1. Introduction

Radio and radar systems are now an integral and essential part of aviation, without which the current intensity of air transport operations would be unsustainable. In the early days of aviation, aircraft were flown with visual reference to the ground; flight at night, in cloud, or over the sea was not possible.

As aircraft complexity increased, it became necessary to design navigational systems that permit aircraft to operate without reference to terrain features. Early systems were very basic and inaccurate by modern standards — providing reasonable en route accuracy over land, but only limited service over the oceans. Until approximately 40 years ago, oceanic flight relied on traditional seafarer techniques of astro-navigation — taking sights on the sun, moon, stars, and planets to determine position.

Developments that commenced in the 1910s accelerated during the 1930s and 1940s, continuing to the present day, producing long-range systems that by the 1970s provided a global navigation service.

The development of radar in the 1930s allowed air traffic control systems to be developed, providing a service capable of identifying and monitoring aircraft at a much higher intensity than would otherwise be possible. Modern satellite technology extends this capability over oceans and areas where conventional radar is not practical.

2. The Radio Navigation Syllabus

The syllabus begins with the nature of radio waves and how they travel through the atmosphere. This is essential to understand:

• Why different radio frequencies are selected for particular applications
• The limitations those frequencies impose
• How radio waves are produced, transmitted, received, and how information is encoded and decoded

3. Electromagnetic (EM) Radiation

If a direct electric current (DC) is passed through a wire, a magnetic field is generated around the wire perpendicular to the current flow.

If an alternating electric current (AC) is passed through the wire, because the direction of current flow is changing, the polarity of the magnetic field also changes — reversing polarity as the current direction reverses. At low frequencies the magnetic field collapses back to zero with the current; but as frequency increases, the magnetic field will not have collapsed completely before the reversed field starts to establish itself, and energy begins to travel outward from the wire in the form of electromagnetic radiation — i.e., radio waves.

The resulting EM energy is made up of two components:

• An Electrical (E) field — parallel to the wire
• A Magnetic (H) field — perpendicular to the wire

4. Polarization

The polarization of radio waves is defined as the plane of the electric field and is dependent on the plane of the aerial:

• A vertical aerial → electrical field in the vertical plane → vertically polarized wave
• A horizontal aerial → electrical field in the horizontal plane → horizontally polarized wave

Circular Polarization

Circular polarization can be produced in a variety of ways — one method uses a helical antenna. In circular polarization, the electrical (and hence magnetic) field rotates at the frequency of the radio wave. The rotation may be right-handed or left-handed, dependent on the orientation of the aerial array.

5. Radio Waves & The Period

The length of time it takes to generate one complete cycle of a radio wave is known as the period, signified by the Greek letter tau (τ), and measured in microseconds (µs).

1 µs = 10⁻⁶ second

Frequency

Frequency is the reciprocal of period:

Frequency Units

6. Wavelength

The speed of radio waves (c) is the same as the speed of light (both are electromagnetic radiation):

c = 300,000,000 m/s (= 300 × 10⁶ m/s)   or   162,000 nautical miles per second

Simplified Formula (for Exam Speed)

7. Frequency Bands

The radio part of the electromagnetic spectrum extends from 3 kHz to 300 GHz. It is divided into 8 frequency bands, each related to its neighbours by a factor of 10.

flowchart LR
    A["3 kHz
(VLF)"] --> B["30 kHz
(LF)"] --> C["300 kHz
(MF)"] --> D["3 MHz
(HF)"] --> E["30 MHz
(VHF)"] --> F["300 MHz
(UHF)"] --> G["3 GHz
(SHF)"] --> H["30 GHz
(EHF)"] --> I["300 GHz"]
    style B fill:#e3f2fd
    style C fill:#e8f5e9
    style D fill:#fff9c4
    style E fill:#fce4ec
    style F fill:#f3e5f5
    style G fill:#e0f7fa
    style H fill:#fbe9e7

8. Phase Comparison

Some radio navigation systems use the comparison of phase between two signals to define navigational information. Two critical requirements:

One signal is designated the reference signal and the other the variable signal.

Graphical Method

1. Identify zero-phase on the reference wave (point A in Figure 1.4)
2. Move in the positive direction from point A
3. Measure the phase angle the reference wave travels through before zero-phase is reached on the variable wave (point B)
4. That angle is the phase difference

In Figure 1.4: starting at zero phase on the reference wave (point A), the reference wave travels through 270° before zero phase is reached on the variable wave (point B). Phase difference = 270°.

Mathematical Method

9. Practice Frequency–Wavelength Conversions

Q10. Determine the approximate phase difference between the reference wave and the variable wave: (The reference wave is the solid line and the variable wave is the dashed line)

1. 045°
2. 135°
3. 225°
4. 315°

✓ Correct Answer: (c) — 225°

Explanation: From the diagram, start at zero phase on the reference wave (solid line). Moving in the positive direction, the reference wave travels through approximately 225° before zero phase is reached on the variable wave (dashed line). The variable wave appears to lead the reference by about 135°, which means the reference wave must travel 360° − 135° = 225° in the positive direction. See Section 8.

Why the other options are wrong:

• (a) — 045° is too small. The variable wave is not just slightly ahead of the reference.
• (b) — 135° would be the phase difference if you mistakenly measured Variable − Reference (in the negative direction). This reverses the formula.
• (d) — 315° would require the variable wave to be only 45° ahead of the reference — not what is shown in the diagram.

Instructor's Note: On graphical phase questions, always identify the reference wave first, locate its zero crossing, then measure how far it travels (positive direction) before the variable wave's zero crossing. Don't measure the gap directly — measure the arc from the reference zero crossing in the forward direction.