HF Propagation (& Related)
© Brooke Clarke 2004 - 2014
Path Loss
Path loss (Wiki)
has to do with the losses between the transmit antenna and the
receive antenna whereas Link Budget (Wiki)
includes the properties of the transmit and receive
antennas. But neither of these contains all the elements
needed. For example an antenna optimized for transmission
is very different from an antenna optimized for reception.
Also modulation gain is not mentioned.
Antenna gain is directly related to antenna pattern. You
can not have a high gain (Wiki)
omnidirectional antenna.
One factor in optimizing a transmit antenna is it's efficiency (Wiki),
where the more efficient the better.
One factor in optimizing a receiving antenna is it's G/T ratio (Wiki),
the higher the better. But if the noise that's in the
atmosphere is high the antenna noise contribution no longer is
important. This is mainly a concern for antennas pointing
up, like for receiving satellite signals.
Modulation Gain
This is also called process gain (Wiki).
I first came across this when working on C-band satellite
receivers (Wiki: TVRO)
where a 5 Mhz wide video signal is sent up and down as a 40 MHz
wide signal. This is done to reduce the transmit power
needed in the satellite. The modulation gain is something
like 10 * LOG( 40 Mhz / 5 Mhz) = 9 dB.
Another example is FM radio where a 40 kHz audio band (two
stereo Hi-Fi channels) are modulated in a 100 kHz bandwidth (4
dB gain).
Magnavox made a direct sequence spread spectrum (Wiki)
hand held radio that covered a very wide band in the VHF
frequency range the modulation was a voice band (3 kHz)
signal. Just a guess, but if it covered 30 to 170 MHz the
process gain would be 10 * LOG( 140 MHz/ 3 kHz) = 47 dB!
HF vs. higher and lower frequencies
H.F. frequencies are special in that the ionosphere will
reflect them depending on the frequency, time of day, time of
year and phase of the sun spot cycle (Wiki).
Frequencies higher than H.F. are not reflected by the ionosphere
(Wiki) and
so are called line of sight (Wiki)
so the Earth's curvature or local hills may block the
signal. At frequencies below H.F. not only are they not
reflected by the ionosphere but the D-layer absorbs signals
during daytime. Also because of the long wavelengths
transmitting antennas are inefficient and very large and
expensive to erect.
Very Basic Antenna Info
First Order Theory
All Electromagnetic waves, like
radio and light, follow the same rules independent of wavelength
or frequency. An antenna in free space works the same for
all of them. But an antenna on Earth is not in free space
and so the effects of the atmosphere, size and shape of the
Earth, dirt composition, time of day, season of the year, sun
spot cycle, and many other things come into play.
Sunspot Cycle (Wiki)
Dirt and Ground
Often people speak of an antenna over "ground". It's a word
that has different meanings. For example in an electrical
circuit a "ground" is a metallic connection that's a perfect
conductor. But under an antenna "ground" is really dirt
and is far from a good conductor. Dirt is made of of:
three components: sand & rock, clay and organic
material. There may be other stuff under the surface like
pipes or wires, but they are not considered part of the dirt.
Sand (Wiki) is
a dielectric not a conductor so while you can walk on top of
sand an H.F. radio signal will penetrate some distance into it
(Wiki: Skin
Effect, penetration
depth). Therefore in a sandy environment "ground" is
many feet below the surface. There are a number of AM
radio station transmitters located near or in the San Francisco
bay because the ground is conductive. There used to be a
major HF radio station there with Beverage and Rhombic
antennas. In this case ground is at the surface.
Most other places is somewhere in between.
1/2 Wave Horizontal Dipole (Wiki)
The theoretical antenna would be located in free space and
would have the most gain at right angles to the long axis (think
donut shaped pattern). But when it's mounted closer to the
Earth there are reflections that change the pattern.
