Global Propagation
Radio Propagation Basics

Radio propagation on the primary frequency is usually good out to three thousand miles or more from the MARSRADIO 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.

Stations operating at night monitor both the primary and alternate 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 alternate frequency should be tried. When able, include your location on the initial call. MARS stations use directional antennas that need to be pointed in your direction for best reception.

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 MARSRADIO station because the radio waves have gone overhead, and are reflected back towards earth hundreds of miles further out. MARSRADIO compensates for the areas of poor propagation by having several stations in different locations around the US in operation at the same time. 

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 alternate 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 MARSRADIO 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.
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 aircrafts 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 aircrafts 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 longer.

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.