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. 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.

Radio Sounding and Imaging of Magnetospheric Plasmas -

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.

Digisonde operations at Millstone Hill - Millstone Hill Observatory

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.


4197500 Automatic channel selection, April 8, 1980, 455/62; 375/347; 455/67.13
5930685 Apparatus and method using fast automatic link establishment waveform for high frequency communications, July 27, 1999, 455/72; 370/477; 370/521; 455/517
4185289 Spherical antennas having isotropic radiation patterns
4746926 Phase Scan Antenn

Introductory Material

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

Educational Institutions

Aurora Page - Northern Lights
Rice University - Space Weather -
University of Lethbridge - Solar Terrestrial Dispatch -
Univ. of Alaska - Poker Flat Research Range -

Government Agencies

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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