Nav Topic 1 basic radio wave properties

AV2220 - Aircraft Communication Systems Chapter 1 1

AVIONICS AVIONICS TECHNOLOGYTECHNOLOGYRadio WavesRadio Waves

Radio is a method of transmitting intelligence from one location to another by means of electromagnetic radiation.

The first radios installed in airplanes were used for communications and it was only much later that navigational radio systems were developed.

Radio systems for other purposes have also been developed, especially in the last 20 to 30 years.


AVIONICS AVIONICS TECHNOLOGYTECHNOLOGY

Some of the uses for radio in modern aircraft can be categorized as follows. Communications Both voice and data.‑

Navigation Many different systems are in use today.‑

ATC Radar The Air Traffic Control system relies on radar.‑

Weather Avoidance Used to avoid areas of adverse weather.‑

Approach Aids A specialized type of navigation to guide an aircraft ‑down to the runway in bad weather.

Altitude Measurement Gives precise altitude above ground level.‑

Airborne Collision Avoidance Warns the pilot of nearby aircraft.‑



The RF portion of the electromagnetic wave spectrum extends from approximately 3 kHz to 300 GHz.

Table radio frequency bands:

Radio Wave Spectrum

30 to 300 GHzExtremely High Frequency (EHF)

3 to 30 GHz (Giga Hertz)Super High Frequency (SHF)

300 to 3000 MHzUltra High Frequency (UHF)

30 to 300 MHzVery High Frequency (VHF)

3 to 30 MHzHigh Frequency (HF)

300 to 3000 kHzMedium Frequency (MF)

30 to 300 kHzLow Frequency (LF)

3 to 30 kHzVery Low Frequency (VLF)

BANDFREQUENCY RANGE



The frequencies are divided into bands which, starting at the low end, are: very low frequency (VLF), low frequency (LF), medium frequency (MF), high frequency (HF), very high frequency (VHF), ultra high frequency (UHF), super high frequency (SHF) and extremely high frequency (EHF).

Radio frequency chart showing the operating frequencies of common aircraft systems:

Radio Wave Spectrum (cont’d)



The range and diversity of electromagnetic waves, or EM waves, is very broad.

The entire spectrum of EM waves includes not only radio waves but visible light, gamma rays, infrared, etc.

Common aircraft radio systems are included on the right side in the Figure. Notice: there is an aviation

application for all of the bands except EHF.




Radio waves are produced by an alternating current that is fed to an antenna to produce radio frequency ‑waves.

Radio waves travel as an energy field from the antenna at the speed of light, approximately 300,000,000 meters per second.

An RF wave is composed of an electromagnetic field and a 90o displaced electrostatic field.




The polarization of an RF wave is determined by the direction of the lines of force in the electromagnetic field.

The direction of these lines of force is dependent on the polarization of the radiating element. A vertical antenna produces a

vertically polarized wave A horizontal antenna emits a

horizontally polarized wave.

Simple vertical antennas have omnidirectional characteristics in that they transmit and receive in a 360o pattern, They are used for

communication. Horizontally polarized antennas are basically directional, They are primarily used for

navigation.




All EM waves consist of two different invisible energy fields that travel through space. The electric field and the magnetic field are at right angles to each other and to the direction of propagation or travel.

Figure below shows the two fields and the direction of propagation.


A radio wave has two components, an electric wave (E) and a magnetic wave (H)



Radio waves are produced when a radio frequency electrical signal is sent down a conductor to an antenna.

The antenna transforms the electrical signal into EM waves which propagate outward from the antenna through space.

The EM waves travel through space at the velocity of light, which is 186,284 miles per second or 300,000,000 meters per second.

The basic operating frequency of a radio is called the carrier frequency, the signal carries the data or

information that needs to be transmitted from one place to another.

The wavelength of an EM wave is the distance from peak to peak for the invisible waves in the electric and magnetic fields. Wavelength is measured in meters and it is inversely proportional to the frequency. The wavelength in meters can be found by dividing the constant 300,000,000 by the frequency in hertz.




The usable range of frequencies has been divided and bands assigned for various communication and navigation purposes.

The frequencies used for aviation communication and navigation are shown in Figure on the right side.

Communication radios use highly sensitive and selective transmitters and receivers for two-way communication between aircraft and ground stations or between aircraft in flight.

Frequency Band Use



The very low frequency (VLF) and low frequency (LF) bands were originally used for radio navigation.

Because the wavelengths were in the kilometer range and higher (30 kHz has a wavelength of 10 kilometers, or about 6.2 miles), enormous antennas had to been used.

Until today, these frequency bands are still used in aircraft for long range navigation (LORAN-C, Omega and Decca) and direction finder navigation (ADF).

VLF and LF Band Communication



The medium-frequency (MF) and high-frequency (HF) bands are not only used by commercial AM broadcasting stations, but portions are also used by aircraft for air-to-ground communication.

Aircraft flying over the oceans typically use HF communication because it can travel great distances.

HF equipment operates in the frequency range of 2 to 25 megahertz and is normally single-sideband.

MF and HF Band Communication



Signal radiation in these frequency ranges have the important property of being reflected by the ionosphere.

When a radio wave in the MF or HF range hits ionosphere layer, it is reflected back to earth.

Multiple reflections between this layer and earth are possible, allowing great distances to be obtained in these ranges, particularly the high-frequency band.

MF and HF Band Communication (cont’d)



The disadvantage of this type of propagation is that it depends on the characteristics of the ionosphere, which varies widely, especially during daylight hours.

As a result of this varying, the waves are reflected differently and take different paths over a period of time. This causes the signal at the

receiver to vary in strength, which causes the output to fade in and out.

MF and HF Band Communication (cont’d)



Signal radiation in these frequency ranges gets very little ionospheric reflection. Communications in these ranges tend to be line-of-sight and over a short distance. Line-of-sight means exactly what the name says the transmitter and receiver must be within a straight visual sighting line from each other. Buildings and uneven terrain may affect the transmission. For commercial use, the lower part of the UHF band and the VHF band is also used for mobile communications and television.

VHF and UHF Band Communication



Very high frequency (VHF) radio transmissions operate in the 118.000 to 135.975 megahertz range. This frequency range is used for air traffic control (ATC) communication and for communication between civil aircraft operated domestically. VHF communication use single-channel simplex operation in which a single frequency is used for both transmitting and receiving (single-channel), but only one person can talk at a time (simplex).

VHF and UHF Band Communication (cont’d)

Nav Topic 1 basic radio wave properties

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