Two fields, at right angles
What are the two components of a radio wave?
Electric and magnetic fields.
What is the relationship between the electric and magnetic fields of an electromagnetic wave?
They are at right angles.
Each field regenerates the other as the wave travels: a changing electric field produces a changing magnetic field, which produces a changing electric field. That mutual regeneration is why the wave keeps going with nothing to carry it. Radio needs no medium.
Polarisation is defined by the orientation of the electric field. Both fields are there and both are essential; the electric one is the one we name, by convention.
One speed, for everything
What is the velocity of a radio wave traveling through free space?
Speed of light — approximately 300,000,000 metres per second.
And the question that catches people:
Which of these frequencies travels at the highest velocity in free space?
All radio frequencies travel at the same velocity.
Frequency does not affect speed. A 3.5 MHz signal and a 1.2 GHz signal arrive together. What changes with frequency is wavelength, because in a fixed travel time a faster-alternating wave fits more cycles into the same distance.
Wavelength and frequency
What is the relationship between wavelength and frequency?
Wavelength gets shorter as frequency increases.
Inversely proportional. And the reason it comes up constantly:
In addition to frequency, which of the following is used to identify amateur radio bands?
The approximate wavelength in meters
Every band has two names, and fluency between them is everyday vocabulary rather than exam trivia.
The frequency ranges
Three ranges to memorise, each a factor of ten:
| Range | Frequencies | Amateur bands in it |
|---|---|---|
| HF | 3 – 30 MHz | 80, 40, 20, 15, 10 metres |
| VHF | 30 – 300 MHz | 6 metres, 2 metres, 1.25 metres |
| UHF | 300 – 3000 MHz | 70 cm, 33 cm, 23 cm |
Note the boundaries fall in convenient places: 6 metres (50 MHz) is just inside VHF, and 10 metres (28 MHz) is just inside HF. That pair sits either side of the 30 MHz line and behaves very differently as a result — which is the subject of the propagation lessons that follow.
Below HF are MF (300 kHz – 3 MHz, where AM broadcast lives) and LF; above UHF are SHF and EHF, the microwave ranges.
Check yourself
- Two signals, 7 MHz and 700 MHz, leave an antenna together. Which arrives first?
- Which range is 146 MHz in? 28.4 MHz? 446 MHz?
- What defines a wave’s polarisation?
Answers
- Together. All radio frequencies travel at the same velocity in free space.
- 146 MHz is VHF; 28.4 MHz is HF; 446 MHz is UHF.
- The orientation of the electric field.