sun's spottedness, is the number of individual spots plus 10 times the number of disturbed regions.
X-Ray Measurements from Spacecraft
Proton Measurements from Spacecraft
More on the solar wind
Interplanetary Magnetic Field Orientation (Bz)NEGATIVE = BAD
Solar wind speedHIGHER = BAD
Solar wind densityHIGHER = BAD
Solar wind temperature
IMF angle PhiRAPID CHANGE = BAD
The interplanetary magnetic field (IMF) is a part of the Sun's magnetic field that is carried into interplanetary space by the solar wind.
IonogramsStations worldwide regularly probe the ionosphere with ionosondes.The outputs are ionograms showing ionospheric profiles.
Recommended Reading and Viewing
Three Wrong Assumptions about the Ionosphere, by Eric Nichols KL7AJMAR 2012 - QST (PG. 40)
Understanding Propagation with JT65, JT9, and FT8, by Carl Luetzelschwab K9LAOCT 2017 - QST (PG. 45)
More Links to ARRL articles here:http://www.arrl.org/propagation-of-rf-signals
Sentinels of the Sun: Forecasting Space WeatherBy Barbara B. Poppe, Kristen P. Jorden (2006)
Tamitha Skov – Space Weather ForecastsYouTube Channel: https://www.youtube.com/user/SpWxfx
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