BECAUSE WE'RE ALL ABOUT RADIO
FM DX PROPAGATION
FORECASTS AND LIVE CONDITIONS
Before you can identify when DX is open, you need to know what it sounds like when it is not. Complete 1-2 full bandscans of the FM band from your location. Note each station you hear, their format, their slogan/brand and how loud they are. Also, note where are the holes where you have no stations or very weak reception of a station. These will be your indicators that alert you when the band is open. Hopefully, your local FM band will allot you empty frequencies in different segments of the band from the low end (in the non-commercial section from 88.1-91.9) all the way through the upper end of the band. This will help you be ready for DX openings because if you hear a station where you know you typically do not, that is a clue that DX is in!
Not all propagation can be forecasted with much accuracy, but Tropospheric (tropo) ducting can be! William Hepburn's DX Info Centre has a large amount of information for DXers but perhaps the most used area is his Tropospheric Ducting forecast. His forecasts cover the entire globe and can provide an idea of the locations and strength of upcoming Tropo openings. Further, he includes a link to live APRS (an amateur radio position system) maps that can also indicate current openings and where they are found.
While sporadic Es can be much more unpredictable (hence the name), there still resources for identifying where and when an opening is occurring. Remember that Es skip will often start in the 12 or 10-meter amateur radio band (there are some that even theorize it can occur on the amateur radio 15-meter band) and as the opening intensifies, it rises in frequency. This is known as the Maximum-Usable Frequency (MUF), which indicates the upper end of the Es skip opening.
Another one of the early indicators that an FM Es opening may be coming is when there is an opening on the amateur radio 6-meter band. This is located in the 50 MHz area, just below the FM band. During the summer months, 6-meter openings are often a daily occurrence, though not all Es openings will be strong enough to venture further up in frequency into the FM band,
There is a site, coveted by amateur radio operators and FM DXers alike, DXMaps.com which will show these openings on a map. They are based on self-reported "spots" provided by DXers to notify other DXers what they are hearing. You can usually see very clearly where the Es "cloud" is by where the largest gathering of spots is clustered around. This site also includes an MUF Es map, which will show you the MUF for a particular area based on what is being spotted online. Between the spot map and the MUF map, you can keep an eye out for FM openings!
WHEN THE BAND IS OPEN
As you learned in my FM/TV DX 101 article posted here, the main two forms of FM propagation are Tropospheric Ducting and Sporadic Es. Tropo can happen at any time of the year, especially near coastal locations where temperature inversions are commonly place between warm air aloft and cooler air near the sea surface. Sporadic Es generally peak in the summer (with another peak in early Winter). So how to know when the bands are 'open?"
First, check check the low end of the FM band, especially during the summer. As an Es Skip opening will start low and work itself up the band as it intensifies. Also, note that generally as the MUF increases, the distance on lower frequencies will decrease. So, let's say the MUF has risen to 104.3 MHz and you are hearing stations in Minnesota. Earlier, you were noticing Minnesota was coming in around 88-89 MHz. If you go back to 88-89 MHz now, the stations you hear will likely be between you and Minnesota. This can be a great way to get stations that are relatively close to you and often too far for Es skip openings, especially in areas where tropo is not commonplace.
Tropo openings tend to be strongest within the first few hours after sunrise although you can get some great coastal tropo after sunset into the early morning hours if conditions are just right. Sporadic Es tend to be strongest during the daytime although in recent seasons, I was DXing an Es opening well into the evening towards midnight before it started to finally break down.
Expect to see DX between 600 and 1400 miles for a typical sporadic Es opening for a typical "single hop" Es opening. There are "double hop" openings where distances can stretch much, much further. Es openings of under 500-600 miles are rare but do occur. They are usually accompanied by a very high MUF, often into the 2-meter amateur radio band (144 MHz-148 MHz) and higher.
As usual, Gordon West (WB6NOA) has a fantastic explanation of the ins and outs of tropospheric ducting in the video below:
OTHER TYPES OF PROPAGATION
Tropospheric ducting and Sporadic Es are not the only ways that distant FM radio signals can make their way into your radio. While not as common (or as easy) these are some of the other propagation methods that you can explore, if interested, for FM radio DX.
Transequatorial Propagation (TEP) - reception of stations between roughly 3,000 and 5,000 miles (4,800 to 8,000 km) across the Equator from the receiving location. Not as common for FM DX especially during periods of minimal solar activity. There are two main periods of TEP reception: evening and afternoon TEP. Evening TEP is when you are most likely to see FM DX openings through this propagation method although it is much more reliant on solar activity than afternoon TEP (which rarely extends to FM DX)
Meteor Scatter Propagation (MS) - occurs when a radio signal encounters the ionized tail of a meteor. Reception comes in burst that lasts anywhere from milliseconds (for weaker ionization) to a couple of minutes for a highly ionized meteor tail. Most effective on open frequencies with weak or no audible stations. Activity is most noticeable during meteor showers as well as during the early morning (when there is the highest concentrations of meteors entering the atmosphere). Check out this excellent primer on Meteor Scatter FM DX including audio examples of what to listen for and tips on how to pull in Meteor Scatter!
Airplane Scatter - Very similar to Meteor Scatter and behaves very similarly. The mechanics are similar, just instead of signals refracting through ionized meteor trails they are literally bouncing off of an airplane! Depending on the angle of the plane and its altitude compared to the inbound signal and your location, determines the length of time the signal is audible. Reception in the 400 mile range is possible with this type of propagation.