Predicted Maximum Useable Frequency

Current Predicted Propagation To The US

United Kingdom

North Africa

Middle East


Click on the above maps for a larger version. The above prediction maps shows possible coverage from four different locations. Red is best where blue is bad. Propagation will deteriorate from the maps if there are Flares or Storms. Classification can be seen here.

Radio Propagation Basics

AFA1RElogRadio propagation on the Primary Frequency* is usually good out to three thousand miles or more from the MARS station. Between 3,000 and 6,000 miles, propagation is less reliable but is often acceptable. Nighttime propagation (US time) on the Primary Frequency* is less reliable than daytime propagation, particularly during or around the winter months.

MARS stations operating at night monitor both the Primary and Secondary Frequency* in order to compensate for the possibility of poor nighttime propagation on the Primary Frequency. It should be noted that many nights the Primary Frequency works just fine.

Attempts to contact MARS should always start with several calls on the Primary Frequency. If there is no answer on the Primary frequency, the Secondary Frequency should be tried. When not classified, include your location on the initial call. MARS stations use directional antennas that need to be pointed in your direction for best reception.

*MARS Frequencies are not published on this website as per MARS regulations. Frequencies may be obtained by contacting Richard Duncan AFN6PP.

More Details

Radio Propagation on the Primary Frequency is able to cover long distances because it is reflected off the ionosphere and back to the ground. Depending on the distance, it may be reflected back and forth between the ground and ionosphere several times. This type of radio propagation is commonly referred to as “skip”. Skip propagation results in an area of poor reception when you are between 200-500 miles from the MARS station because the radio waves have gone overhead, and are reflected back towards earth hundreds of miles further out. The MARS Phone Patch Net compensates for the areas of poor propagation by having several stations in different locations around the US in operation at the same time. If you are very close (

The Primary Frequency is the best overall frequency for long distance communication. During the winter months, the ionosphere tends to be less reflective at night, and this results in the radio waves continuing straight off into space. During these nighttime periods, the net monitors the Secondary Frequency as well as the Primary Frequency.

Radio Propagation on the Secondary Frequency tends to follow the curvature of the earth via “ground wave”. During daylight hours, this frequency is reliable to 1000 miles or so. At night, the range improves to 3000 miles or more. You will notice the same effect when listing to AM Broadcast radio. At night, you are able to hear AM Radio stations from around the country, while during the day only stations in nearby cities can be heard.

While the USAF Phone Patch net has two frequencies that it monitors, it also has eight other frequencies that are available for use when propagation favors a different frequency, or traffic levels require.

Propagation Related Links:

Hourly HF Radio Propagation Charts (from the Australian Govt.)

Aircraft that come up to the net might be asked their general location IF NOT CLASSIFIED. This can be important with aircraft to determine which station should handle the patch. While this may take a minute, the goal is to provide the best quality patch based on location and propagation.


This illustration shows how radio propagation works on HF. There are several zones involved.

Ground Wave

Ground wave is the area that can be seen from the patch station to the aircraft, or line of sight.  HF travels further than line of site depending on the frequency and conditions.  This area changes constantly depending on weather, propagation and height of the aircraft.

Sky Wave

Sky wave is the area covered by the radio signal being bounced off the ionosphere.  This area changes constantly based on a multitude of conditions including sunspots, time of day, solar flares, etc.  The coverage of the sky wave can be small like several hundred miles to global.

Skip Zone

Also called the silent zone is an area not covered by the patch station.  There is almost always a skip zone on HF and again is affected by conditions as with the sky wave.  Aircraft within the skip zone of a patch station will not be able to communicate or be extremely weak.  The skip zone is a natural phenomenon that cannot be influenced by technical means. Its width depends on the height and shape of the ionosphere and, particularly, on the local ionosphere.


Sky waves can bounce many times even to the point of circling the earth.  This allows communication over long distances, but also can include skip zones in between.  An aircraft could have good signals and then fade out only to return back to good signals after traveling some distance.

These conditions are the primary reason why it is desired to know the aircraft’s position but only to the general location like a state or large body of water.  The net control needs to determine the best coverage at that time, but also consider the aircraft’s movement to try and avoid the skip zone.  There are two main calling frequencies for the phone patch net but many others that can be tried for better coverage of either ground or sky wave.  Night is most effective for long distance communication but the skip zone becomes significantly larger

This is also why when there is no answer by the net to wait 10 – 20 minutes and call again.  The aircraft may be in a skip zone for the patch stations but then flown out of the zone into either ground or sky wave coverage or propagation could have changed.  Calling several times slowly and distinctly when there is no response is important at times to reach the net when conditions are not favorable.  Stations will be trying to determine your callsign and location.

There is no way to predict propagation conditions without considering solar and geomagnetic indices. As the K factor increases, the quality of propagation reduces. Solar flares, CME and other sun events also have an affect on propagation and can change conditions rapidly.

Handiman’s Guide to Propagation

By Paul Harden, NA5N

Geomagnetic Indices and Conditions

0 0-2 Very Quiet S1-S2 None
1 3-5 Quiet S1-S2 Very Low
2 6-9 Quiet S1-S2 Very Low
3 12-18 Unsettled S2-S3 Low
4 22-32 Active S3-S4 Moderate
5 39-56 MINOR Storm S4-S6 High
6 67-94 MAJOR Storm S6-S9 Very High
7 111-154 SEVERE Storm S9+ Very High
8 179-236 SEVERE Storm Blackout Extreme
9 300-400 EXTREMELY SEVERE Blackout Extreme

Kp – Planetary K-index, averaged over past 3 hours and tends to be a measure of current conditions

Ap – Planetary A-index, 24-hour average and represents overall geomagnetic field conditions for the UTC day

HF Noise – Approximate “S-meter” noise level <10 MHz

Aurora- Approximate level of auroral activity
* High conditions usually extends to: Latitude 45 deg.
* Very High conditions extends to about: Latitude 35 deg.
* Extreme conditions can extend to below: Latitude 35 deg.

Solar Wind-averages 350-450 km/sec and density <10 p/cm^3 >500 km/sec or high density can trigger geomagnetic activity

Shock Wave- from a solar flare or Coronal Mass Ejection (CME) arrives at the Earth about 55 hours after the solar event.

Solar Flare Classifications

A Very Small None None
B Small None None
C Moderate *Low Absorption *Active to Minor
M Large *High Absorption *Minor to Major
X Extreme *Possible Blackout *Major to Severe

(*) – Conditions cited if Earth is in trajectory of flare emissions

Flare class further rated from 1-9, ex. M1, M2, M3 … M9

The larger the number, the larger the flare within that class

An X7 – X9 is considered a “Grand daddy” flare. Only a few have occured over the past 30 years, causing total dispruption to communications, huge aurora’s, power grid failures, etc.
Radio and x-ray emissions from a flare effect the Earth for the duration of the solar event, usually 30 minutes or less.

Earth is 8 light-minutes from the Sun.

Sunspot/Active Region Classifications

Alpha Unorganized, unipolar magnetic fields Little threat but watched for growth
Beta Bipolar magnetic fields between sun spots C class flares and possible large M class
Delta Strong, compact bipolar fields between spots High potential for a M of X class major flare. Major Flare Alert issued

© 1999 by Paul Harden, National Radio Astronomy Observatory, Socorro, New Mexico
“Above information may be freely used in other newsgroups, ham radio reflectors, non-profit journals, etc., without permission, providing credit is cited.”

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