PROPAGATION MODES

There are a number of different propagation modes that hams are interested in using. Some of them are mainly HF long distance modes, some span HF and VHF and other go well into the microwave range. A basic understanding of each mode might well help you make more contacts. Once you understand the different modes, you can also look at solar prediction numbers and try to maximize the DX you are able to work. Of course you should also keep in mind that some modes, especially VHF and above can be influenced by weather. More on this topic later. There other modes not described below but this page will give you a good solid understanding of what is out there and how they may assist you in reaching some goal or objective. You can also find additional information in the ARRL Handbook of any recent year. They have a complete section detailing these and other propagation modes. So read on.......

SPORADIC E (E_S): This is a somewhat common long distance mode using the lowest of the three ionization payers in the atmosphere. Most all 6 meter DX over a couple hundred miles is done using Sporadic E, but not always. 10 meters can also be highly enhanced with Sporadic E. There is some very strong evidence to suggest that 2 meters is also subject to this mode. Typical contacts on 6 meters can be up to around 1200-1400 miles using what is generally referred to as single. With "double hop" these distances can be doubled. Most Sporadic E openings in North America are in June and July though other months at times offer Sporadic E propagation. It is rare in the fall and winter months, or even early spring since the sun energy reaching that portion of the ionosphere is greatly reduced from later spring and summer months. The "E" region of the ionosphere is about 65 miles above Earths surface. Signals can also be subject to slow or rapid fading as the ionization level changes from minute to minute

F₁: This layer is one that is generally split off from the F₂ during daytime hours yet disappears at night. It is certainly the lower of the F layers, being about 100 miles high, but it also much weaker then the higher F₂ layer. It actually does not play a major role in long distance skip but does tend to act more like Sporadic E then F₂.

F₂: This is the big on for the HF DXer. This layer, which can be as much as 300 miles high is the one that allows bands such as 20 through 10 meters to be wide open or "dead as a door nail." HF contacts out to thousands of miles are routinely made using this mode. Signals can also be subject to slow or rapid fading as the ionization level changes from minute to minute. It has little if any impact on VHF signals.

GRAY-LINE: This mode is a north - south propagation mode that is also known as twilight zone. Basically what Gray-Line propagation provides is a north top south mid day path along the "terminator" line which is what an astronomer would refer to the phenomenon. So what is it then? Actually the term gray line describes it quite well. Think if the earth as it rotates during the 24 hour cycle. There is not a clearly defined line between night and dark is there? Due to light scatter in the atmosphere there is a gray area, or twilight zone between bring sun and complete darkness . Kind of a 'gray" area isn't it? The sunlight hitting this "gray" area can cause sufficient ionization to allow good HF propagation across the equator. So gray-line is now not so gray.

TROPO DUCTING: This mode of propagation is a VHF/UHF'ers dream. This is the way to work Very long distances on say 2 meters. In the June 2001 Contest I was able to work BK29 (Hawaii) from CM97 (California) on 2 meter SSB. Tropo generally is weather induced when inversion layers are formed. To really simply tropo an relatively thin air mass gets trapped between two other air masses at greatly different temperatures. This creates a duct that if (for example) a 2 meter signal gets into it continues in the duct until the duct ends. Hopefully there is another station within that duct somewhere near the other end. If the duct is above your location, then you will not hear anything. Same if it is below your location. You must be in the duct to be able to hear the signal. Of course it goes without saying that the stations at both ends need to be within the duct. In some parts of the country ducts are known to form on a somewhat regular basis. Learn to listen to weather forecasts and figure out from then when ducting might be possible. See you on 2 meter tropo. Maybe even 432 or another band.

AURORA: This is a terrific mode for the VHF ham. Although it does not happen all that frequently it is well known in the VHF weak signal groups. What happens here is an unusual amount of energy is ejected by the sun and heads towards Earth. When it reaches the planet it tends to concentrate around the North Pole (or South Pole in the Southern Hemisphere) forming aurora conditions. The closer your station is to the pole, the more frequent and/or stronger aurora enhance you will get. This more allows for long distance VHF communications, mostly on 6 and 2 meters. An oddity of this mode is antenna pointing. For any of the above modes antennas are generally pointed at the station being worked. For aurora the antenna is pointed at the aurora, generally northerly (or southerly for the Down Under folks). High power is quite helpful here and fading can be extreme and rapid. The VHF signal hits the highly ionized field and is bounced back at a different angle from what it arrives. This mode would allow, for example, a 2 meter station in Philadelphia to work a station in Chicago with both antennas pointing north. If they both pointed their antennas at each other nothing would be heard. You know you are working aurora when you here the buzzing on the received signal. This buzz is provided courtesy of the aurora itself. As the ionization level drops off, signals disappear and propagation returns to normal. For additional information go to the web page on the sun and learn more about this mode. Click HERE to go there now.

