VOR Principles of Operation

CHAPTER ONE

INSTRUMENT FLIGHT RULES WORKBOOK

mega (M) for 1,000,000 Hz. For example, a frequency of 225,900,000 Hz would be written

225.9 MHz.

The four main frequency ranges used with air navigation and communication radios are:

Low Frequency/Medium Frequency (LF/MF)

High Frequency (HF) (not discussed)

Very High Frequency (VHF)

Ultra High Frequency (UHF)

Military air navigation and communication radios operate in specific bands within each

frequency range as indicated below:

LF/MF 100 to 1750 (kHz)

HF 2.0 to 29.999 (MHz) (not discussed)

VHF 108.00 to 136.975 (MHz)

Military aircraft use LF/MF radios, which are more commonly known as automatic direction

finders (ADFs), primarily as a backup navigational radio. Aircraft LF/MF radios are similar to

automobile radios; they are only capable of receiving. An ADF receiver is used to receive

bearing information from ground transmitters called non-directional beacons (NDBs). These

facilities operate in the 190 to 535 KHz band and transmit a continuous three-letter Morse code

identifier. Those NDBs which have a class code of AB (listed in IFR Supplement) transmit

continuous automatic transcribed weather broadcasts in addition to the three-letter identifier. An

ADF receiver can also receive commercial broadcasting stations operating in the AM frequency

band from 540 to 1650 kHz. The ADF is very susceptible to atmospheric disturbances and is

severely limited in areas of thunderstorms or other severe weather. TACAN and VOR, however,

are not greatly affected by weather. The T-34C is one of the few military aircraft that has no

ADF installed.

The VHF frequency band of 108.00 to 136.975 MHz serves the purposes of air navigation and

communication. The complete frequency band is used by civilian aircraft for direct pilot to

controller two way VHF voice communications.

103. VOR PRINCIPLES OF OPERATION

The lower half of the VHF band is used in the VHF Omnidirectional Range (VOR) air

navigation system; the frequency range is from 108.00 to 117.95 MHz. The present system of

federal low altitude airways in the United States is based on a network of VOR stations.

1-4 INTRODUCTION TO AIRBORNE NAVIGATION AND COMMUNICATIONS

EQUIPMENT AND PRINCIPLES OF OPERATION