Background: On December 4, 2017, I was making some casual FT8 QSOs on 40 meters when out of the blue, JR7AMZ answers my CQ. It was 6 pm local and the band was full of the usual Europeans. What a totally unexpected surprise! Working Asia from New England is a huge challenge usually requiring multi-element direct beams. And my verticals were beaming toward Europe.
Today, December 15th, I was up at 6 am, a little earlier than usual and for some reason, I had a desire to work Asia. So I configured my verticals for broadside bi-directional pattern of 160 and 340 degrees. ( If not the short path, maybe I can catch an Asian on the long path.)
Contesters and DXer tend to be very knowledgeable about radio propagation from their own extensive observations. Knowing what bands to operate on and at what times given the current state of the ionosphere can give a “contester” a winning edge or help a DXer contact an elusive country. Let’s look at the mechanics of long skip propagation:
If the frequency is too high, your signal will not be reflected and your RF will be ‘out of this world’. But there is one ideal frequency wherein your signal will be reflected for the longest distance. With the proper vertical wave angle, you can get up to 1800 miles. Beyond that, you need multiple hops. The more hops, the lower the signal strength due to reflection losses.
Typical ground reflection losses for DX hops are 3 dB for poorly conducting ground and 0.5 dB for sea water. 3dB represents a loss of half the signal.
With 11-bands to choose, how do I determine the proper band to use on a real-time basis? I could go to the Reverse Beacon Network (http://www.reversebeacon.net/dxsd1/dxsd1.php?f=17106) and configure it to display 100 spots received from spotters in Zone 5 and scroll down till I find Asian stations spotted. But remember these are reports of stations already received, worked, and spotted.
Or I could go to VOACAP (http://www.voacap.com/p2p/index2.html) and see the “prediction” for the New England to Asia path. See: https://www.n1fd.org/2017/12/03/propagation-prediction-websites/
An ionosonde results from an “ionospheric sounder” instrument used to monitor and measure the ionosphere. You can think of Ionosondes as ‘fish finders’ that find, instead of schools of fish, regions of electrons and electrically charged atoms and molecules in the upper atmosphere.
The first ionosondes were invented in the 1920s, grew in sophistication during the 1930s, and were used by both sides during WWII to identify the best shortwave communication frequencies. Ionosonde systems incorporate a transmitter tunable from as low as 500-kHz to as high at 40-MHz (1.6 to 12-MHz sweeps are a more typical range), antennas usually pointed straight up, and a receiver that tracks the transmitter listening for echoes reflected back to earth. It is, in other words, a radar system.
The Ionosphere is in constant flux. The global ionosonde network is periodically mapping the ionosphere measuring the highest frequency that reflected back to earth (this is Fc, the critical frequency) and at what height above the earth that occurs. The critical frequency is proportional to charged particle density in each ionospheric layer. Signals at frequencies above Fc at the F2 layer (highest ionospheric layer) continue off into space instead of coming back to earth.
Knowing the critical frequency at various points around the world enables calculation of MUF (Maximum Usable Frequency) for shortwave radio broadcast and two-way radio communication in those regions. A useful rule of thumb is the MUF will be around three times the Fc. So, for a Fc of 6.2-MHz, the MUF for signals transiting that region of the ionosphere would be around 18.7-MHz. In such conditions, the amateur 17-meter band, centered on 18.1-MHz, would be a great choice for long distance communication, as would the 20-meter band (14-MHz). The 15-meter band (21 MHz), on the other hand, would likely be ‘dead’ for paths across that region.
Why is the MUF so much higher than the Fc? Radio waves propagated over long distances are refracted (bent) back to earth at acute angles, not ‘bounced’ back to earth like a handball off a wall. Less ionization is needed for refraction at low angles than for a return of a signal transmitted straight up. (See https://www.linkedin.com/pulse/ionosondes-fish-finders-ionosphere-how-ham-radio-can-help-bill-hein.
Now the magic begins. To determine the MUF for your location is one thing. But for a circuit, you need the MUF at the destination. Then, for obvious reasons, your band choice would be the lower of the two.
From this website, you will find Sounders around the world: http://af7ti.com/stations.shtml.
We are lucky as one sounder is in our backyard: MHJ45 Millstone Hill, Westford, MA. For Asia, I use JJ433 Jeju Island, Korea which is an island off the South Coast of Korea.
It’s 5 am. Millstone shows MUF 9.68 MHz and Jeju 8.57 MHz so 40 meters would be the frequency. Sure enough, the FT8 decoder shows JE7JDL and JH0INP working many stateside QSOs. No luck! Now at 6 am, Millstone shows MUF 15.11 MHz and Jeju 9.15 MHz. Time to switch to 30 meters. There’s JA7WND, JK1IQK, and JE8CIC! I keep calling not to no avail. Maybe next time!
At Least I know I’m on the right frequency! I’m confident I work my share of Asian this winter!
73, Layne AE1N