Up 1/4 Wave
If a horizontal antenna is one quarter wavelength above ground
(or less) then it's pattern is mostly vertical. This was
named Near Vertical Incidence Skywave (NVIS) late
in or just after the Vietnam era, but has been known and used
back at least to W.W.II when
German vehicles used this mode. The advantage is
that the signal goes more or less straight up and when it
bounced back covers an area that extends out several hundred
miles. But the proper selection of frequency is
vital. The 2 to 12 MHz region is used here and that's why
many military portable radios have a truncated HF frequency
range, since they were designed for this mode of
operation. Note that a field deployed wire type antenna
will typically be <= 1/4 wave above the ground. In the
case of the Eyring antenna it may
be buried in the dirt or a few inches above it.
Since the main lobe is pointing up it works in canyons and
ravines where Line Of Sight (Wiki)
communications do not reach and where satellites only can work
if they are not blocked by a mountain. There is no
requirement to aim the HF antenna like you need to do for a
satellite and so works while a vehicle is in motion. A
simple way to get NVIS from a whip antenna
is to either tie it down so most of the whip is horizontal, or
better fold it down parallel to the ground. The down side
is that for this frequency range (2 to 12 MHz) a 1/2 wave
antenna is between 75 meters to 12.5 meters long. Whip
antennas (Wiki)
shorter than this can be used with loading coils but they are
not as efficient as a full size antenna.
A horizontal loop <= 1/4 above the ground also has a similar
radiation pattern.
Up 1/2 Wave
The same dipole when raised so that it is 1/2 wavelengths
above the reflecting surface (this may be different that the
surface you walk on) has a pattern where most of the signal
goes horizontally. This is called a DX antenna (distance
antenna) in ham radio circles.
1/4 Wave Vertical Antenna (Wiki)
One of the simplest antennas is a vertical conductor 1/4
wavelength long situated over a conductive ground plane.
Electrically is half of a dipole antenna with the ground plane
inserted at it's center. These are popular antennas for a
range of frequencies from the AM broadcast band where they are
guyed towers over a hundred feet tall to WiFi radios were they
are a small fraction of an inch long.
Reflection from Ionosphere
If a horizontal antenna is one half wavelength above ground
it too will have a horizontal pattern like the vertical antenna
above. An advantage of this arrangement is that ground
losses much lower than in the case of the vertical.
Scale Drawing showing Earth and layers.
Choosing the best frequency
In the early days of radio (about the time of the Vietnam ear)
it took an experienced radio operator to know about how the
phase of the sun spot cycle, time of year, time of day, etc.
effected the ionosphere.
Fixed Frequency Beacons
A practical way was to listen to each band and see if it was
dead or if there were stations heard that were located in the
area where you wanted to transmit. Even today there are a
number of fixed frequency beacon transmissions that you
can listen to to determine band conditions.
Northern California DX Foundation, Inc.
The NCDXF
maintains a system of HF beacons that work on a set of single
frequencies and are time multiplexed. This is done in
co-operation with the International
Amateur Radio Union. These transmitters step their
power. The main information that a receiver gets is
weather or not the signal is heard. No ionagrams are
available at this time.
The NCDXF Active
Beacon Wizard ++ -WIN95 or higher program
Propagation
Research
by Amateurs: Introduction -
BeaconSee- PC DSP
based reception of NCDXF beacons
This program is very powerful but also difficult to
get working properly.
Tuning to a strong WWV station helps you to see what's going
on.
I found that in order to get the display to be mostly black I
needed to turn down the PC audio mixer volume sliders for
"Line" and "Record" to about 5% or less of full scale.
Note that "Record" must be active.
The receiver needs to have the CW BFO set for one of the
frequencies used by BeaconSee, I used 1 kHz.
The receiver AGC should be set for as long a time constant as
possible (5 seconds for the NRD-545).
Here is the NRD-545 radio
setup information. I just paste this file to my
Windows desktop and cut and paste the radio commands into the
BeaconSee program. It's much easier to edit in Notepad
then in the BeaconSee program.
The clock in my PC (and probably yours) drifts about 1 second
in a few minutes. This means that you need some type of
active PC clock correction, not just a one time setting.
I use the TAC-2
software package that senses a 1 Pulse Per Second TTL
signal from a GPS and resets the PC clock. Note: you do
not need the TAC-2 hardware to do this, just a special RS-232
cable from your GPS to a computer COM port with the 1 PPS
signal.
