Why does the best band change with the time of day, and how do I pick one?
The ionosphere is not constant. It changes with the sun, and that changes which bands work.
The sun and the ionosphere
- How strongly the ionosphere is ionised depends on sunspot activity on the sun.
- More sunspot activity means better sky-wave propagation.
- The Sunspot cycle is the long-term variation in the number of sunspots. Its average length is 11 years.
Which bands are affected?
- The bands most affected by sunspot activity are 10 m and 15 m.
- Sunspots generally do not affect VHF propagation. The exception is a solar flare, which may cause intermittent VHF interruptions.
40 m and 80 m: quieter by day, longer by night
On 40 m and 80 m, signals are quieter by day and travel longer distances by night.
Why is 40 m poor for local contacts at night? Because signals travel too far and there is too much interference.
Picking a band
The tutorial gives these four worked cases. Learn them as written, because exam questions are likely to use the same wording.
| Situation | Use |
|---|---|
| Station 200 miles away at 5 a.m. | 3650 kHz (80 m) |
| Station 200 miles away at noon | 7.075 MHz (40 m) |
| Station more than 1000 miles away at noon | 28.310 MHz (10 m) |
| Station 500 metres away at noon | 145.555 MHz (2 m) |
You now know
- Sunspot activity decides how ionised the ionosphere is; more sunspots mean better sky-wave propagation.
- The sunspot cycle averages 11 years; 10 m and 15 m are most affected.
- VHF is generally not affected by sunspots, apart from solar flares.
- 40 m and 80 m are quieter by day and longer by night.
- 200 miles at 5 a.m. is 3650 kHz (80 m); 200 miles at noon is 7.075 MHz (40 m); more than 1000 miles at noon is 28.310 MHz (10 m); 500 metres is 145.555 MHz (2 m).