Understanding Propagation Mechanism in Various Amateur Radio Bands

 This post is in response to a question by VU3CDK from Mumbai regarding the mechanism of propagation on various amateur radio bands. He also wishes to know the best time of the day, difference between bands etc. I am going to base the discussion on my experience over the years in ham radio as well as the information which I have gained from Elmers and friends from on air and online discussions. These days, when we think of propagation on HF or High Frequency bands, what we usually do is to have a look at QRZ.COM to see the Solar Terrestrial Data by N0NBH. As of now, it shows that 40-80m propagation is poor at day time and good at night. 30-20m is listed as poor at day time and good at night. Luckily, 17-15m is listed as good during day and night. 12-10m is listed as good at day time and poor at night. These may change on a day to day basis and vary with solar activity over varying periods of time.

Those who have been operating HF for a long period know that 80m and even 160m are mainly night time bands while 15m and 10m are day time bands. This is because higher level of ionization in the ionosphere is needed for reflection of higher frequencies back to Earth. That brings us to the concept of Maximum Usable Frequency or MUF. Frequencies above MUF go straight to the space without being reflected back to the Earth for being received as Sky Waves. Reflection to Earth and back to the sky is the basis of long distance communication with radio waves on a terrestrial basis. Frequencies above the MUF can be used for satellite communication if the satellite transponder picks them up and re-transmits back to Earth. Those waves can also be used for line of sight and groundwave propagations. Typically VHF, UHF and above are used for line of sight communication, communication through repeaters and through satellites.

All those who have worked on 40m, which is the popular amateur radio band for regional communication and having plenty of roll call nets, would have noted the pattern in which the band opens up. As a typical example, 40m can open up early in the morning for relatively long distance stations like between North and South India, at a distance of 2000 km. The time at which it opens could vary from 5.30 am to 6.30 am with the current level of solar activity. Things will be different when the solar activity comes down during the minima of the 11 year solar cycle 25, which we are going through. After some time, we in the South India stop hearing North Indian stations gradually and start hearing South Indian stations very strong. During early morning time we are unable to hear our local stations, though North Indian stations can hear our neighbouring stations.

As the day progresses, by about 8.30 am, local stations also start fading on 40m, with only the nearest stations being heard well. Moreover, the background noise also increases remarkably, typically to S9 to S9+10 or even 20 dB, in city regions. In rural areas, most often the noise, which is mostly due to several radio frequency interference generating devices around, is lower. Things can be quite different if you have plenty of solar panels around with solar inverter installations which are not compliant with RFI suppression guidelines. As one DX ham said jokingly, when we started off in ham radio long back, we were the cause of RF interference, often with our homebrew radios radiating harmonics and causing interference to analog TV reception. Now we radio amateurs are at the receiving end, often not hearing any DX stations most of the time. 

The cycle repeats in 40m by evening in the reverse order. We start hearing local stations first and later on long distance stations. That is why during AIRNET India at 7.30 pm stations from Indonesia and Maldives are able to check in and even become alternate net controls. Similarly, stations from Oman also check in to AIRNET India on 7.150 MHz, from a distance of nearly 2300 km. We do hear 40m Indonesian nets as well in the evening. In fact, if we hear a foreign language station in the evening on 40m, it is likely to be an Indonesian station. Sometimes we do get far away stations also on 40m SSB. I have been able to work stations from Spain and Belgium in the morning on 40m with my '100W and wire' setup. Decades back I had the fortune to work several DX stations including those from the United States of America, on 40m during evening and morning hours.

That brings us to the concept of Grey Line propagation. Simply put Grey Line is the line separating daylight and night zones of the globe. Due to the difference in the ionization of various layers of the ionosphere, dawn and dusk are the best times for long distance communication, typically on 40 and 20m bands. Though I have not heard many US stations during grey line propagation, mainly because of high noise level these days, I have been able to work some super stations on SSB in early morning and evening. Most of you who are regular on the 20m band would know that KE5EE is regularly heard on 14.507 kHz starting from 5.30 pm local time. But he has a super station and antenna yard, which very few radio amateurs have. Many stations in this region are able to contact him daily on that frequency. I have also been able to have an occasional contact.

Moving to the higher bands, 15m and 10m are open mostly during day time, though occasionally they are open at night also. Usually 15m opens first during day time followed by 10m when the solar activity increases. In the evening, 10m band closes first, followed by 15m. Incidentally, I was able to have an SSB contact with an Indonesian station on 10m at around 10 pm local time during the last year's solar eclipse in the United States. I was on the looking for and anomalous propagation during the solar eclipse, though I was way beyond the path of the eclipse. Another aspect on 10m propagation is the Winter Anomaly. 10m propagation is better during winter time. That is why I was finding 10m as the best band in January 2024 when I bought my HF radio while the band was almost dead by June 2024. In June we have Summer Doldrums which indicates peak solar activity and the reverse of Winter Anomaly cuts down the MUF below the 10m band. It is supposed to recover by the end of September. Still, I have been able to have occasional 10m contacts in July as well.

When we move on to 6m, which though strictly a VHF band, is there in my HF radio. 6m or 50 MHz band is known as the Magic Band and sometimes the Tragic Band. That is because the usual propagation rule which apply to other HF bands do not apply to the 6m band. It can be silent for months together and open up suddenly to give unusually long distance contacts, making it the Magic Band. These are due to special propagation modes like tropospheric Duct Propagation, Sporadic E propagation, Trans Equatorial Propagation and Auroral Propagation. Auroral Propagation and Sporadic E Propagation are listed at the Solar Terrestrial Data by N0NBH on QRZ.COM. Currently those are not active. Sporadic E Propagation was seen near the Summer Doldrums when the 10m became inactive. Auroral Propagation usually follows a huge solar plasma discharge producing a geomagnetic storm, which we had in May and in second week of August 2024. We in the tropics usually do not have a chance to see Aurora, leave alone direct our beam antennas to Auroral discharge for contacts by reflecting the beam off the plasma display.