Your monitor needs to be 1152x864 or higher resolution to see
the display for all 5 frequencies at the same time.
HIGH
FREQUENCY ACTIVE AURORAL RESEARCH PROGRAM (HAARP)
monitors some of the NCDX beacons
in near real time.
G3USF's
Worldwide List of HF Beacons -
Oblique Sounding
This is newer than making use of fixed frequency beacons
and has a number of advantages. For example you can get a
metric that tells you the signal/noise ratio as a function of
frequency and so can help select an ideal frequency. The
down side is that the equipment is expensive.
This is done with the transmitter and receiver at opposite ends of
the path. The BR Communications "chirp" sounders are in this
category.
These transmissions are almost all at an upward frequency sweep
rate of 100 kHz per second. They can be monitored in a
number of ways.
- Using a receiver manually that's tuned to a fixed
frequency (this is how I got started)
- Using a receiver designed receive the chirp sounders like
the RCS-5 from BR Communications
- Using an amateur type HF receiver tuned to a fixed frequency
and a dedicated
chirp
demodulator that time stamps each chirp using GPS to get
microsecond accuracy. Based on work by Peter Martinez
G3PLX the father of PSK-31. This method also allows
separate software to read the demodulator output and might be
combined with receiver frequency control to product results
very similar to the RCS-5.
- Using an amateur HF receiver tuned to a fixed frequency and
a PC sound card running specialized DSP software to time stamp
each chirp. This method is called "Chirp
View".
The fixed frequency methods are all good for discovering chirp
stations. The two automated versions of these also allow the
location of the stations to be determined by using time of arrival
methods with 3 or more stations. This is much easier for the
bulk of stations because the more modern stations are using GPS to
keep their start times very accurate.
The BR receiver is good because you see a CRT display of the
propagation time vs. frequency and can see the various layers
and ionosphere conditions.
The two methods compliment each other well.
Vertical Incidence Sounding
These are not easy to monitor by someone other than
the one sending the signals. These signals are aimed
mainly at research rather than at facilitating communications.
Most of the work on this page relates to sounders that
have the transmitter and receiver at about the same location.
They are sending the signal straight up and looking at the
reflected signal.
The transmission nevertheless will propagate to other locations.
Digisonde
Station List - locations, model numbers, organizations
& many links
Digisonde Main Page
-unfortunately this is a frames page and if you are behind a
firewall you can NOT see the menu in the left frame.
There are a large number of these stations all around the
Earth. The soundings are vertical so the data is localized
to the sounder location. Maybe it could be used if a
reflection point was near the sounder to predict a path.
URSI
(International Union of Radio Science)- Commission G -
Ionospheric Radio and Propagation
Air Force - Ionospheric
Hazards Branch (VSBI) - Links - Communication/Navigation
Outage
Forecasting System (C/NOFS) -
ARPL - Aeronomy
and Radio propagation Laboratory - Aeronomy
-
SPIDER (Space
Physics Interactive Data Resource) - Ionospheric Data
- Vertical
Incidence Soundings (Ionograms) - Real Time
Ionograms (long load time) Many Links - FAQ
Rutherford Appleton Laboratory - Ionospheric
Monitoring
Group - History of
Ionospheric Science at RAL -
Dynasonde at
Tromsų - Freq
-vs- Time for 3 modes - QuickTime
movie -
Warszawa - Helio-Geophysical
Predictions
Service -
latest ionogram - Ionospheric
Research (in Polish)
University of Leicester - Radio and Space Plasma Physics
Group -
Automatic Link Establishment (Wiki:
ALE)
The radio periodically transmits and listens to a number of
channels and keeps track of which are open, maybe even
remembering by time of day, so that the operator can just
press the mike button and the radio chooses the optimum
frequency. This requires radios and antenna systems that
can change frequency in milliseconds and this speed
requirement eliminates many older radios that are not agile
enough.
