# Propagation

### Explanation of question 17

In the HF bands

- i there is less refraction of radio waves at higher frequencies
- ii at higher frequencies less ionisation is required to cause refraction

1. i and ii are both true
2. i is true but ii is false
3. i is false but ii is true
4. i and ii are both false

The answer is :- 2 To cause refraction of a radio wave, greater ionisation is required as the frequency increases. There is therefore less refraction of radio waves at higher frequencies. A too high frequency can pass straight through the ionosphere.

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