Ham Radio General Class Practice Test

Which of the following amateur radio HF frequencies are least reliable for long distance communications during periods of low solar activity?

1. 3.5 MHz and lower

2. 7 MHz

3. 10 MHz

4. 21 MHz and higher

The correct answer is: 21 MHz and higher

The correct answer identifies that frequencies of 21 MHz and higher are least reliable for long distance communications during periods of low solar activity. This is primarily due to how solar activity affects the ionosphere, which is critical for HF (high frequency) radio propagation. During periods of low solar activity, the ionosphere does not reflect HF signals as effectively, particularly at higher frequencies. Frequencies above 20 MHz, such as those in the 21 MHz and higher range, are more dependent on a well-ionized ionosphere for long-distance propagation. When solar activity is low, the higher HF bands can often experience significant signal loss and reduced propagation range, leading to less reliable communications. In contrast, lower frequencies, such as those at 3.5 MHz or even 7 MHz, tend to perform better under these conditions. These bands can take advantage of lower ionospheric layers, making them more conducive for long-distance communications when solar conditions are poor. This is why operators often rely on lower frequencies for consistent communication during times of low solar activity. Frequencies around 10 MHz can still see some reliability but are not as consistent as 3.5 MHz or 7 MHz under similar conditions.