I had an interesting telephone conversation with a friend and fellow DXer today and we were discussing the FM List website. Of the things we discussed, he asked me if I knew for certain which transmitter sites I was receiving and could I be wrong with some of them. I tried to answer his question and I think some of the points we discussed are worth mentioning.
So there's an opening to Italy and I get a signal on 107.9 from Radio Maria. How do I know which of the billions of transmitters I am receiving? I don't - initially anyway, but after some time it can become apparent which transmitter it is. It is also possible that mistakes can be made, but with a little care and experience, I think it's possible to be almost 100% certain with this in most situations.
You also have to be aware that the DX path can change, so you realise nothing can be absolutely guaranteed, but there is a logic to this. Italy is the best example. Finding a station with multiple transmitters on one frequency is common.
The Radio Maria network is one of the best examples of multiple sites-per-frequency I can think of. You could also look at RAI stations, where there could be dozens of low power relays on a single frequency, together with one or more higher powered outlets. It is important not to assume that you are always receiving the highest powered transmitter on the frequency. Sporadic E openings can be very confined, allowing reception of only a small town or city. Of course, sometimes you could be receiving a wide area, including several regions. So what needs to be done to ensure the correct transmitter is logged?
To begin with, I might check a DX Cluster. DX Maps is a good place to start. There are various others too, but they all tend to be interlinked these days. Just keep your eye on them and check they are updated regularly. For sporadic E on VHF band 2 you need to be checking the 50, 70, "FM DX" and 144 MHz pages. Look to see where the paths are open to.
OK, so I now know that the band is open to southern areas if Italy. I am still no wiser regarding my Radio Maria transmitter. My next plan is to consult the FM List. I call up 107.9 and sorted the stations into alphabetical order or "Programme". There are approximately three billion transmitters for Radio Maria on 107.9 (only a slight exaggeration!). My personal experience tells me that Santu Lussurgiu is the most likely candidate but I know Asti is also a possibility. Maybe some of the other low power sites are possible too. My gut reaction at this stage might be to go for Santu Lussurgiu. I click on "Santu Lussurgiu" and get a pop-up menu which lists other stations which use that site. I see there is a higher power outlet of Radio Dimensione Suono on 87.7 and another on 105.8. Let me check them ...... Nope. Nothing there. So maybe I am not receiving Santu Lussurgiu. Still keeping an open mind, I click on Asti and repeat the same procedure. Ah, there is only one station coming from this transmitter - Radio Maria, so this is not helpful. Let's look at Ancona. Bingo! They also have Veronica My Radio on 88.0 and that is coming in too. So the chances are now very strongly in favour of Ancona being the site of my Radio Maria on 107.9. I am now going to tune around the band, looking for other stations coming from this site.
This situation is only going to be successful if a small area of Italy is coming through. When multiple regions and provinces are present there is usually little you can do. Those conditions probably move around so there is still a degree of guesswork to begin with and, as is the case with Italy, many station's sites remain unidentified.
Next thing is to look at the FM List Worldwide Visual Logbook. It is clear to see how the path of DX tends to fall in a straight line. This not only helps increase that percentage of certainty but can also clearly show if you have made a mistake. Having studied propagation paths on FM List for a number of years I can see that these lines of propagation can often reveal the exact are which is being received. There is something called the parallax effect. It's easy to see how this works when you look at the FM List's Visual Logbook. The paths of different DXers intersect in such a way so all those lines meet roughly in the middle, exactly where the refraction point is. On days of multiple Es clouds this positioning becomes blurry as many reception paths open up at the same time.
Two other great benefits of using the FM List are: Once you have logged a particular transmitter, it is highlighted with an orange background on other frequencies where that transmitter exists. Also, future frequency searches are narrowed down to a smaller compass beamwidth, thus eliminating transmitters which are well off beam and therefore probably not coming through.
Don't neglect those PI codes either. You can enter these into FM List and conduct a search to find out which stations use that PI code. Each one is a unique identifier for that station and they are all clues. Sometimes a transmitter might have a local PI code and so you know which site you have received straight away.
Another thing I have found essential to do is log everything! Why? That might sound excessive but it helps to build up an even clearer picture of the propagation path.
This might still not be 100% foolproof due to the sheer number of sites in Italy, but keep all these things in mind and just take your time. I looked at Italy in the example here. Other countries are less complicated. All this comes with experience too I suppose, but these methods form the basis of how I decide which sites I have received.
One other rather more obvious way to identify a site, but one which isn't always available, is to use the AF (Alternative Frequency) list, which is (sometimes) Sometimes it's quite obvious. In the case of me checking a recent meteor scatter bust from Finland on 87.9. The RDS is complete, 6201 and YLE_YKSI. The AF list says "87.9 89.2 93.2 88.5 89.5 90.9 89.8". Clearly this is YLE 1 from one of the two sites listed on 87.9. But which one? Espoo (60 kW) in the south of the country or Ainiovaara (3 kW), half way up the country and on the Swedish border. All those frequencies have transmitters in the near vicinity of Espoo but not of Ainiovaara. It's Espoo!carried within the RDS data.