1. VHF Direction Finder
1.1 The VHF Direction Finder (VHF/DF) is one of the common systems that helps pilots without their being aware of its operation. It is a ground.based radio receiver used by the operator of the ground station. FAA facilities that provide VHF/DF service are identified in the Airport/Facility Directory (A/FD).
1.2 The equipment consists of a directional antenna system and a VHF radio receiver.
1.3 The VHF/DF receiver display indicates the magnetic direction of the aircraft from the ground station each time the aircraft transmits.
1.4
DF equipment is of particular value in locating lost aircraft and in helping to identify aircraft on radar.

2.
Nondirectional Radio Beacon (NDB)

2.1 A low or medium frequency radio beacon transmits nondirectional signals whereby the pilot of an aircraft properly equipped can determine bearings and “home” on the station. These facilities normally operate in a frequency band of 190 to 535 kilohertz (kHz), according to ICAO Annex 10 the frequency range for NDBs is between 190 and 1750 kHz, and transmit a continuous carrier with either 400 or 1020 hertz (Hz) modulation. All radio beacons except the compass locators transmit a continuous three.letter identification in code except during voice transmissions.
2.2 When a radio beacon is used in conjunction with the Instrument Landing System markers, it is called a Compass Locator.
2.3 Voice transmissions are made on radio beacons unless the letter “W” (without voice) is included in the class designator (HW).
2.4 Radio beacons are subject to disturbances that may result in erroneous bearing information. Such disturbances result from such factors as lightning, precipitation, static, etc. At night radio beacons are vulnerable to interference from distant stations.
Nearly all disturbances which affect the aircraft’s Automatic Direction Finder (ADF) bearing also affect the facility’s identification. Noisy identifica-tion usually occurs when the ADF needle is erratic; voice, music, or erroneous identification will usually be heard when a steady false bearing is being displayed. Since ADF receivers do not have a “FLAG” to warn the pilot when erroneous bearing information is being displayed, the pilot should continuously monitor the NDB’s identification.
3. VHF Omni.directional Range (VOR)
3.1 VORs operate within the 108.0 . 117.95 MHz frequency band and have a power output necessary to provide coverage within their assigned operational service volume. They are subject to line.of.sight restrictions, and range varies proportionally to the altitude of the receiving equipment.
NOTE.
Normal service ranges for the various classes of VORs are given in GEN 3.4, TBL GEN 3.4.1, VOR/DME/TACAN Standard Service Volumes.
3.2 Most VORs are equipped for voice transmission on the VOR frequency. VORs without voice capability are indicated by the letter “W” (without voice) included in the class designator (VORW).
3.3 The effectiveness of the VOR depends upon proper use and adjustment of both ground and airborne equipment.
3.3.1 Accuracy. The accuracy of course alignment of the VOR is excellent, being generally plus or minus 1 degree.
3.3.2 Roughness. On some VORs, minor course roughness may be observed, evidenced by course needle or brief flag alarm activity (some receivers are more subject to these irregularities than others). At a few stations, usually in mountainous terrain, the pilot may occasionally observe a brief course needle oscillation, similar to the indication of “approaching station.” Pilots flying over unfamiliar routes are cautioned to be on the alert of these vagaries, and, in particular, to use the “to.from” indicator to determine positive station passage.

Federal Aviation Administration Twentieth Edition
3.3.2.1 Certain propeller RPM settings or helicopter rotor speeds can cause the VOR Course Deviation Indicator (CDI) to fluctuate as much as plus or minus six degrees. Slight changes to the RPM setting will normally smooth out this roughness. Pilots are urged to check for this modulation phenomenon prior to reporting a VOR station or aircraft equipment for unsatisfactory operation.
3.4 The only positive method of identifying a VOR is by its Morse Code identification or by the recorded automatic voice identification which is always indicated by use of the word “VOR” following the range’s name. Reliance on determining the identifica-tion of an omnirange should never be placed on listening to voice transmissions by the FSS (or approach control facility) involved. Many FSS remotely operate several omniranges which have different names from each other and, in some cases, none have the name of the “parent” FSS. (During periods of maintenance the facility may radiate a T.E.S.T code (-  -) or the code may be removed.)
3.5
　
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