BACK SCATTER & SIDE SCATTER: These are very interesting propagation modes that few hams ever experienced. It is quite useful on 10 meters during low sun spot activity as well as 6 meters during sun spot peaks. To explain it in very simple terms it is the result of the RF signal that leaves the antenna hitting the ground at some distant point, scattering in random directions with some of it winding up at the other end of a QSO. This is a real hit and miss mode of communications but it does make for band activity. The old adage really applies here; no matter how good band conditions are, there will be no contacts made unless someone transmits and another person receives the signal and responds. So make some noise!

METEOR SCATTER: Somewhat related to back and side scatter in concept, this mode is a little more defined and predictable. Meteor showers such as Leonids and Persieds are well known and documented. However, meteors enter the atmosphere all the time. All it takes is one good meteor burn to get information from one end of a QSO to the other. Getting information back will most likely require another meteor or so. A mode such as High Speed CW is perfect for this type of work, especially when random contacts are attempted. During a known meteor shower CW and even SSB have been successfully used though contacts can take some time. High power and high antenna gain is very helpful for this. The other keys to success are accurate clocks and frequency. You have to know when to transmit and when and where to listen or it is nearly impossible to make a two way contact. For additional information on meteor showers you can go to that web page by clicking HERE and reading more about them.

RAIN SCATTER: Very similar to meteor scatter but instead a signal is bounced off of rain drops. This is almost always done on 10 GHz. That is not to say to would not work on other frequencies. In fact, rain scatter has been observed on 5760 MHz, and even 24 GHz though much less often. According to Tom Williams, WA1MBA, rain scatter occurs when the water droplets are 1/10 wavelength or less in size. Doppler shift will be very apparent making CW the mode of choice.  SSB simply would not work well with Doppler shift. Besides CW has a several dB advantage when compared to SSB.

See QST, January 2003, page 88 for more information on this subject in Tom Williams column.

KNIFE EDGE: You may not find this one listed in a reference book but I have included it because it does work. This mode is fairly simple and works very well at microwave frequencies and into visible light. It is usable at lower frequencies as well. All it really does it take a signal that hits the very top of a mountain (for example) and actually gets bent by the edge. It is simple to demonstrate with a knife. Hold a sharp edge of a knife out in front of you while looking at a point light source, say an LED. Move the blade up and down slightly and watch the effect. Picture a microwave signal pointed at the top of a mountain from the valley below. There is another station on the other side of the mountain that you can not see because of the mountain but yet you can work him. Your signal is hitting the top of the mountain and being deflected down into the next valley over and low and behold, contact is made. This is a fairly reliable thing to do.

BOUNCE/REFLECTION: Somewhat related (though loosely) to knife edge, this is a mode very commonly used at microwave frequencies. Send a signal from one valley to a mountain side that can also be seen by another valley and contacts can often be made. Signals can be bounced off the side of building, towers, just about anything both ends of the contact can see even though they can't see each other. This works well at 10 GHz but it also quite usable at other microwave frequencies.

LINE OF SIGHT: This is not actually a mode but I included it from a practical standpoint. It is pretty self explanatory. If you can see the station you are working you can make a contact with little trouble. The VERY high microwave frequencies can be an except due to very lower available power availability and quite high atmospheric absorption of signals.

GROUND WAVE: This mode of propagation is primarily a low frequency mode. Basically it is a lower frequency signal that travels near the surface and does not leave the atmosphere. Ground wave can also include waves that reach the troposphere and bend around the curvature of Earth which is only a few miles up. There is a lot of attenuation of ground wave during the day but at night signals can reach very long distances. This is what allows the "Top Band", 160 meters to be a real night time DX band. Now atmospheric noise is another issue but we will not get into that topic here.

MICROWAVE SIGNAL ABSORPTION EFFECTS: At frequencies of 10 GHz and higher, especially 24 GHz, different path loss issues come into play that to a large extent do not affect lower frequencies nearly as much. For example, a 10 meter signal sees little attenuation going through a 6" diameter tree trunk but at 2304 it might be a real concern. At 24 GHz, the contact might not be worth even trying from that location. Water drops absorb 24 GHz signals like they were made for each other. At higher frequencies the oxygen molecule has high absorption rates. These are all issues to be dealt with but suffice it to say that the higher in frequency you go, the bigger the problem. Then as you continue going higher some of those problems sort of go away but others take over. There is still some real frontiers out there to be bridged.

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