Patents
e-layer
· University of Propagation - archive from mail list
Propagation: An
Introduction-
The
Solar
Guide - with links
Principals
of
Radio Propagation by Simon Collings
Radio Netherlands - propagation
related sites -
Mobile
Aeronautics Education Laboratory - part of NASA
University
of
Propagation - email list server archive
-
Tom's
Astro-Ham page - sun spot observation info
Web movie about HF propagation
and the AN/PRC-137 radio
Aurora
Page - Northern Lights
Rice University - Space
Weather -
University of Lethbridge - Solar Terrestrial
Dispatch -
Univ. of Alaska - Poker Flat Research Range
-
Communications
Research
Laboratory, Japan - Solar-Terrestrial
Research Center - Ionospheric data
& Observatories
-
EISCAT Scientific Association,
Scandinavia - coherent Scatter Radar systems, an Ionospheric
Heater and a Dynasonde
UCLA-High Power Auroral
Stimulation Observatory (HIPAS) - study of the ionosphere
through the use of high power transmissions
Cornell Univ. - National Astronomy and Ionosphere
Center at Arecibo
Observatory -
National Center for
Atmospheric Research - mostly weather
- DRAO
10cm Solar Radio Noise Patrol -
Geophysical
Alert
Message - as on WWV
27-day
Outlook outlook - Explanation
-
Report of
Solar-Geophysical Activity - one pg. report from the Space
Environment Center - Radio User's
-
Geomagnetic
kp
and ap Indices -
Hiraiso Solar Terrestrial
Research Center, Japan -
HLMS
VHF Auroral Radar - measures reflections from intense
patches of ionization in the E-region of the ionosphere
IPS Radio & Space Services,
Australia -
Transition
Region and Coronal Explorer (TRACE) - explore the
three-dimensional magnetic structures that emerge through the
visible surface of the Sun
Solar and Heliospheric
Observatory (SOHO) - spacecraft to study the internal
structure of the Sun
Solar Data Analysis Center
- analysis of SOHO and other spacecraft data
NOAA FTP
site has, among other items, annual solar flux data
back to 1947
Solar-Terrestrial
Physics (STP) - Ionospheric Data
- HF Internet
Resource List -
Sunspots and the Solar
Cycle - all about & forecasting
Royal Observatory of Belgium
- Sunspot Index Data
Center (SIDC) -
Stanford -
Estimating the Sun's Rotation Rate -
Blackout!
- all of a sudden, all the signals go away. What happened?
Propagation Forecast
by AD5Q - and extensive links
Real-time
Day/Night
Terminator - looks like a photograph, but is computer
animation, + neat zoom in feature
RSGB Propagation News
- Propagation Studies Committee -
Shortwave Radio
Propagation Tables by Jerry Hall, K1TD -this month's
estimated median signal strengths and s/n ratios
Solar and
Ionospheric Weather Report by Wolfram Hess, DL1RAX -
Skycom -
Sunspot cycles and
planetary tides by Jean-Pierre Desmoulins -
W1AW
Propagation Bulletins - ARRL
Propagation
Of
Long Radio Waves - by J A Adcock VK3ACA
AE4TM HF
Propagation Study - PACTOR allows measuring delay time
R_Meteor -
PC DSP meteor detection
Advanced
Stand Alone Prediction System -
Communication Analysis
Prediction Manager -
HamTools - a number of
programs & lots of links
GeoClock -
HFx - Windows based
high-frequency (HF) propagation prediction application for the ham
radio user
NTIA/ITS - High Frequency
Propagation Models -
UFsight
Propagation
Prediction Software -
PROPLAB-PRO
- ray-trace signals through a realistic three-dimensional
ionosphere in three dimensions
Voice Of America - voacap ftp -
VOACAP is IONCAP modified by USIA/VOA for broadcasting Signal to
Noise Predictions using VOCAP by George Lane
MultiNEC
- combines MiniNEC & VOCAP - allows visualization of the
combined antenna design & propagation conditions
MINMUF - published in QST - very simple MUF forecasting program
Rockwell-Collins - Kristine
M.
Larson 2000 - based on VOACAP - Demo
- HF
CPS -
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